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                            THE EARTH SEEN
                             FROM THE AIR

[Illustration: FIG. 1--The National Capitol, Washington, D. C. A view
obliquely downward from a position over the National Botanical Garden,
showing the western front of the Capitol at the approach to it from
Pennsylvania Avenue. In the background, at the right, can be seen a part
of the Library of Congress and, at the left, a part of the Senate Office
Building. The radiating avenues of approach are of interest as well as
the character of the district surrounding the Capitol, as indicated by
the apartment houses and tree-lined streets.]




                     AMERICAN GEOGRAPHICAL SOCIETY
                       SPECIAL PUBLICATION NO. 4
                       W. L. G. JOERG, _Editor_

                         THE FACE OF THE EARTH
                         AS SEEN FROM THE AIR

                A Study in the Application of Airplane
                       Photography to Geography

                                  BY

                             WILLIS T. LEE
                        U. S. Geological Survey

                            [Illustration]

                     AMERICAN GEOGRAPHICAL SOCIETY
                       BROADWAY AT 156TH STREET
                               NEW YORK
                                 1922


                            COPYRIGHT, 1922
                                  BY
                   THE AMERICAN GEOGRAPHICAL SOCIETY
                              OF NEW YORK


                   CONDÉ NAST PRESS GREENWICH, CONN.




CONTENTS


CHAPTER                                                             PAGE

INTRODUCTION                                                          ix

   I THE VIEWPOINT                                                     1

  II FAMILIAR SCENES FROM A NEW ANGLE                                  7

 III ARCHITECTURE, LANDSCAPE GARDENING, AND ENGINEERING               11

  IV THE MOSAIC                                                       20

   V GENERAL ASPECTS OF THE SURFACE AS SEEN FROM THE AIR              22

  VI MARSHES AND MARSH DRAINAGE                                       27

 VII COASTAL MUD FLATS                                                41

VIII SUBMERGED LAND FORMS                                             45

  IX THE PLAIN FROM THE AIR                                           50

   X MOUNTAIN FEATURES                                                57

  XI AIR CRAFT IN THE STUDY OF ROCKS AND ORES                         69

XII MAPPING AND CHARTING FROM THE AIR                                 72

    INDEX                                                            105




LIST OF ILLUSTRATIONS

_(o) indicates an oblique_, _(v) a vertical airplane photograph_


FIG.                                                                PAGE

THE VIEWPOINT, AND FAMILIAR SCENES FROM A NEW ANGLE

1 The National Capitol, Washington, D. C. (o)               Frontispiece

2 Symbols of automatic register in the Eastman mapping camera (v)      3

3 The United States Military Academy, West Point, N. Y. (o)            8

4 The United States Naval Academy, Annapolis, Md. (o)                  9


ARCHITECTURE, LANDSCAPE GARDENING, AND ENGINEERING

5 Monument Avenue, Richmond, Va. (o)                                  12

6 The United States Naval Observatory, Washington, D. C. (o).         13

7 Shipyards at Newport News, Va. (o)                                  14

8 The New York Connecting Railroad Bridge (o)                         15

9 A part of Washington, D. C. (o)                                     16

10 Part of Rockaway Beach, Long Island, N. Y. (v)                     17

11 Part of Long Branch, N. J. (v)                                     18

12 Benning, D. C., and the Anacostia River (v)                        22

13 The land along the Anacostia River on the eastern edge of Washington,
D. C. (v)      facing                                                 22

14 Map of the same area as in Fig. 13                        facing   22


GENERAL ASPECTS OF THE SURFACE

15 Southeastern part of Mulberry Island on the lower James River,
Maryland (v)                                                          24

16 Map of the same area as in Fig. 15                                 25

17 Columbus, Ga. (v)      facing                                      26

18 Map of the same area as in Fig. 17                        facing   26


MARSHES AND COASTAL MUD FLATS

19 Tidal marsh and ocean beach at Corsons Inlet, New Jersey (v)       28

20 Details of marshland, Lee Marsh, lower Pamunkey River, Virginia
(v)                                                                   30

21 Details of marshland, Cousaic Marsh, lower Pamunkey River,
Virginia (v)                                                          31

22 Barrier beach from Corsons Inlet to Atlantic City, N. J. (v)       32

23 A river system in miniature near Hampton, Virginia (v)             33

24 Sweet Hall Marsh on the lower Pamunkey River, Virginia (v)         34

25 Map of the same area as in Fig. 24, with cross section             35

26 Eltham Marsh on the lower Pamunkey River, Virginia (v)             36

27 Map of the same area as in Fig. 26, with cross section             37

28 A stream system of the mud-flat area off the Eastern Shore of
Virginia (v)                                                          42

29 Mud-flat streams (v)                                               43


SUBMERGED LAND FORMS

30 Stove Point Neck at the mouth of the Piankatank River, Virginia
(v)                                                                   46

31 Gwynn Island at the mouth of the Piankatank River, Virginia (v)    46

32 Map showing the location of Figs. 31 and 32, with cross section    47

33 A drowned valley: Lambs Creek, Chesapeake Bay, Virginia (v)        48


THE PLAIN FROM THE AIR

34 The Chattahoochee River south of Columbus, Ga. (v)        facing   50

35 Map of the same area as in Fig. 34      facing                     50

36 A river channel in the Great Plains: The Red River northeast of
Wichita Falls, Tex. (v)                                               51

37 A glacial drift plain in southwestern Michigan (v)                 52

38 Map of the same area as in Fig. 37                                 53

39 Schoolcraft, Mich. (v)                                             54

40 Map of the same area as in Fig. 39                                 55

41 Kettleholes in glacial till southwest of Schoolcraft, Mich. (v)    56


MOUNTAIN FEATURES

42 Mt. Shasta, California (o)                                         58

43 A glacial gorge on the northeastern face of Mt. Shasta (o)         59

44 Yosemite Valley, California (o)                                    60

45 Map showing the angle of vision of Fig. 44                         61

46 Cinder Cone on the eastern edge of the northern Sierra Nevada,
California (o)                                                        62

47 Map showing the angle of vision of Fig. 46                         63

48 The top of Cinder Cone (o)                                         64

49 Simi Hills northwest of Santa Monica, Cal. (v)                     65

50 Part of Santa Monica Mountains north of Santa Monica, Cal. (v)     66

51 Map of the region between the center of Los Angeles and Santa
Monica, Cal., showing the location of Fig. 50                         67

52 A young mountain gorge in the San Joaquin Hills, a coastal range
in Southern California (v)                                            68

53 Canyon in sedimentary rocks near the mouth of the Pecos River,
Texas (v)                                                             70


MAPPING AND CHARTING FROM THE AIR

54 View across the western end of Lake Erie (o)                       73

55 Map showing the angle of vision of Fig. 54                         73

56 Rochester, N. Y. (v)      facing                                   74

57 Map of the same area as in Fig. 56      facing                     74

58 Index map showing the location of the airplane photographs in
this book taken on the Atlantic seaboard of the northeastern
United States                                                         75

59 Marshlands on Chesapeake Bay south of the mouth of the York
River, Virginia (v)                                                   76

60 Left shore of the York River northwest of Gloucester Point, Va.
(v)                                                                   77

61 The northwestern tip of Sandy Hook, New Jersey (v)                 78

62 Beach cusps under water near Beach Haven, N. J. (v)                80

63 First stage in the formation of an inlet through a barrier beach:
near Beach Haven, N. J. (v)                                           81

64 A tidal delta in Shark River Inlet, Belmar, N. J. (v)              82

65 A tidal delta, Popes Creek on the lower Potomac River, Virginia
(o)                                                                   83

66 A double tidal delta at Barnegat Inlet, New Jersey (v)             84

67 A tidal inlet through the barrier beach south of Beach Haven,
N. J. (v)                                                             85

68 Beach between Brigantine and Little Egg Inlets, New Jersey (v)     86

69 A simple spit: Lower Cedar Point, Maryland, on the lower Potomac
River (o)                                                             87

70 A recurved spit south of Brigantine Inlet, New Jersey (v)          88

71 A recurved spit showing interference with natural growth: The
northern end of Ocean City, N. J. (v)                                 89

72 New Point Comfort at the tip of the York-Rappahannock peninsula,
Virginia (v)                                                          90

73 Spit at Tucker Beach, New Jersey (v)                               91

74 A land-tied island: Napatree Point, near Watch Hill, R. I. (v)     92

75 Powells Creek, Virginia, on the lower Potomac River (o)            93

76 Roberts Creek, a drowned river valley southeast of Yorktown, Va.
(v)                                                                   94

77 The underwater channel in Quantico Bay on the lower Potomac
River, Virginia (v)                                                   95

78 Natural channels and shoals near Miami. Fla. (o)                   96

79 A dredged channel at Miami, Fla. (o)                               97

80 A shoal in Hereford Inlet north of Wildwood, N. J. (v)             98

81 Sand bars at Cape Charles, Va. (v)                                 99

82 Bars, channels, beaches, and marsh near Far Rockaway, Long
Island, N. Y. (v)                                                    100

_All of the airplane photographs in this book, both oblique and
vertical, were taken by the United States Army Air Service, except Figs.
78 and 79, which were taken by the United States Navy Air Service, and
Figs. 10, 65, 69, 75, 77, and 82, which were taken by the author. To
these two services the author is indebted for the permission to
reproduce their photographs, and this acknowledgment is made with the
same force as if made individually under each illustration._

_As a guide to the evaluation of the scale of the vertical photographs,
which is expressed under each photograph in the form of the natural
scale, or representative fraction, the following approximate equivalents
may be borne in mind_:

        _1:10,000 = 800+ feet to the inch_
        _1:16,000 = ¼ mile to the inch_
        _1:21,000 = ⅓ mile to the inch_




INTRODUCTION


Scarcely a generation has passed during the evolution of the airplane
from a ridiculous dream to a practical factor in the work of the world.
Men who once read with derision, or only passive interest at best, of
the experiments of Langley, Chanute, and the Wrights have seen the
airplane developed suddenly into an indispensable instrument of war and
an agency of demonstrated value and of such diversity of application
that its future is hard to estimate.

The navigation of the air has accomplished much in many fields. Not only
does it offer a new means of efficiency in military reconnaissance,
rapid delivery of mail, fire patrol of forests, and the constantly
increasing number of commercial and scientific pursuits to which it is
being adapted; but it has also opened a new world to the geographer, the
physiographer, and the geologist.


AIRPLANE PHOTOGRAPHY: ITS DEVELOPMENT AND APPLICATION

Very early in the war the airplane was recognized as a useful, in fact a
necessary, means of observing enemy positions and movements. But the
speed of the airplane was found to preclude the taking of more than the
most hurried of notes during a flight, and notes written from memory are
not the most satisfactory. Photography was found to obviate this
difficulty. The ability of the camera to make instantaneous exposures
and fix a clear image on a photographic plate enabled the observer to
obtain a record not only of the scenes that he had viewed but also of
many that he might have missed while engaged in the necessary business
of watching the sky for the enemy--a record that for detail and accuracy
could not be approached by the most elaborate notes or the most graphic
description. Immediately inventive genius was set at work to adjust the
mechanism of the camera to the demands of air photography and to prepare
the rapidly working films and highly sensitized paper necessary for the
best results.

So satisfactory were the results and so great are the possibilities of
further adaptation that there is an unfortunate tendency on the part of
certain enthusiasts to make exaggerated claims that may react to retard
progress. This is particularly true in the use of the air photograph in
mapping. There are limitations to this use of air photography. It cannot
be reasonably expected to do away entirely with the ground work of the
surveyor. Rather, the camera is to be regarded as one of the instruments
of the surveyor. Observation from the air can never take the place of
close examination of the ground, but it can be of great use in the
location and study of land forms and geologic relations. Air photography
is only an added means of obtaining information, although it promises to
become a very important means.

Observations from the air described in numerous reports and articles in
geographic magazines during the war and since its close indicate that
air craft, especially in connection with air photography, can be of
great use in studying the physical features of the face of the earth. In
order to make a practical test of the use of the airplane in the study
of geography the writer spent about nine months during the year 1920
making flights, taking pictures from air craft, and gathering
information from various sources. This book embodies the chief results.

The material presented here is by way of illustrating the possibility of
using the airplane and airplane photography as a means of securing
information that should become increasingly useful in the study of
geography, and of showing geographic and geologic features better than
in any other way. The views have been chosen to illustrate the three
uses of air photographs with which this book deals--the presentation of
new views of subjects of popular interest and the practical value of
such views; the study of land forms from a new and advantageous point
of view; and the use of the air photograph as an aid in mapping.

In presenting these illustrations there is no intention that the list of
types should be considered in any sense complete. Physiographic
observation from the air is a relatively new undertaking, and results
are limited and imperfect. As improvements in mechanism and technique
are made, observations will be extended and better photographs and a
greater variety of them will be secured. Such as are presented here,
however, serve to demonstrate that the air photograph will come to be
recognized as a valuable source of information for the student of
geography and geology.


ACKNOWLEDGMENTS

The results here presented were secured by the co-operation of the Air
Services of the United States Army and Navy. Hydroplanes were placed at
my disposal on several occasions, and a number of flights were made over
water bodies, particularly over the Potomac River, Chesapeake Bay, and
New York Harbor. But the information was gathered chiefly through the
Army Air Service. Many flights were made in army planes, some for
general observation, others for photographing specific objects. Also the
army photographers, particularly those at Langley Field, near Newport
News, Va., made several photographic trips at my request, and a large
number of prints were furnished from negatives stored at this and other
flying fields.

In this connection I wish to express appreciation for the many
courtesies extended by Major General C. T. Menoher, U. S. A., Chief of
Air Service at the time the work was done, and by Major J.W. Simons,
Jr., A.S., Acting Administrative Executive, Air Service. These officers
placed at my disposal every facility of the service that I could use. It
would be a pleasure, if space allowed, to mention the names of the
numerous pilots and other officers to whom I am directly indebted for
the safe completion of some of the most thrilling adventures of my life.
I must, however, mention the officer to whom I am perhaps more indebted
than to any other. My introduction to this study was through Major J.W.
Bagley of the United States Army Engineering Corps, who has done much
toward making the camera a valuable instrument in mapping.[1] Through
his active interest I became acquainted with the officials of the Army
Air Service, who gave the necessary authorization for flights and for
securing most of the photographs used to illustrate this book. During
the time spent at this work I retained my position as geologist of the
United States Geological Survey. Hence the work is one of co-operation
chiefly between the United States Army Air Service and the United States
Geological Survey, and to a lesser degree with the United States Navy
Air Service.




CHAPTER I

THE VIEWPOINT

(FIGS. 1 TO 4)


OBLIQUE AND VERTICAL AIRPLANE PHOTOGRAPHS

Air photographs are, in general, of two sorts, depending upon whether
the photograph was taken with the camera pointing vertically or
obliquely downward. In either case the air photographer is free from the
limitations that hamper the ground photographer in choosing a point of
view. For he can ascend to any desired height and not only select an
advantageous position from which to photograph the feature which he
wishes to emphasize but also, at the same time, avoid obstacles which
might obstruct his view from the ground. Vertical photographs are
preferable where the accurate location of objects is desired. When
properly taken they serve many of the purposes of maps and are, in many
ways, even more useful than maps. They furnish the untrained mind with
much of the information that the trained mind reads from a topographic
map and, in addition, supply details and relations that a map cannot
depict. Exact accuracy, however, cannot be claimed for them until they
have been corrected for distortion and adjusted to some system of
controls.

Where the photograph is to be used as a means of securing a more
advantageous view of a subject than can be had from the ground rather
than as a map on which distances are to be scaled off, the oblique
photograph is probably the more desirable, since it is as easily
intelligible as a photograph taken laterally. The advantage of such
photographs is obvious. To the architect, the landscape gardener, the
city planner is given the opportunity to study their projects free from
all obstructions yet in such perspective that their relations to their
surroundings are brought out as would be possible by no other means.
Views like that of West Point (Fig. 3) are occasionally to be had from
some hilltop, but the limited choice of position on the ground contrasts
sharply with the unlimited choice in the air.


ELEMENTS TO BE RECORDED

Air photography is by no means simple. Much still remains to be done by
way of adapting the camera to its peculiar demands. Its present degree
of perfection, of course, is largely due to the impetus given its
development during the war because of its great importance in military
reconnaissance. The adaptation of the camera to operation from the
airplane might be described with profit but will be passed with slight
mention because it is the results of air photography rather than the
mechanism that are to be considered here. Technically, a photograph of
the earth’s surface may not be a map, but, given certain means of
interpretation, it can be made to serve as such. In using air
photographs, particularly the vertical ones, it is desirable to know the
scale, which is dependent upon the altitude at which the exposure is
made; the angle of the lens; and the variation from the vertical, in
order to make corrections for distortion. Therefore, it is desirable
that each photograph show the altitude, date, time of day, and position
of the lens at which the exposure was made. Cameras have been
constructed that automatically record these data on each negative. This
information is illustrated in Figure 2. The circular symbol at the left
in the white strip at the top of the photograph represents a circular
level, or inclinometer. The small round dot close to the center of the
inclinometer indicates that, at the time the exposure was made, the axis
of the lens was very nearly vertical. The symbol in the center of the
white strip indicates an altitude of about 9,800 feet, and that at the
right, that the exposure was made 7 seconds after 11 A.M. The other
symbols record that this photograph was No. 13 of a series made at
Rochester, N. Y., October 23, 1920, with an Eastman mapping camera known
as K-2. The symbol 8-P is non-essential and records that this negative
is No. 8 of panchromatic film.

[Illustration: FIG. 2--Symbols of automatic register in the Eastman
mapping camera, photographed with the body of the picture showing roads,
streams, orchards, cultivated fields, etc. For explanation of the
symbols, see the text.]

The information given by the symbols is corroborated by the picture.
Orchard and shade trees appear as circular dots in place of the
elongated images characteristic of pictures taken obliquely downward,
and the short, squat shadows denote exposure near midday. Shocks of corn
standing in the fields show that the season is autumn.


HOW TO READ AIRPLANE PHOTOGRAPHS

Not all the features, however, are so easily recognizable. Oblique
photographs are often more readily interpreted than ordinary
photographs, since they combine with the usual view the essentials of a
plan; but in vertical photographs very few objects present an appearance
that is natural in the light of our experience as lateral observers. The
uninitiated, on attempting systematically to identify the features of a
vertical photograph, find a very large number that are foreign in
appearance. A necessary preliminary is an acquaintance with the ground
photographed or with similar regions and features. Without such a key
the air photograph is not always self-interpretative and is often
unintelligible. Military observers are carefully trained to recognize
features of military significance. It is not to be expected, however,
that they should be trained in the observation of land forms except such
as are of military importance. Consequently, whereas a great variety of
photographs is now easily obtainable at many flying fields, the
information that a scientist would desire concerning them is not so
easily available. Most of the photographs used in this paper were taken
by men who were not trained in observing land forms. Many were taken
simply as a requirement in practice flights and meant so little to the
observer that no record was made concerning them. For several not even
the location was recorded.

It is of primary importance that the picture be held in the right
position. Not only must the observer imagine himself looking directly
down on the scene but he must hold the photograph in the position in
which experience has shown that the image appears the most natural.
Otherwise a depression will appear as an elevation and an elevation as a
hollow. It is a well-known fact that in telescopic photographs of the
moon the craters appear like hollows when the print is held in one
position and like elevations when the position is reversed. Experience
shows that if the print is held _so that the shadows fall toward the
observer_ the objects appear natural. The reason is that the observer
sees only those shadows that are caused by light falling towards him.
Consequently, the only interpretation that the brain can give to shadows
on a photograph is that they are cast by an elevation between the eye
and the light. In a picture, therefore, in which shadows fall away from
the eye instead of towards it valleys are seen as hills and hills as
valleys. In the northern hemisphere this prescribed orientation
conflicts with the convention of placing the north side of a map at the
top of the page and also with the modern shaded map on which the light
is represented as coming from the upper left, or northwest, corner of
the map.


FAILURE OF AIR PHOTOGRAPHS TO SHOW RELIEF, AND MEASURES TO REMEDY THIS
DEFECT

In photographs taken from the ground the lights and shadows are such
that a high degree of naturalness is possible. But objects seen from
directly above, and even those viewed obliquely, though to a lesser
degree, are illuminated so uniformly that photographs of them are apt to
appear flat. To some extent this has been overcome by the use of
extra-sensitive emulsions, special ray filters, and printing papers
adapted for accentuating contrast. Many of the photographs used in this
book did not allow satisfactory reproduction till the contrast of the
negatives was greatly increased by the arts of the photographic
laboratory. But, even at its best, no photograph taken vertically
affords an adequate idea of the height of hills or the depth of hollows.
Only shadows that are particularly well defined can be distinguished as
shadows, while small elevations and depressions affect the negative no
differently than a difference in marking or color. In military defenses,
if the mere surface of the camouflage is sufficiently realistic, the
ordinary camera is even more easily deceived than the human eye. It is a
well-known fact that man and other animals of the higher order see
objects in relief, within a certain range of vision, because the eyes
convey to their respective retinas slightly different images of the same
object which the brain combines into a relief image. The stereoscopic
camera has long been used for the same purpose. Its principle, with
certain adaptations that need not be discussed here, has been to some
extent employed in airplane pictures, with such excellent results that
it is claimed by some that by further development actual contouring will
be possible by this means. It is reported that in military
reconnaissance stereoscopic pictures render ordinary camouflage useless
and that bridges, observation towers, gun emplacements, etc., are shown
in relief and, therefore, easily detected.[2]




CHAPTER II

FAMILIAR SCENES FROM A NEW ANGLE

(FIGS. 1, 3, AND 4)


Pictures of well-known buildings are of wide appeal. In so far as they
create an interest in the activities for which the buildings stand they
are distinctly educative. Such widely known buildings as the National
Capitol and the Library of Congress are used repeatedly for
illustration. They are as welcome as the sight of a familiar face. Any
unusual circumstance connected with them is seized upon as an excuse for
republishing pictures of them. Views of them from a new angle are always
in demand. Not only do air photographs offer a welcome novelty, but they
have the added advantage of lifting the subject out of the clutter of
surrounding buildings and making it really the central figure of the
picture. It would be difficult to get a more impressive view of the
National Capitol than that of Figure 1 or a more attractive glimpse of
the Naval Academy at Annapolis than that of Figure 4. The objects of
chief interest occupy the center of view without distracting
obstructions. In the former, the imposing structure of the Capitol
building appears in a pleasing setting of minor details. The proximity
of the Senate Office Building and the Library of Congress is at once
apparent, and the radiating systems of the avenues of approach.
Strangers may have wondered as to the nature of the environs of the
Capitol. The tree-lined streets and the apartment houses seen in the
picture answer the question. In the view of the Naval Academy the
buildings occupy the center of the scene, with the beautiful dome of the
memorial to John Paul Jones, the first great American naval fighter,
prominently in view. Spa Creek in the foreground, a part of the capital
city of Maryland at the left, and the Severn River, with its low wooded
banks, stretching away

[Illustration: FIG. 3--West Point, N. Y., and the Hudson River. An air
view of the United States Military Academy and the gorge of the Hudson.
The picture shows the commanding view of the river to be had from the
point of land 180 feet above the river on which the Military Academy is
located and shows the wisdom of the choice of this spot as one of the
chain of redoubts by which the river was fortified during the
Revolution.]

[Illustration: FIG. 4--The Naval Academy at Annapolis, Md. Oblique view
from an airplane from a position over Eastport in a general
northwesterly direction. The water in the foreground is Spa Creek. The
Severn River, spanned by the county bridge and the Baltimore and
Annapolis Railroad bridge, stretches away to the left. The buildings in
the middle of the picture are those of the Naval Academy. The domed
mausoleum built in honor of John Paul Jones, which serves as his final
resting place, appears at the left. Still farther to the left lies
Maryland’s capital city. Of interest is a comparison of the low-lying
and stream-cut banks of the drowned valley now occupied by the Severn
River with the mountains through which the Hudson River has cut its
gorge at West Point (see Fig. 3).]

in the distance, spanned by the county bridge and the Baltimore and
Annapolis Railroad bridge, form an interesting setting and show, without
detracting from the importance of the academy itself, its advantageous
location with regard to the city and the water approaches.




CHAPTER III

ARCHITECTURE, LANDSCAPE GARDENING, AND ENGINEERING

(FIGS. 5 TO 14)


Only a few photographs are necessary to show how valuable to the
architect, the construction engineer, the city planner, or the landscape
gardener the air photograph, both vertical and oblique, is destined to
become. Pictorial records of progress in the construction of buildings,
bridges, ships, canals, reservoirs, etc., that partake also of the
nature of ground plans, as do air photographs, furnish an admirable
means of study and comparison. No photograph of the great shipyards at
Newport News taken from the ground would show the relation of the shops
and drydocks to the deep-water approaches as does Figure 7. Figure 8
gives an unusually comprehensive idea of the location, magnitude, and
construction of Hell Gate Bridge; and Figure 10, Rockaway Beach, now a
densely populated town where a few years ago was a barren strip of sand,
suggests that photographic records of construction in rapidly growing
communities where changes are being made in streets, railroads, and
buildings, will come to be a part of the equipment of the city engineer
and architect.


ARCHITECTURE AND LANDSCAPE GARDENING

Equally useful will the air photograph become to the landscape gardener
and architect. Heretofore, in order to get a comprehensive conception of
his task and a definite picture of its completion, the landscape
gardener has had to depend upon the use of maps and such views as could
be made by the sketch artist or the ordinary lateral photograph. In the
future, from vertical and oblique photographs of the area to be
developed, he will have the means of studying its features in their
correct proportions and relationship. By means of similar photographs of
completed projects he can choose and combine until he has developed the
plans best suited to his purpose. He can bring to his aid first-hand
studies of gardens and grounds the world over whose beauties have made
them famous.

[Illustration: FIG. 5--Monument Avenue, Richmond, Va., and the statue of
Robert E. Lee. An oblique photograph illustrating the use of aerial
photography in landscape gardening and street planning.]


ENGINEERING PROJECTS COVERING LARGE AREAS

Where the project covers large areas, the “mosaic,” or group of matched
photographs, can be used in the study of problems of construction or
improvement. Figure 13, a mosaic of the Anacostia flats, the site of
improvements under way in the District

[Illustration: FIG. 6--The United States Naval Observatory and grounds,
Washington, D.C., as seen from an airplane at a height of a few hundred
feet above the ground, showing an unusually attractive arrangement of
shrubbery and trees.]

of Columbia, shows the Anacostia marshes as they appeared in the autumn
of 1920, after the changes effected since 1915, as can be seen by
comparison with Figure 14, the topographic map of the same area. To the
right is the terraced slope rising to a height of about 150 feet above
the river--an elevation so low that the air photograph does not properly
reproduce it. Near the foot of the principal terrace lie the tracks of
the Pennsylvania Railroad, on which can be seen Benning, Deanewood, and
Kenilworth. Between the railroad and the Anacostia River are the Benning
race track and the swampy lowland and tidal marshes of the Anacostia
flats. The river and the marshland on either side of it from the
Pennsylvania Avenue Bridge to Benning Road have been modified by
dredging, but north of

[Illustration: FIG. 7--Shipyards at Newport News, Va., showing docks and
deep-water approaches, steamships, and drydocks, in one of which is a
vessel for repairs.]

[Illustration: FIG. 8--The New York Connecting Railroad Bridge, which
affords an all-rail passenger and freight route between Boston and
Washington. The bridge, which was completed in 1917, starts on the
mainland in the Port Morris section of southern Bronx Borough, New York
City, seen in the background, then crosses Bronx Kill, Randalls Island,
Little Hell Gate, Wards Island, and Hell Gate to reach the Long Island
shore, seen in the extreme lower right corner, at Long Island City,
Queens Borough. The tracks continue towards Washington by way of tunnels
under the East River and the Hudson.]

[Illustration: FIG. 9--A part of Washington, D.C., showing the White
House, Treasury, State-War-Navy, and other public buildings in the
foreground; the Ellipse, Washington Monument, and new War and Navy
offices in the middle ground; and the Tidal Basin, Potomac Park, and the
Potomac River in the distance. By no other means could so informative a
glimpse be given of a spot of such wide interest. Every feature in the
picture is more or less familiar to a large number of Americans, but
their familiarity is with the individual features rather than with their
situation and relation to one another as shown here.]

[Illustration: FIG. 10--Part of Rockaway Beach, Long Island, N.Y.,
showing city blocks, streets, and buildings covering the sand which a
few years ago was barren and unoccupied. Scale, about 1: 6,700.]

[Illustration: FIG. 11--Landscape gardening. An airplane view of a part
of Long Branch, N.J., taken from a height of 10,000 feet, showing the
beach and surf at the right, and the streets, mansions, driveways, and
lawns in the body of the picture--an example of the development of a
barrier beach of little value before the exploitation for summer homes.
Scale, about 1: 15,000.]

this road the surface appears in its natural state. In the mosaic are
shown at the left the highlands west of the marshes, wooded in some
places but cleared and improved in others. In the northern part can be
seen land wooded north of the District line but cleared south of it. So
comprehensive a view of the field of the project and of the progress to
date should be of great service to the engineers and promoters.




CHAPTER IV

THE MOSAIC

(FIGS. 13 AND 22)


In its simplest form, the mosaic is made by mounting overlapping prints
so that the corresponding details coincide. This type of mosaic is quite
adequate for relatively small areas or where a high degree of accuracy
is not required. For larger areas and greater accuracy, an accurate
outline map is used as a base upon which the prints are mounted so that
recognizable features coincide with their location on the map. When the
prints are properly arranged, the better print of each overlapping pair
is selected, the excess paper removed, and the whole mounted and
photographed. Figure 13 is left untrimmed to illustrate the method of
matching the overlapping prints. The differences in shade are due to
difference in printing and developing the pictures which make up the
mosaic. The slight offsetting of line at the junction of the prints may
be due to errors in mounting, shrinking, or stretching of the
photographic paper, tilting of the camera at the time of exposure, or
other cause. Such errors and imperfections illustrate the difficulty of
using these photographs in the making of maps.

A skillful manipulation of both airplane and camera is necessary to the
success of the mosaic. To prevent distortion and variation of scale, the
camera must be maintained at the same altitude at all times and pointed
directly downward. This can be accomplished by flying with an even keel
at a uniform altitude. Mechanical devices are also being perfected to
accomplish the same result. Still greater skill is necessary when
consecutive rows of exposures are made for the purpose of placing strips
of photographs side by side to cover a large area. It is difficult under
the varying conditions of wind and weather to fly so evenly and so
nearly at the same level that distortions and differences in scale are
not noticeable. Strong objection to the mosaic is frequently raised
because of inaccuracies due to difference in scale in neighboring
prints. Until these defects are overcome, such a group of matched
photographs cannot take the place of an accurate map. Much, however, is
being done to correct these defects, and, even in photographs where
inaccuracies in scale are many, the value of the photograph for the
portrayal of detail cannot be denied.




CHAPTER V

GENERAL ASPECTS OF THE SURFACE AS SEEN FROM THE AIR

(FIGS. 12 TO 18)


[Illustration: FIG. 12--Benning, D.C., and the Anacostia River, showing,
from right to left, cultivated lands 40 to 20 feet above sea level, an
elevation too slight to be shown in a vertical photograph; a brushy
slope running from 20 feet to sea level; and marshland along the stream.
The checkered pattern of the upland fields is caused by
different-colored crops. Shocks of corn, spaced evenly in rows,
buildings and shade trees, and light-colored roads and a race track are
shown. The light-colored areas along the stream are occupied by tidal
marsh and are free from brush but covered with vegetation of annual
growth. The figure is one of the photographs used to make the mosaic
shown on Fig. 13. It should be compared with Fig. 13 and with the
topographic map, Fig. 14. Scale, 1: 11,000.]

When a region is viewed from an altitude of several thousand feet the
observer can readily imagine himself looking down on a large map. The
chief features stand out prominently, the smaller to a lesser degree.
Mountains, rivers, and the seashore are

[Illustration: FIG. 13]

[Illustration: FIG. 14]

[Illustration: FIG. 13--Vertical photograph of the land along the
Anacostia River on the eastern edge of Washington, D.C., made up of
several photographs matched together and adjusted to points located by
ordinary survey methods, and reduced in size to correspond with the map,
Fig. 14. The photographs were taken from an airplane with a so-called
mapping camera at such intervals of time that the prints overlap, thus
making it possible to adjust them to each other and to form a continuous
picture of the area. The region shown is the site of improvements that
are at present under way, mainly the regulation of the Anacostia River.
The channel has been widened by dredging and part of the bordering marsh
areas filled in. The photograph shows that this work had progressed to
the Benning Road bridge by the autumn of 1920, when the photograph was
taken, while in 1915, when the area was surveyed for the map, it had
been carried out only as far as the Pennsylvania Avenue bridge. Such
airplane photographs furnish an incomparable tool in the handling of
large-scale engineering projects, both in the study of the territory in
its unimproved state and to follow the progress of the work after
operations are under way. Scale, about 1: 28,000.]

[Illustration: FIG. 14--Part of the topographic map of Washington and
Vicinity, 1: 31,680, published by the U.S. Geological Survey, showing
within the irregular line the same area shown in Fig. 13. Scale, 1:
28,000.]

[Illustration: FIG. 15 (on page 24)--Mosaic of the southeastern part of
Mulberry Island, on the left bank of the James River about 11 miles
northwest of Newport News. Va., showing an area portrayed by many
photographs matched together. Slight differences in shade indicate the
junction of the separate prints. The higher land, about 10 feet above
sea level as determined by surveys on the ground, is shown at the right
by roads and cultivated fields. It is to be noted that roads outline the
dividing line between the high ground and the marsh. At the left are
lower areas of wooded or brushy swampland and of grassy marsh. They
contain a number of abandoned channels: some completely silted up,
others containing small thoroughfares, and still others drained by
meandering streams which seem to have developed after the channels were
definitely abandoned by the streams which originally occupied them.

The streams which drain the marshes have many characteristics of streams
which drain higher lands. They have dendritic patterns, so called from
resemblance to the forking branches of a tree; channels which widen
downstream; and winding or meandering courses. The island terminates in
a long spit composed of silt and fine sand. The banks to the left on
James River are low and marshy: those to the right on Warwick Creek,
except for one small marsh, form low bluffs.

In order that the mosaic may be compared with the map, Fig. 16, it has
been placed with the northerly part at the top of the page, with the
result that, until the page is reversed, the trees in the swampland
appear like hollows in the earth. Scale, 1:14,000.]

especially conspicuous. Streams appear as smooth, winding
ribbons--glistening if the sunlight reflected from them enters the eye,
dark if the bright rays are reflected away from the eye. Railroads can
easily be traced and towns recognized by their form. Concrete roads and
others of light-colored material are plainly visible. Those built of
dark-colored material appear less prominently. Something even of the
character of the forests can be ascertained--whether evenly timbered or
partly of primary and partly of secondary growth; whether intact or
partly burned over; whether consisting chiefly of one species of trees
or of many. The cultivated fields and their relations to roads, streams,
and forests are conspicuous. Towns and cities are spread out like
panoramic views in which are strikingly visible the residence and
manufacturing centers, the layout of streets, the systems of parks, the
position of suburbs, and the relation of these to routes of
transportation and travel--roads, railroads, and waterways. These and
many other features of the landscape--swamps, marshes, buildings, trees,
orchards, and many lesser details--are recognizable and are all recorded
on the

[Illustration: FIG. 15. (For explanation, see p. 23.)]

[Illustration: FIG. 16--The same area as shown in Fig. 15 reduced from a
section of a map on the scale of 1:10,000 by the Corps of Engineers,
U.S.A. The photographs shown in Fig. 15 were used for mapping certain
small features on this map, such as small streams. Scale, 1:14,000.]

photographic negative. So faithfully does the camera reproduce all the
horizontal features within its range of vision that it is conceivable
that a photograph correctly dated might become a valuable record in
cases of boundary disputes or other litigations involving the position
of fences, fields, roads, or even streams, at a given date.

[Illustration: FIG. 17]

[Illustration: FIG. 18]

[Illustration: FIG. 17--Columbus. Ga. A part of a mosaic made at Camp
Benning near-by in 1909 showing the town, river, and surrounding
country. The scale is so small that buildings and trees appear as dots,
city blocks as small parallelograms, streets and roads as light-colored
lines. The cultivated fields appear as irregularly checkered areas, and
the concentric lines of the terraced slopes have the appearance of
contour lines on a topographic map (see Fig. 18). The picture
illustrates many of the features of city geography. The comparatively
straight course of the river and the heavy growth of trees and bushes
along its edges indicate a minimum of flood-plain and steep banks--an
inference supported by the fact that the principal business center of
the city, shown by large, closely set roofs, is built close to the
river. Surrounding this section is the most densely populated district,
which in the northern part of the city gives way to a district of houses
set farther apart and separated by lawns set with trees. Other less
extensive business centers are shown as small spots of closely grouped
buildings. A variety of suburban types is to be seen: some quite
city-like, with a business center, a densely populated residential
district, and a district of houses separated by grounds; others more
village-like in their lack of a well-developed center but still more or
less completely separated from the city proper; still others, sporadic
scatterings of houses and grounds extending from the city for some
distance along the principal roads. The railroad center is located
conveniently near the business center, and the radiating lines of road
and railroad communications are in strong contrast to the rectangular
arrangement of the city streets. Factories, indicated by large,
light-colored roofs, are located along the railroad in the southern part
of the city and along the river to the north. Those along the river are
operated by power from the falls which the picture shows. The terraced
slopes are characteristic of the region, the farmers here and elsewhere
in the South making these terraces in their plowed fields to prevent
rain water from washing away the soil. Scale, about 1:38,000.]

[Illustration: FIG. 18--Map of the same area shown in Fig. 17 enlarged
from the corresponding sections of the 1:62,500 Columbus and Seale,
Ga.-Ala., and the 1:125,000 Talbotton and Opelika, Ga.-Ala., topographic
sheets surveyed mainly in 1906 and 1907 and published by the U.S.
Geological Survey. The cross section at the bottom lies along the line
indicated on the map and extends somewhat beyond the right border of the
map. The section shows the broad shelf upon which the city rests and its
relation to the river and to the terraced hillsides east of it. Scale,
1:38,000.]




CHAPTER VI

MARSHES AND MARSH DRAINAGE

(FIGS. 19 TO 27)


Mention has been made of the objects seen better from the air than from
any viewpoint on the ground; but there are some objects which as a whole
can be seen only from above. Swamps, parts of everglades, peaks in the
midst of difficult country, precipitous canyon walls, and many volcanic
craters cannot be seen from the ground without undue effort. Photographs
of bluffs, terraces, and other slopes facing bodies of water have
hitherto been adequately obtainable only from the water. All of these
can be readily viewed and photographed from the airplane. Pictorial
representations of drainage systems were rare until photographs such as
Figure 19 were taken from airplanes. The intricate drainage of marshes
like those along the Pamunkey River in Virginia pictured in Figure 20
was never accurately shown until photographed from the air.

Of frequent occurrence on the Atlantic Coastal Plain of the United
States are swamps and marshes inaccessible from the ground. Much of the
surface material is so soft that they cannot be easily traversed; and,
even where firm enough to support a man’s weight, few of the details are
deemed of sufficient importance to warrant the trouble and expense of
mapping by ordinary methods. Yet the trapper would scarcely admit that
these details are unimportant, and, to the student, they are an
interesting feature of marsh topography that has thus far received
little attention.

Figure 25 is part of the excellent New Kent, Va., sheet of the
topographic map and is probably as detailed as a map of this character
should be when made from ground surveys only. However, on comparison of
the map with a photograph of the same area (Fig. 24), there is no
difficulty in detecting errors; and it is probable that, had the
photograph been available when the map was made, the marshes would have
been represented differently.

[Illustration: FIG. 19--Stream development in a tidal marsh, showing, at
the right, the northern end of Ludlam Beach, about 6 miles south of
Ocean City, N.J., and the mouth of Corsons Inlet leading to Ludlam Bay,
and, at the left, the marsh just west of the inlet, with streams rising
close to the bank of the larger stream at the extreme left and flowing
in meandering courses across the marsh. The great variety of types of
vegetation probably is one cause of the remarkable meandering of these
drainage lines by reason of the fact that the accumulated remains as
well as the annual growth of different weeds and grasses offer varying
resistance to the current of the streams. Scale, about 1:10,000.]


MARSH DRAINAGE

One of the most striking characteristics of marsh topography illustrated
by the photographs presented here is the great wealth of drainage lines
and the resemblance of the drainage patterns to those of river systems
developed on higher ground. The dendritic patterns, the meanders, and
the sharply outlined divides are surprising in areas which have
altitudes varying from only a few inches to a little more than a foot at
times of ordinary high tide and which are wholly submerged at times of
maximum tide. Some of the streams have gently winding courses suggestive
of normal stream development. Others, particularly the smaller, have a
conspicuous angularity of course. It is possible that the latter may
have originated as the trails of animals. Some of the lines are observed
to cross the larger streams and are probably tracks made by muskrats.
Some of the streams rise close to the river’s brink and lead to
through-going waterways near the center of the marsh. This suggests the
deposition of silt on the brink of the river at times of high water. The
notched appearance of the shore in Figure 20 seems to be due to
overhanging bunches of sedge grass and, in some instances, to the
breaking away of the surface mat or crust of the marsh formed by the
interlacing roots of grass. The mottled appearance of the marsh in this
picture may be partly due to shadow of clouds, but to some extent, at
least, the difference in shade is caused by differences in the character
of the plants.

The marshes used for illustration here are typical of many along the
Atlantic Coast. They are situated near West Point, Va. The Pamunkey and
the Mattaponi Rivers both rise in the Piedmont Plateau, flow
southeastward through the tidewater portion of Virginia, and join about
midway of the Coastal Plain to form the York River[3] (see Fig. 58).

[Illustration: FIG. 20--Details of marshland. A part of Lee Marsh near
West Point, Va. (cf. Fig. 26), as photographed from a height of 2,000
feet, June, 1920. Local observers report that this marsh has been
submerged only twice in nineteen years. The drainage systems are well
entrenched. The larger stream channels are cut 1 to 5 feet or more below
low tide (the tidal variation at West Point is about 3.4 feet), and
their form is made stable by the tough surface crust of the marsh,
consisting of the matted roots of the luxuriant sedge grass. The
intricate, veinlike appearance of the drainage lines and the furry
appearance of the edges of all the waterways, showing overhanging
vegetation, are of interest. Drainage systems flowing in opposite
directions slow connecting tributaries apparently silted up. Scale,
about 1:4,000.]

[Illustration: FIG. 21--Details of frequently submerged marshland. A
part of Cousaic Marsh on the Pamunkey River, near Sweet Hall, Va. (cf.
Fig. 24), as photographed from a height of 2,000 feet, June, 1920. The
surface of the marsh is covered with water several times each year,
according to local report. It is relatively soft, and a comparison with
Fig. 20 shows an apparently different, less dense vegetation than that
of Lee Marsh, which is rarely submerged. The stream channels are less
definitely fixed and lack the evidence of overhanging vegetation. Scale,
about 1:4,000.]

[Illustration: FIG. 22--Atlantic City and Ocean City, N.J. Strips of
photographs taken from an airplane, March, 1920, from a height of 10,000
feet, showing, in order from east to west: the ocean water, which
appears dark-colored; the surf, white where it breaks into foam; the
light-colored beach sand; the cities laid out on the sand of the barrier
beach; and the marshes, channels, and drainage systems west of the
barrier. West of Peck Beach in the strip of photograph at the right many
features characteristic of salt marsh areas of the Coastal Plain are
shown back of the barrier beach. The right strip forms the southern
continuation of the left strip. Scale, about 1:75,000.]

[Illustration: FIG. 23--A river system in miniature. A small stream near
Hampton, Va., showing flood plain, meanders, an ox-bow lake and cut-off,
abandoned channels and a delta partly under water. Scale, not known.]

[Illustration: FIG. 24--Sweet Hall Marsh on the lower Pamunkey River,
near West Point, Va., as photographed from a height of 10,000 feet at 11
A.M., December 11, 1920. Cousaic Marsh lies to the left and Hill Marsh
to the right of the central meander. Some of the watercourses in these
marshes are thoroughfares, or channels opening to the river at both
ends, that can be traversed by boat at high tide. But many of them are
quite different in nature, beginning as minute rills and broadening
toward the mouth in a manner suggesting typical drainage channels on
higher land. Scale, about 1:31,000.]

[Illustration: FIG. 25--The same area as shown in Fig. 24, enlarged from
the New Kent, Va., topographic sheet, 1:62,500, published by the U.S.
Geological Survey. The cross section at the left lies along the line
indicated on the map and extends somewhat beyond its borders. The
somewhat greater height of the map than of the photograph, although both
cover exactly the same area, is due to the unavoidable slight difference
in tilt of each of the exposures of which the photographic mosaic is
made up. This illustrates the fact that airplane photographs cannot be
directly used as equivalent to maps, until the necessary adjustments
have been made. Experiments in camera construction are under way to
overcome these difficulties by automatic devices. Scale, 1:31,000.]

[Illustration: FIG. 26--Eltham Marsh on the lower Pamunkey River, as
photographed from an altitude of about 10,000 feet at 11 A.M., December
11, 1920. At the right lies the town of West Point, Va., at the junction
of the Mattaponi and Pamunkey Rivers, and at the left appears a part of
Lee Marsh. Eltham Marsh, in the center of the illustration, is traversed
by a so-called thoroughfare, through which boats of light draft make
their way at high tide. At one point in the middle of the marsh the
thoroughfare is perceptibly broader than elsewhere, and the tidal
currents entering from opposite ends of the thoroughfare meet there and
cause slack water in which silt is deposited, forming mud flats exposed
at low tide. The cultivated fields south of the marsh are on a bench
about 10 feet higher than the marsh. Scale, about 1:31,000.]

[Illustration: FIG. 27--The same area as shown in Fig. 26, enlarged from
the New Kent, Va., topographic sheet, 1:62,500, published by the U.S.
Geological Survey. It is obvious that many interesting details shown by
the photograph are missed or neglected as unimportant in the most
careful mapping. The cross section at the left lies along the line
indicated on the map and extends somewhat beyond its borders. Scale,
1:31,000.]

The York is one of the estuaries of the tidewater portion of Virginia,
and the water level at West Point, the junction of the two tributaries,
rises and falls about 3½ feet under tidal action. The Pamunkey is
affected by the tide 53 miles by channel above West Point, and the
Mattaponi 42 miles. Much of the broad lowland along these rivers is
marshy, but the largest marshes are found near West Point, where the
river current in swinging from side to side has formed great meanders.
For some reason the valleys eroded long ago by these streams have filled
with sediment here to a greater extent than farther downstream; perhaps
because this is essentially the head of sea water, so that the checking
of the current of the river causes it to deposit much of its load. Sea
water regularly mingles with the river water as far upstream as West
Point, but above this point the water is chiefly fresh. The marshes
consist of soft mud and muck to a considerable depth. A well driven in
Hill Marsh to an underlying artesian horizon penetrated 50 feet of this
soft material before entering rock such as is exposed in the river bank.
The thickness of the mud is comparable to the maximum depth of the York
farther downstream and suggests that the old valley which there is
filled with water is here filled to a depth of 50 feet or more with
sediment brought down by the river. Only a small part of the marsh near
the landward margin has surface material firm enough to support the
weight of large animals except when the surface is frozen.

Many kinds of marsh plants grow here, among which is sedge grass
(_Spartina cynosuroides_ (L.) Willd.), which grows to a height of 10
feet or more and forms dense thickets. Its roots interlace to form a
tough mat which in some places will support the weight of a man. In
other places the soft muck reaches to the surface.


“THOROUGHFARES”

These marshes are cut by a few waterways open at both ends, known as
thoroughfares, or tidal runs, which also serve as the trunk streams
through which the marsh is drained. Some of the thoroughfares may be
trunk streams modified by tides, or they may be silted remnants of
abandoned river channels. Some seem to be channels in the last stages of
silting. The incoming tide enters the down-river end but ascends the
thoroughfare more slowly than it ascends the river. The tide in the
river reaches the upper end of the thoroughfare, enters it, and meets
the opposing tide within the marsh near the upstream end of the
passageway. Where the tides meet, thus causing slack water, silt is
deposited and mud flats are formed. In Eltham Marsh (Fig. 26) these
flats are well within the marsh. In the larger thoroughfares of Sweet
Hall Marsh (Fig. 24) the tide passes entirely through while the tide in
the river is making its long way around, so that slack water and the
deposition of silt occur at the extreme upper end of the passage. In all
of the thoroughfares the silting has reached the stage that precludes
their use by boat, except at times of high water. Even at high tide some
are navigable only by small skiffs, although throughout much of the
course the water is many feet deep.

Some of the thoroughfares become narrow and shallow upstream in a manner
that suggests that they originate as two normal streams flowing in
opposite directions from a common point and that they were later united
by the breaking down of the divide between their headwaters. Such a
junction might be affected by an unusually high tide breaking through a
divide and cutting a channel. Such a divide, be it noted, consists of
soft mud only a few inches above the general level and might readily be
broken down. In some instances the connection may have originated as an
animal trail, as we have seen. Muskrats, otters, and other marsh
animals use the waterways as lines of travel and make paths in between
them from one to another. Apparently many of the small drainage lines
originated in this way, but in some instances stream systems of
considerable size and complexity are independent of all others and
possess all the characteristics of normally developed river systems.




CHAPTER VII

COASTAL MUD FLATS

(FIGS. 28 AND 29)


Of frequent occurrence along the Atlantic Coast of the United States are
low mud flats which are practically at sea level and which are covered
with water at times of high tide. Where these tracts are exposed to the
air during ebb tide for so short a time that plants have not taken root
and where the surface material is fine-grained and soft, the tracts are
known as mud flats. In the part of the peninsula between Delaware and
Chesapeake Bays belonging to the state of Virginia which is called the
Eastern Shore a low barrier beach of sand has formed on the ocean side
several miles off shore, and the space between this and the mainland is
occupied by mud flats, broad, shallow lagoons, and an intricate maze of
interlacing channels and winding, branching, interlocking, vermicular
streams.

The mud flats are exposed for a short time during low tide, and, as the
surface of the water here rises and falls with the tide more than 4
feet, with a maximum fluctuation considerably greater, large volumes of
water are continually flowing backward and forward over the flats. As
the tide rises, strong currents of sea water set in through the inlets,
flow up the main channels and through the thoroughfares, and gradually
find their way into the countless small channels and out of them over
the broad level stretches of soft mud. As the tide falls, this action is
reversed, and the broad sheet of water finds its way by devious paths
through the winding watercourses out to sea. The larger channels extend
considerably below the surface at times of highest water and may be
quite deep even at times of low water. They are, perhaps, stream courses
excavated before the region was drowned. Many of the smaller channels
also have the general form characteristic of normal stream channels,
although others show peculiarities not common to subaerial drainage. The
origin of these submarine and tidal features is not well understood, but
the photographs of them show their form and furnish some basis for a
study of them.

[Illustration: FIG. 28--A stream system of the mud-flat area on the
ocean side of the Eastern Shore, Virginia. The light-colored area is
beach sand above water. The treelike form is a stream system of
subnormally developed pattern. Note the seeming uncertainty of course,
some of the branch streams rising close to the mouth of the trunk
stream; the junction of branches at the head; and the “frostwork”
patterns. Scale not known.]

[Illustration: FIG. 29--Mud-flat streams, showing curious frostwork
pattern at the head of underwater channels. Note the pools and the
veinlike drainage lines from them. Scale not known.]

The photographs reproduced as Figures 28 and 29 were taken northeast of
Cape Charles, Virginia, in the summer of 1920 at low tide. The
light-colored ribbon-like bands represent water-filled channels; and the
darker-colored areas, either wet mud exposed to the air or mud slightly
submerged. However, photographs taken under certain conditions of light
may show the exact line between the exposed and the drowned portions of
a land surface.




CHAPTER VIII

SUBMERGED LAND FORMS

(FIGS. 30 TO 33)


Heretofore the study of beaches, deltas, and other partly submerged land
forms has been chiefly confined to the exposed parts, the underwater
forms being largely matters of conjecture. By means of air photographs
not only can the exposed parts of the delta and beach be studied, but
the forms of shoals and terraces, the underwater portions of river
deltas, tidal deltas and their underwater distributaries, and many other
submerged forms can be shown clearly. Sand bars, terraces, and other
submerged forms appear in many of the photographs already presented; but
a few so taken that the bars and terraces appear to be the chief objects
in the picture may be useful for illustrating the underwater land forms
and for demonstrating that these forms can be successfully photographed.
Unfortunately not many photographs could be found which were taken with
the express object in view of illustrating underwater land features. In
most of the available photographs these features were only incidental,
the chief purpose in taking them being to photograph the shore.

Much has been written concerning the physiographic history of the
Atlantic Coastal Plain of the United States, and the question is still
being debated whether the land is rising, sinking, or stationary. To
some extent these questions are answered by the exposed land forms. The
submarine forms are imperfectly known. The possibility of recognizing
shoals and channels from a photograph and of determining in some measure
the shapes of the submerged land forms opens a new avenue of approach to
the study of submarine geography. In some places, especially in regions
of drowned topography, it is possible that, by using the air photograph
in working out the physiographic processes that have produced the land
forms that are now under water, some of the vexing problems of earth
history may be solved.

[Illustration: FIG. 30 (left)--Sand bars and drowned terrace about Stove
Point Neck, at the mouth of the Piankatank River, Virginia, as
photographed from a height of about 10,000 feet at 11:30 A.M., December
11, 1920. West (left) of the neck, at the outer edge of the terrace, the
water is 2 to 3 feet deep at low tide, or 5.7 feet and 6.7 feet at high
tide, but deepens abruptly westward, where it is 20 to 30 feet deep in
Fishing Bay (see Fig. 32). To the south and east of the point the abrupt
descent is at the side of the deep channel of the Piankatank River. To
the right, the bottom, having a wavy appearance because of sand bars,
fades off more gradually under deep water. The mottled area in the
middle of the neck is wooded, and the smoother parts near the point and
in the upper part of the neck are cleared land. Scale, about 1:30,000.]

[Illustration: FIG. 31 (right)--Drowned terraces at Gwynn Island at the
mouth of the Piankatank River, Virginia, as photographed from a height
of about 10,000 feet at 11:30 A.M., December 11, 1920. At the right is a
part of the island, showing trees, fields, and houses. Bordering the
land area is a narrow band of light-colored beach sand, expanded at
Cherry Point into a conspicuous sharply recurved hook. Under the shallow
water can be seen wave marks resembling large ripple marks. The water is
2 to 3 feet deep at low tide at the outer edge of the light-colored
submerged shelf, beyond which the bottom descends abruptly toward the
left to a depth of about 20 feet. North of Cherry Point the waxy bottom
shades off more gradually to the deep channel of the Piankatank. Scale,
about 1:30,000.]


THE BEST CONDITIONS FOR PHOTOGRAPHING UNDERWATER LAND FORMS

[Illustration: FIG. 32--Part of the Kilmarnock and Mathews, Va.,
topographic sheets, 1:62,500, published by the U. S. Geological Survey,
showing the location of Figs. 31 and 32; and a cross section along the
line indicated on the map, showing a terrace 26 feet above sea level at
the left, one less than 5 feet above water level on Gwynn Island, one 5
feet or less below water level; and the river channel with abrupt banks
between the shoals. Scale, 1:70,000.]

The photographic study of underwater land forms is relatively new, and
little information concerning it is available. It is annoyingly obvious
to the air observer that at times he can see nothing beneath the surface
of the water, whereas at other

[Illustration: FIG. 33--A drowned valley: Lambs Creek, 8 miles southeast
of Yorktown, Va., one of the estuary-streams tributary to Chesapeake
Bay, showing the broad mouth narrowing upstream and the irregular
margins caused by partial submergence of the valley slopes, eroded
before the rise of the water to its present height. Even the vertical
photograph, which does not register relative elevations, shows a
distinct difference between the shore line of this type of body of water
and rivers with broad, low flood plains. The large trees close to the
margin of the river and the cultivated fields just back of them indicate
a relatively high bank. Scale, about 1:9,000.]

times he can see with great distinctness. In trying to ascertain the
most favorable conditions for such observation, it was found that
submerged objects are seen best when the sky is evenly overcast or when
it is uniformly clear. Sometimes when the sky is only partly cloudy the
surface of the water seems to act as a mirror and nothing is seen but
the reflection of cloud and sky. Waves have less effect on the
visibility of objects beneath the surface than was expected, although
they diffuse the reflected light to some extent and consequently weaken
the image on the negative. But the reflected light from the surface of
the water is stronger than that coming from objects under water. Hence,
to photograph underwater features successfully, a time should be chosen
when direct reflection of light from the sun or from a brightly
illuminated cloud will not enter the lens.

Experience in both the air and the laboratory shows that the best
results are likely to be obtained when the sunlight strikes the surface
at an oblique angle. In summer favorable times are mid-forenoon or
mid-afternoon under an evenly illuminated sky. In winter the sun is low
enough at midday to avoid direct reflection into the lens. But
experience also indicates that often photographs taken at moments when
the eye caught the image of a submerged object show only the surface of
the water.




CHAPTER IX

THE PLAIN FROM THE AIR

(FIGS. 34 TO 41)


A RIVER ON THE GREAT PLAINS

The difficulty of photographing a plain from a point on its surface
needs no emphasis, but its successful representation by means of air
photographs is illustrated by many figures in this book. The Great
Plains of the west-central part of the United States are illustrated
here by a view of the Red River (Fig. 36), which shows the flat surface
of the land and the broad sandy bed of the river only partly covered by
the intricately woven strands of the braided channels--a scene
characteristic of the Great Plains.


MEANDERING STREAMS ON THE COASTAL PLAIN

The ox-bow curves of meandering streams are among the features of the
earth’s surface most familiar to the student of physical geography; yet,
heretofore, they have been illustrated only by maps, constructed at
great labor and expense. Comprehensive photographs of them are rare and
are, at best, imperfect and unsatisfactory for purposes of illustration.
On the other hand, meandering streams lend themselves admirably to air
photography. Equally familiar to the student of geography and
physiography is the term “abandoned meander.” These ancient stream
courses, many of which are now occupied by marsh, brush, or forest, have
been still more difficult to illustrate by means of photographs. In some
instances wooded meanders like those near Columbus, Ga. (Fig. 34), long
ago abandoned by the stream that formed them, are shown in air pictures
in a manner but little less conspicuous than the meanders of the
present-day stream. It is believed that instructors will find Figure 34
useful, not only in illustrating meandering streams and abandoned
meanders but also in showing how meanders develop.

[Illustration: FIG. 34]

[Illustration: FIG. 35]

[Illustration: FIG. 34--The Chattahoochee River south of Columbus, Ga.,
showing the results of progressive lateral shifting of a meandering
stream. In the upper part of the illustration to the left (west) of the
stream are light-colored concentric markings which probably represent
the gradual shifting of the stream toward the right. As interpreted from
the information at hand, this section of the stream at one time occupied
a position much farther west than now. It cut away the bank on the east,
forming a curved course, depositing sand and mud on the inside of the
curve. This typical feature of stream erosion and deposition is to be
noted from the picture of the present course of the stream. At the
outside of each meander stretches of the bank appear light-colored and
denuded of the trees and bushes that line the bank elsewhere. These are
scours, a slipping away of the bluff caused by the cutting of the stream
into the foot of the bank at points where the velocity of the outside of
the current, and consequently its corrosive power, is increased as it
swings round the curve. The inside of the sharpest meander shows also
the deposit of material due to the fact that the velocity of the inside
of the current is checked by the bank, causing it to deposit some of its
load. Added to this deposit is much of the material brought by
cross-currents from the opposite-lying scour. The light-colored banks
are probably successive deposits. Finally, either by a gradual wearing
away or by some whim of the current at flood tide, the river chose a
shorter course, leaving its old channel as an abandoned meander. Farther
south several abandoned meanders may be distinguished, each
distinctively marked by a steep bank on the outside of the curve and
concentric bandings on the inside. The abandoned channels are especially
marked by the trees and brush that fill them in many places. It appears
that a well-developed growth of trees is to be found only along the
river banks in this region and the growth in the abandoned channels is
probably due to the fact that in flood time there is much seepage of
water into these old channels if not an actual overflow from the present
course of the stream. At the bottom of the picture is to be seen the
recently made land under cultivation. The fields appear striated and
checkered, obscuring the concentric banding. The illustration is from a
mosaic made up at Camp Benning near-by of many photographs matched
together, hence there are certain differences in shade due to dark and
light prints. Scale, about 1:38,000.]

[Illustration: FIG. 35--Map of the same area shown in Fig. 34 enlarged
from the corresponding sections of the Columbus and Seale, Ga.-Ala.,
topographic sheets, 1:62,500, published by the U. S. Geological Survey.
The cross section at the bottom lies along the line indicated on the map
and extends somewhat beyond the right border of the map. The section
shows between the hills the broad lowland over which the Chattahoochee
River has meandered. Scale, 1:38,000.]

[Illustration: FIG. 36--A river channel in the Great Plains. The Red
River northeast of Wichita Falls, Tex., as photographed from a height of
8,000 feet, September 12, 1918. Between the bluffs is seen the
dark-colored water of the braided stream flowing on a broad sandy bed
more than a mile wide, which is completely covered with water only at
flood time. The river forms the Texas-Oklahoma boundary, and frequent
changes in the position of the channel during periods of high water make
the exact position of the interstate boundary uncertain and give rise to
disputes and litigation over the ownership of land. North of the river
(top of figure) to the right are sand dunes with a sprinkling of trees
and bushes; in the middle of the channel there is an island of
light-colored sand. The stream channel bites sharply into the southern
bluff, which is cut by many strong gulches. Across the river is the
familiar sand flat built of the material washed downstream at flood time
and spread out by the subsiding water. The channel at this point shows
the changes that have taken place in the position of the stream and,
where the stream crosses the sandy floor, affords an example of
braiding. Scale, about 1:23,000.]

[Illustration: FIG. 37--A characteristic glacial drift plain in
southwestern Michigan. There appear, at the left, the round surface of a
terminal moraine and gullied slopes, which show mottled in the picture;
morainic hollows and kettleholes once partly filled with water but now
filled with peat or occupied by marshes formed by the accumulation of
peat from plant growth until carbonaceous matter has replaced the water
of the original lake; in the center, a relatively smooth outwash plain
characterized by straight roads and well-cultivated fields; and, at the
right, a brush-lined creek, a small reservoir, and the town of
Flowerfield. Scale, about 1:20,000.]

[Illustration: FIG. 38--The same area as shown in Fig. 37, enlarged from
the advance edition, 1:48,000, of the Schoolcraft, Mich., topographic
sheet to be published by the U. S. Geological Survey. This advance sheet
results from an experiment in the use of airplanes for mapping. The area
was photographed with a mapping camera. From the photograph a base map
was constructed, which was verified on the ground; on this base the
contour lines were added by instrumental survey. Scale, 1:20,000.]

[Illustration: FIG. 39--Schoolcraft, Mich., a town typical of the
agricultural portions of the north-central United States, showing the
characteristic features--roads, fields, town blocks, and others--by
which the aviator can recognize a locality from a distance. The mottled
appearance of the land surrounding the village is characteristic of air
photographs of glacial moraine regions. The picture of the village
itself might be taken as a prototype of the American village with its
fairly regular layout of streets, its business center indicated by a few
larger roofs along the widest street, its lawns, trees, and gardens, the
bordering farm lands, and the scattered extensions of the village into
points in the direction of the main roads. Scale, about 1:14,000.]


THE GLACIAL DRIFT PLAIN

[Illustration: FIG. 40--Map of the town of Schoolcraft, Mich., for
comparison with Fig. 39. Enlarged from the advance edition, 1:48,000, of
the Schoolcraft, Mich., topographic sheet to be published by the U. S.
Geological Survey. Scale, 1:14,000.]

Some of the characteristics of a third type of plain, the glacial drift
plain, are shown in Figures 37 to 41. Here are pictured glacial lakes,
bogs, marshes, moraines, and outwash plains, peat-filled depressions,
kettleholes and gullied slopes--typical features of a glaciated region.
The views show, also, many of the familiar aspects of the central and
western parts of the United States: the rectangular pattern formed by
the land subdivisions established by the United States Land Office, the
checkerboard pattern being emphasized by the section-line roads; the
minor subdivisions into fields; and the cultivation of a variety of
crops.

[Illustration: FIG. 41--Kettleholes and other depressions in glacial
till, on the Grand Trunk Railway about 5 miles southwest of Schoolcraft,
Mich. The distance between the eastern (right) edge of this view and the
western (left) of Fig. 37 is about 1 mile. Scale, about 1:15,000.]

These photographs were selected from a series taken as an experiment in
map-making. In June, 1920, the United States Air Service sent a plane
equipped with a K-1 camera from Dayton, Ohio, to Schoolcraft, Mich,
where in seven hours’ flying time a fifteen-minute quadrangle, about 220
square miles, was photographed. The prints were matched together and
reduced to a scale of 1:48,000. From them such features as roads,
streams, forests, land corners, etc., were transferred to plane-table
sheets, which the topographic engineers on the ground then used for
contouring the relief. Figure 38 is a part of the preliminary proof of
this map. It may be added that the experiment is regarded as highly
favorable to the use of the airplane camera as an instrument in
mapping.




CHAPTER X

MOUNTAIN FEATURES

(FIGS. 42 TO 52)


In obtaining photographic illustrations from the ground of mountains,
canyons, and associated land forms, the same difficulty, but in
exaggerated form, is encountered that obtains in securing an
advantageous point of view for small objects. The difficulty is overcome
in large measure by the use of aircraft. In an airplane the observer can
rise above the obstructions which interfere with the view desired; can
look an isolated mountain peak squarely in the face, as in the case of
the photograph of Mt. Shasta (Fig. 42); can study the details of its ice
cap (Fig. 42) and gaze downward on the lateral and recessional moraines
left by the retreat of the mountain’s glaciers (Fig. 43). Few volcanic
craters, occurring as they do at the top of cones, have been
successfully photographed unless some higher mountain stands near-by on
which a favorable viewpoint can be found. From an airplane, however, one
can look into the very throat of a crater, as into that of Cinder Cone
(Fig. 48), near Lassen Peak, California.

Much attention has been given to the interrelations of canyons, gorges,
and mountain ridges, but these relations have hitherto been illustrated
chiefly by means of maps and charts. Figures 49, 50, and 52 picture
three relations more expressively than any map. To the experienced
geographer a map may illustrate perfectly the action of a stream working
headward into higher land; but the student to whom the conception of
headward erosion is new will certainly grasp the idea more readily from
the picture presented in Figure 52. No map could give so clear a
conception of a maturely dissected highland as does a photograph like
that of the Santa Monica Mountains (Fig. 50).

[Illustration: FIG. 42--A glaciated volcanic Cone: Mt. Shasta,
California, 14,162 feet high, as seen by an airplane observer from the
northeast, showing Hotlum Glacier in the foreground and Wintun Glacier
at the extreme left. The monadnock which separates the two main lobes of
Hotlum Glacier appears as the dark-colored mass of rock in the midst of
the ice. To be noted are the many indications of movement in the
glaciers shown by curved lines, eddies, and crevasses, and the glacial
streams flowing away from the ends of the glaciers. The long lobe at the
left center shows the formation of both lateral and recessional as well
as terminal moraines.]

[Illustration: FIG. 43--A glacial gorge on the northeastern face of Mt.
Shasta, California, below Hotlum Glacier (see Fig. 42), the lower end of
which is to be seen in the upper part of the photograph. At the left are
two ridges, one the edge of a sheet of flow lava, the other, in part at
least, a lateral moraine. In the center, at the bottom of the gorge,
between the two white lines which represent glacial streams, is a system
of concentric ridges which are probably recessional moraines. At the
right is the western slope of the gorge. (This figure is the lower
overlapping continuation of Fig. 42.)]

[Illustration: FIG. 44--Yosemite Valley, California, a typical
ice-shaped gorge, showing at the left the granite face of El Capitan,
about 3,000 feet above the bottom of the famous gorge, and, at the
right, the pinnacle of Sentinel Rock and the well-known form of Half
Dome. At the sky line in the center of the picture is Clouds Rest, and
well down in the gorge Washington Column and the Royal Arches can be
distinguished.]

These photographs have the advantage of appealing to the mind through
the sense of vision and will serve the same purpose as plaster models.
Thus, in Figure 52, a variety of topographic forms are to be
distinguished, including slightly dissected highlands with sharply
incised gorges; maturely dissected highlands made up now of canyons and
ridges; a mountain valley broadening out toward an intermontane plain;
several arroyos; and many minor features.

In the interpretation of the features shown in a vertical view of a
mountainous country the orientation of the photograph is of prime
importance. When viewed in proper orientation, that is, as already
pointed out (p. 5), with the shadows falling toward the observer,
mountains and valleys appear in their correct relation. But, if the
position of the picture is reversed, a mountain will look like a
depression and a valley like a ridge. This reversal of the image can be
tested by looking at Figures 49 or 52 from both viewpoints. However,
since the vertical photographs will be compared with maps of the same
area, it is thought better to place them on the page as if they were
maps. In order to make them appear natural the prints can be turned in
the necessary direction.

[Illustration: FIG. 45--Map of the Yosemite Valley, showing the area
included within the angle of vision of Fig. 44. The map, a reduced
section from the Yosemite and Mt. Lyell, Cal., topographic sheets,
1:125,000, published by the U. S. Geological Survey, is oriented for
direct comparison with the photograph. Scale, 1:167,000.]

[Illustration: FIG. 46--Mountains of volcanic origin: Cinder Cone with,
in the distance at the right, Lassen Peak in the northern Sierra Nevada,
California, as seen from an airplane over Lake Bidwell. Beyond the lake
appears the rough surface of lava poured out as molten rock less than
two hundred years ago (see _U. S. Geol. Survey Bull. 79_, 1891).
Surrounding the cone is a light-colored ash field, sparsely forested at
the right, which was formed about two hundred years ago. The mountain in
the middle of the photograph having a smooth surface is Cinder Cone,
rising 640 feet above the general level of the ash field and consisting
of fragments of lava--the so-called ash and cinders--blown from the
crater at times of eruption.]

[Illustration: FIG. 47--Map of the region between Cinder Cone and Lassen
Peak in the northern Sierra Nevada, California, showing the area
included within the angle of vision of Fig. 46. The map, a reduced
section from the Lassen Peak, Cal., topographic sheet, 1:250,000,
published by the U. S. Geological Survey, is oriented for direct
comparison with the photograph. Scale, 1:307,000.]

[Illustration: FIG. 48--The top of Cinder Cone, looking from an airplane
down into the crater, showing a large saucer-shaped crater 750 feet
across, with a deeper crater formed at the time of a later volcanic
explosion, which looks like a cup in the middle of the saucer and
extends to a depth of 240 feet below the outer rim. On the barren cinder
slopes at the right is the pathway by which the crater can be reached.]

[Illustration: FIG. 49--Mountain, valley, and plain in the Simi Hills
about 15 miles northwest of Santa Monica, Cal. (see Calabasas, Cal.,
topographic sheet), showing, in the right center of the picture,
headward erosion from two parallel valleys, in strong contrast with the
gently rounded, slightly dissected part of the mountain (left center)
into which the streams have not yet eaten their way. Farther up the
mountain is more maturely dissected and the divides are narrow and
steep. On its top the mountain shows little effect of stream erosion
(right). Strongly cut gorges and arroyos appear where the streams enter
the plain (left). Probably north is at the bottom of the photograph.
Scale, probably about 1:20,000.]

[Illustration: FIG. 50--A maturely dissected highland: Santa Monica
Mountains north of Santa Monica, Cal., as photographed from a height of
nearly 10,000 feet at a midday in January, 1919. The light-colored
irregular line at the left is Sepulveda Canyon; and the similar line at
the right, Stone Canyon (for location, see Fig. 51). These mountains
rise nearly 1,600 feet above sea level and about 700 feet above the
bottom of the canyons.

To obtain the proper impression of ridges and valleys the figure should
be reversed. Such photographs as this of the actual ground can hardly be
distinguished from photographs of good relief models; they strikingly
confirm the correctness of this and similar methods of representing
relief on maps, developed intuitively, as it were, such as the Swiss
school of hill shading. Scale, about 1:17,000.]

[Illustration: FIG. 51--Map of the region between the center of Los
Angeles and Santa Monica, Cal., showing the location of the area covered
in Fig. 50 (the double-ruled rectangle in the upper left corner).
Reduced from the Santa Monica, Cal., sheet, 1:62,500, of the
“Progressive Military Map” of the United States being published by the
Corps of Engineers, U.S.A. This sheet, which is the equivalent of the
Santa Monica topographic sheet surveyed in 1893 and published by the
U.S. Geological Survey, was revised in 1920 by airplane photography. A
comparison of the 1893 and 1920 editions brings out strikingly the rapid
urban development in this region. Scale, 1:123,000.]

[Illustration: FIG. 52--A young mountain gorge showing an erosional
hollow developing headward into the less deeply eroded highlands: San
Joaquin Hills, a coastal range in Southern California about 45 miles
southeast of Los Angeles, near the mouth of Aliso Creek. North is at the
left (see Corona, Cal., topographic sheet). Scale, probably about
1:10,000.]




CHAPTER XI

AIR CRAFT IN THE STUDY OF ROCKS AND ORES

(FIG. 53)


The admirable manner in which air photography lends itself to the
observation of geographic relations and physiographic processes suggests
its use as a valuable addition to the instruments of geologic
reconnaissance; for, not only is the study of geology inseparable from
that of physiography, but, in large measure, geology is applied physical
geography and many conclusions of a geologic nature are drawn from
observed surface relations.

Probably, in most cases, the actual character and composition of rocks
cannot be determined from air photographs; but, just as on a good map an
area of crystalline rocks can be distinguished from one of sedimentary
rocks by means of the topographic expression, so areas of different
rocks can be distinguished on photographs. For instance, an area of
upturned sedimentary rocks would be readily distinguished from one of
horizontal rocks. Figure 42 shows how the character of glaciated
mountains is revealed, and Figures 37 to 41 of the Michigan area show
well the familiar features of continental glaciation.

It is perhaps premature to say much of the use of the airplane in the
study of geology until it has been thoroughly tested. But it should be
possible from the air to locate and map ore bodies, metalliferous veins,
and outcrops of rock: for it is well known that rocks at the outcrop
differ in color, in the forms of erosion developed in them, and in the
kind of plants which they support. It is of interest that Colonel Alfred
H. Brooks, who was Chief Geologist of the American Expeditionary Forces
in France during the war, found that geologic boundaries could be
recognized on air photographs and that by means of these photographs he
could correct existing geologic maps and identify

[Illustration: FIG. 53--Canyon in sedimentary rocks near the mouth of
the Pecos River, Texas. The rocks consist of flat-lying strata, and the
tortuous lines resembling the grain in wood denote the outcrops of hard
layers and the benches formed on these layers by erosion. This
photograph illustrates the use of air photography in geological
reconnaissance. Scale not known.]

formations in inaccessible areas within the enemy lines. His method was
to use air photographs in the study of the geologic formations of areas
accessible to him. Then, having familiarized himself with the appearance
of the different rock formations and structures on the photographs, he
was able to recognize the same features on photographs of areas held by
the enemy and so project his mapping over into inaccessible
territory.[4]

The prospector should effect a great saving of time by using air
photographs to guide him to places where he can find exposures of rock
and to help him to avoid places where it would be useless to look for
exposures. Particularly in wooded regions air photographs are valuable
in indicating localities where exposures can be found in areas so
covered with forest that examination on the ground would not be worthy
of consideration. Prospectors for oil are planning to use airplanes for
this purpose in northern Canada, in South America, and in other places
where much of the country is so densely wooded that much time is usually
spent in looking for clear space.


USE IN EXPLORATION

Exploratory work should benefit in many ways. General reconnaissance has
been carried on to a considerable extent in foreign lands with airplanes
and to some extent also in America. Wide areas along the Mexican border
have been photographed for the making of new maps and for the correction
of existing maps. The same photographs would be useful in geologic
reconnaissance. The new photographs of southern Arizona are said to show
mountain ranges many miles away from their location on existing maps.
Such corrections are of importance to the geologist as well as to the
geographer and the map-maker. Amundsen intends to employ several small
planes in his Arctic work now under way. Mjöberg[5] has projected an
expedition to New Guinea in which the use of airplanes is a fundamental
condition.




CHAPTER XII

MAPPING AND CHARTING FROM THE AIR

(FIGS. 54 TO 82)


Mention has already been made (p. 56) of the experiment in map-making
carried out by the Army Air Service and the United States Geological
Survey at Schoolcraft, Mich. The results of that experiment and of
others of the sort are sufficient to establish the fact that the air
camera is destined to become a valuable addition to the map-maker’s
equipment. The extent to which it will be used depends, of course, upon
the degree to which its present imperfections are corrected and its
possibilities developed. The Board of Surveys and Maps of the United
States government has recently published the results of its study of air
photography for use in map-making.[6]

[Illustration: FIG. 54--View across the western end of Lake Erie,
looking in a northeasterly direction (see Fig. 55). Oblique photograph
taken from 18,000 feet above Port Clinton, Ohio, by Lieut. G. W.
Goddard, showing, in the foreground at the right, Catawba “Island,” a
part of the mainland, and, at the left, Put-in-Bay and the islands
around it. In the distance below the white clouds are a small island
(Middle Island) and a large one (Pelee Island). In the upper left-hand
corner is seen Point Pelee and the Canadian shore to the northeast of it
about 30 miles away. At Put-in-Bay was fought, September 10, 1813, the
Battle of Lake Erie, in which Commodore Perry defeated the British. The
monument commemorating this victory can be distinguished in the
photograph as a white shaft.

Although most vertical airplane photographs are in the nature of
large-scale maps, this view illustrates how a large area can be covered
in an oblique view taken at a high altitude--an area, when transformed,
of appreciable size even on a small-scale map, such as, for example,
Fig. 55.]

[Illustration: FIG. 55--Map of the western end of Lake Erie showing the
area covered within the angle of vision of Fig. 54. Scale, 1:1,400,000.

FIGS. 54 (upper) and 55 (lower). For explanation, see bottom of opposite
page.]


SCALE AND HORIZONTAL CONTROL OF VERTICAL PHOTOGRAPHS

The vertical photographs taken with an air camera are, of course, of the
order of large-scale maps.[7] For a lens of 6-inch focus the scale at an
elevation of 2,500 feet will be 1:5,000; at 5,000 feet, 1:10,000; and at
10,000 feet, 1:20,000.[8] Air mapping, therefore, lends itself best to
the production of such maps as engineering maps, city plans, topographic
maps, and coast charts. In all of these maps a degree of accuracy is
demanded that will give the exact location of all the features included
on the map and permit the precise measurement of distances between them.
To obtain such accuracy necessitates an elaborate system of control
stations as a basis on which the surveyor works out his triangulations
and traverses. In the United States these controls have been established
principally by the United States Coast and Geodetic Survey.[9] To
construct a map from air photographs, varying in scale and distorted as
they often are because of the impossibility of holding the plane at an
absolute level and because of the stretching or shrinkage of the
photographic paper, would require a great amount of triangulation and
traverse in order that the control might be sufficiently detailed to
permit the accurate mounting of the photographic prints. But, given
these controls, the air camera can, without further adaptation, supply
details that heretofore required the laborious processes of plane-table
mapping. The topographer can place the two-dimensional details from
photographs and then go into the field with only the contouring to be
done.

[Illustration: FIG. 56]

[Illustration: FIG. 57]

[Illustration: FIG. 58--Index map showing the location of the areas
shown on airplane photographs in this book within the Atlantic seaboard
of the northeastern United States, except those whose exact location is
unknown (Figs. 21, 23, 28, 29, and 77). Scale, 1:2,800,000.]

[Illustration: FIG. 59--An area where charting of the coast line is
difficult: Marshlands on Chesapeake Bay which are exposed at low tide
and submerged at high tide. Location: South of the mouth of the York
River, Virginia, between the estuaries of Poquoson River and Back River.
Scale, about 1:2,500.]

[Illustration: FIG. 60--Beach and bluff: Left shore of the York River
north-northwest of Gloucester Point, Va., showing a tied island (one of
the Mumfort Islands) and a narrow band of beach between the water and
the bluff, which is 20 to 50 feet above the water. The plain back of the
bluff is recognized by the checkered pattern made by the cultivated
fields. Scale, about 1:9,000.]

[Illustration: FIG. 61--A sandy beach with beach cusps forming the
extreme northwestern end of Sandy Hook, New Jersey, showing, at the
left, the end of the Hook surrounded by light-colored sand shading off
to shoals and bars; at the right, a broad belt of sand where a new point
is beginning to form; and, between them, six cusps arranged like saw
teeth. Note that a wave breaks into foam at the point of each cusp.
Scale, about 1:9,000.]


USE IN CITY MAPPING

In city mapping, even though time be taken to establish a very elaborate
system of controls, the air camera can accomplish in a few hours a task
of years by ordinary methods. In fact it is only by means of air
photographs that maps of a growing city can be kept at all up to date.
Paris was mapped with 800 plates in less than one day of actual flying.
Washington was completely mapped in two and a half hours with less than
200 exposures.[10] For the mosaic of Rochester, N. Y. (Fig. 56) 82
photographs were made in one hour and twenty minutes. There is no reason
why such a mosaic with an original survey or even a number of accurately
located points as a basis of control should not be sufficiently accurate
for all purposes.


USE IN REVISION OF EXISTING MAPS

Another immediate use of air photographs in mapping is in the correction
and revision of existing maps. So far as individual features are
concerned, the air photograph is an exact record of the area exposed to
its lens, and natural and artificial features are easily transferred
from the picture to the map. Its great value in the saving of time and
money has been demonstrated in the rapidly developing territory near Los
Angeles. In 1893 the Santa Monica quadrangle was surveyed, and houses,
roads, etc., as they existed at that time, are shown on the map. This
area was later built up and so changed that the map was practically
worthless. From information derived from air photographs the map was
revised in 1920 (Fig. 51). Evidence has already been given of the
efficiency of the air photograph in elaborating maps where the
importance of the region is not sufficient to warrant the expense of a
detailed survey of minor features, and in mapping areas inaccessible
from the ground.

[Illustration: FIG. 62--Beach cusps extended under water and showing the
interference of waves off the curved beach on the bay side north of
Beach Haven, N. J. Scale, about 1:3,000.]

[Illustration: FIG. 63--First stage in the formation of an inlet through
a barrier beach, where ocean waves during some storm, probably at high
tide, broke over the sand barrier 2 miles north of Beach Haven, N. J.,
and washed some of its sand into Little Egg Harbor. (For another good
example of wash-overs, see Fig. 68.) Scale, about 1:9,000.]

[Illustration: FIG. 64--A tidal delta in Shark River Inlet, Belmar, N.
J., as photographed from a height of 10,000 feet, showing shoals at the
left, which appear shadowy because they are under water, and the small
channels which radiate outward from the narrow inlet like the ribs of a
fan. They are the distributaries of this underwater delta. Scale, about
1:11,000.]


USE IN COAST CHARTING

[Illustration: FIG. 65--A tidal delta built up until it is partly above
water: Popes Creek, Virginia, on the right bank of the lower Potomac
River, 4 miles southeast of Colonial Beach, as seen obliquely from a
height of 4,000 feet at 3:30 P.M., August 31, 1920. The tidal currents
from the Potomac have built hook-shaped bars nearly across the outlet of
the creek, and the inflowing currents have built a delta from the mouth
of the creek upstream.]

It is fortunate for those engaged in the study of shore features and the
mapping of coasts that, being flat, shore features are particularly well
adapted to representation by air photographs, for on coasts exposed to
the wind and waves the channels, shoals and bars are continually
changing. Air photography offers a quick and convenient means of keeping
charts up to date. The intricacies of the water line in some places
makes accurate charting by the ordinary survey methods a slow, laborious
process. When bluffs or relatively steep slopes, like those of York
River, Virginia, near Gloucester Point, shown in Figure 60,

[Illustration: FIG. 66. (For explanation, see next page.)]

[Illustration: FIG. 66--A double tidal delta at Barnegat Inlet, New
Jersey, as photographed from a height of 10,000 feet. To the east
(right) the breaking waves and shadowy depths indicate the position of
shoals. West of the surf belt are the light-colored beach sand, shading
off from the conspicuous hook at the southern end of Island Beach into
underwater shoals and bars, and older surfaces made dark-colored by the
growth of plants. South of the hook are the inlet leading into Barnegat
Bay and the northern end of Long Beach, at the point of which stands a
lighthouse whose long shadow is to be seen across the beach sand. The
mottled appearance of the bay to the left is due to shoals slightly
submerged or perhaps exposed at low tide, where dark-colored drainage
lines appear, and shading off to deeper water, where the submerged land
forms have a shadowy appearance. The distribution of the shoals
indicates that this is a double tidal delta, an infacing part west of
the inlet and an ocean-facing part to the right. Scale, about 1:17,000.]

[Illustration: FIG. 67 FIG. 67--A tidal inlet through the barrier beach
south of Beach Haven, N. J., connecting the Atlantic Ocean to the right
(east) with Little Egg Harbor to the left. The beach sand south of the
inlet is little above water level and is frequently washed by waves,
which shift the sand and produce the clouded appearance of the sandy
surface. The light-colored ragged belt at the right is surf; the
continuous narrow belt, beach sand; the clouded areas, recently washed
wet or slightly submerged sand. (The ordinary tidal variation here is
4.2 feet.) This inlet does not appear on the 1914 edition of U. S. Coast
and Geodetic Survey Chart 1216, and sand hooks had little more than
begun to form then. Scale, about 1:14,000.]

[Illustration: FIG. 68. (For explanation see bottom of next page.)]

[Illustration: FIG. 68--Beach between Brigantine and Little Egg Inlets,
New Jersey, showing a variety of features characteristic of a wave-built
sand barrier. The upper (northern) part of the illustration shows
several places where waves have broken over the sand barrier and washed
the sand westward, where it was redeposited at the left in the quiet,
protected water. Farther south are older wash-overs, where the enclosed
bay is nearly filled with sand. At the left are numerous islands,
streams, and flats characteristic of the salt marshes west of the
barrier beach along the New Jersey coast. Scale, about 1:7,000.]

[Illustration: FIG. 69--A simple spit: Lower Cedar Point, Maryland, on
the left bank of the lower Potomac River, 6 miles north of Colonial
Beach, Va., as seen obliquely downward from a height of 4,000 feet. The
white line on either side of the point is sand at the foot of bluffs.
Houses and fields are seen at the left.]

occur along the shore, the water line varies little from year to year.
But on very low lands, like those along Chesapeake Bay south of the
mouth of York River, shown in Figure 59, the strand may migrate over a
broad belt between high and low tide. For this reason it is desirable
that photographs of areas affected by the tide be accompanied by a
record of the date and time of day at which the exposure was made, in
order that the height of the tide at the time of exposure can be
computed. As the shore on the Coast and Geodetic Survey charts denotes
the water line at high tide, a photograph taken at low tide might be
interpreted as indicating an error on the chart. Where the water
migrates over such a broad belt of sand or mud, the problems of
charting become very troublesome. Photographs of such areas could be
taken at both low and high tide, and from these the belt of daily
flooding could be charted.

[Illustration: FIG. 70--A hook, or recurved spit, south of Brigantine
Inlet, New Jersey, as photographed from a height of 10,000 feet, showing
the strong curving upstream characteristic of spits on this ocean-facing
coast. The growing end of the spit, resembling a lily bud, shows an
underwater extension beyond the light-colored beach sand. To be noted is
the filling in of the lagoon behind the reef and its pools and drainage
lines. This figure is practically a southern continuation of Fig. 68.
Scale, about 1:9.000.]


EXPERIMENTS BY THE UNITED STATES, FRENCH, AND OTHER COAST SURVEYS

The use of photographs in charting the coast line was tested by the
United States Coast and Geodetic Survey.[11] A flight was

[Illustration: FIG. 71--A hook, or recurved spit, showing interference
with natural growth: The northern end of Ocean City. N. J., as
photographed from a height of 10,000 feet. At the right (east) appears
the curved body of sand, light-colored where dried, darker-colored where
bathed by the waves and fringed by surf. To the left certain
“improvements” seem to have interfered with the natural growth of the
spit, and a small bay of shallow water has been enclosed by the
formation of a bay-mouth bar across the outlet to the north. Scale,
about 1:14,000.]

made over the coast of New Jersey by Captain A. W. Stevens of the United
States Army Air Service, March 20, 1920, in a plane equipped with a K-1
camera of 10-inch focal length, which makes negatives 18 by 24
centimeters in size. During the flight the camera was maintained at an
altitude of about 10,000 feet. The course was covered by 183 exposures
made at such intervals of time that the prints overlap. Unfortunately
the exposures were not sufficient to give all the details desired for
marsh and water areas, but prints were made on developing paper suitable
for showing extreme contrast. These were matched together and a
continuous picture obtained. A part

[Illustration: FIG. 72--New Point Comfort, a complex recurved spit at
the tip of the peninsula enclosed by the York and Rappahannock Rivers,
Virginia. A vertical view taken from an altitude of about 10,000 feet,
showing in order from right to left (east to west): the wavy surface of
Chesapeake Bay; a light-colored band of beach sand curving westward in a
compound hook made dark-colored in some places by trees and brush; a
shallow bay west of the beach in which may be seen shoals, channels, and
sand bars under water; and low-lying areas showing woodland and
cultivated fields. The photograph was taken at a time of day when
reflected light from the partly enclosed body of water was dispersed,
allowing the submerged forms to appear. Scale, about 1:12,000.]

[Illustration: FIG. 73--Lines of growth in a sand spit: Tucker Beach,
New Jersey, as photographed from a height of 10,000 feet, showing the
growth southward by successive ridges, which probably began as sand
bars, grew to be barriers by wave action, were heightened by wind-blown
sand, and finally were added to the main body of the spit by the final
filling of the enclosed lagoons. To the right (east) is the Atlantic
Ocean, showing waves and surf near the light-colored beach sand. Farther
to the left are the ridges of sand made dark-colored by vegetation,
bordered by light-colored beach sand on Little Egg Inlet, which appears
to the south and west. Scale, about 1:14,000.]

of this picture, greatly reduced, is reproduced as Figure 22. Several
characteristic shore and salt marsh features are illustrated by this
series of photographs, and these are reproduced in separate figures
together with illustrations of special features in other places.

[Illustration: FIG. 74--An island developing toward the stage of being
tied to the mainland by a double tombolo, or connecting bar: Napatree
Point, as photographed from a height of about 10,000 feet, connected to
the east by Napatree Beach with the mainland at Watch Hill, R. I., and
approaching connection to the north with the mainland near Stonington,
Conn. On the outer side of the tombolos underwater shoals and bars are
seen dimly at the right (south, Block Island Sound side) and more
clearly at the left (west, Fishers Island Sound), where the boat landing
is situated at the edge of a submerged shelf. The surf appears as a thin
white line, the beach sand as a narrow light-colored belt, and the
higher land as dark-colored areas on which houses and other structures
stand. Scale, about 1:24,000.]

The main features illustrated in detail, all of which are continually
liable to change, making the keeping of a map of the area at all up to
date impossible by ordinary means, are as follows: coast of low-lying
mainland (Fig. 60); mud or peat-covered beach (Fig. 59); sandy beach
(Fig. 63); barrier beach (Fig. 67); beach cusps (Figs. 61 and 62);
recurved spits or sand hooks (Fig. 70 and others); compound hook (Fig.
72); lines of growth in the development of hooks (Figs. 73); tombolos
and tied islands (Figs. 60 and 74).

[Illustration: FIG. 75--Oblique view of the mouth of Powells Creek,
Virginia, on the right bank of the lower Potomac River, 5 miles west of
the Naval Proving Grounds at Indian Head, showing at the left the inner
channel winding through the slightly submerged shoals and fading out
toward the right where the channel crosses the submerged terrace of the
Potomac.]

Another experiment was made by the Coast and Geodetic Survey off the
coast of Florida, where the water is clear, in an attempt to photograph
“the small coral heads and pinnacle rocks” which may be disastrous to
boats. The report states that the results were unsatisfactory and
concludes that airplane pictures are useful in “aerial photo-topography”
but not in

[Illustration: FIG. 76--Channels, shoals, and terraces in a drowned
river valley: Roberts Creek, 9 miles southeast of Yorktown, Va., an
estuary tributary to Chesapeake Bay. Under the water in the drowned
valley is seen the channel, “braided” in some places, and extending out
through sand bars to the deep water of Poquoson River shown at the top
(north) of the illustration. At the left the dark-colored forest area is
fringed with a narrow strip of white beach sand, then with a belt of
shallow water at the outer edge of which the waves form an irregular
white line, and beyond this with a belt which represents a submerged
terrace. Scale, about 1:14,000.]

[Illustration: FIG. 77--The underwater channel in Quantico Bay. Quantico
Creek is a tributary of the Potomac River entering from the Virginia
side, about 28 miles downstream from Washington. The “Bay” is the
widened part of the stream at its entrance into the Potomac, which is
entered by the tide. Forested land appears at the right and left, with
light-colored fields at the left. The space between the wooded areas is
occupied by shallow water, beneath which appears the relatively deep
channel, which forks after the manner of streams. The branching channel
is wholly under water and differs in some respects from the channel of a
normal stream. Among the more obvious peculiarities is the “fanning
out,” of the headwaters. Photograph taken from a height of about 18,000
feet. Scale, about 1:21,000.]

[Illustration: FIG. 78--Natural channels and shoals near Miami, Fla.: an
oblique view eastward to the ocean across Bear’s Cut, which is situated
between Biscayne Key and Virginia Key. Because of the greater depth of
water, the channels with a maximum depth of 17 feet appear darker than
the shoals. The photograph was taken at a height of 3,000 feet, April
20, 1918.]

[Illustration: FIG. 79--A dredged channel at Miami, Fla.: an oblique
photograph showing part of the city, the boat landing, the sandy beach,
and the shallow water, through which a straight ship channel has been
cut, which appears dark-colored in the illustration because of the
greater depth of water. The photograph was taken April 22, 1918, from 21
height of 3,000 feet.]

[Illustration: FIG. 80--A shoal in Hereford Inlet north of Wildwood, N.
J., as seen from a height of 10,000 feet. The breaking waves indicate
that the shoal is only slightly submerged. (Tidal variation is here
about 4 feet.) The sand spit north of the shoal, the south end of
Sevenmile Beach, like other spits of the New Jersey coast, is building
southward and may in time annex the shoal. Scale, about 1:13,000.]

[Illustration: FIG. 81--Sand bars at Cape Charles, Va. South of the town
at the right is the harbor, into which a tug-boat is towing a barge.
West of the town (left) is a belt of light-colored beach sand and sand
bars ending at the south against the breakwater, north of the harbor.
The sand bars appear somewhat dim because they were photographed through
water. They are under water 1 to 6 feet deep at low tide, or 4 to 9 feet
at high tide. Scale, about 1:6,000.]

“aerial photo-hydrography.”[12] On the other hand, Volmat reports the
successful use of air photography for similar purposes on the French
coast, where photographs of objects down to a depth of 17 meters (about
56 feet) were found useful in several ways--among others, the discovery
of points of rock which had

[Illustration: FIG. 82--Bars, channels, beaches, and marsh near Far
Rockaway, Long Island, N. Y., as photographed from a height of 7,000
feet at 11 A.M., September 15, 1920. In order from the bottom of the
picture upward are: East Rockaway Inlet with a shoal to the left and the
sand hook at the end of Long Beach to the right of it; the beach south
of Far Rockaway with streets and houses; a group of boats in the inlet
at the right of the beach; an area of salt marsh that is filling the
lagoon behind the barrier beach; and a small section of the village of
Far Rockaway. Scale, about 1:12,000.]

escaped attention during very detailed surveys. He states that with
proper plates and ray filters the presence of objects invisible to the
eye is revealed by the camera.[13] Similar use of air photographs has
been made by the English in charting reefs, shallows, and harbors.
Thomas says: “In 1917 aeroplane photography was successfully used for
charting the harbor of Rahbeg on the Arabian coast.”[14] It is a
well-known fact that, under proper conditions, objects submerged to a
considerable depth under clear water can be seen from points high above
the surface. During the war, submarines were detected and followed by
observers in airplanes, and sunken vessels, mines, and other submerged
objects have been located by observation from the air. Illustrations in
this paper show the possibility of using this method of observation, to
some extent at least, in detecting and mapping shoals, channels, and
other features under water.

Photographs of channels like those of the Potomac River and its
tributaries will be commercially as well as scientifically valuable. The
deep-water channel of the Potomac is well known and has been charted;
but very little is known of many of the small tributary channels, such
as that of Powells Creek (Fig. 75). Where the channels are not well
known, such a photograph could be used to advantage in avoiding the
shoals, and, by surveyors, first in exploratory work and later as a
general guide in charting. Small boats entering this channel could use
the photographs either for the original location of the deep channel in
case no chart were available or for detecting changes in its course
after the chart was made. For uncharted channels, like those of many of
the tributaries of the Potomac River, air photography furnishes a quick
and accurate means of location.

No amount of sounding, charting, or description could produce so
accurate a mental picture of a drowned valley as that produced by Figure
76. In Figures 78 and 79, both of which were taken near Miami, Florida,
is illustrated the difference in appearance between natural and
artificial channels. The straightaway course and regular outlines of the
dredged channel contrast sharply with the winding course and merging
outlines of the natural channel. To the student of physiography and
earth history the photographs furnish a means of observation of a
definiteness heretofore quite unthought-of. On them the actual shape of
the channels, submerged terraces, and drowned land forms are shown in
detail.


IMPROVEMENTS UNDER WAY POINT TO PROMISING OUTLOOK FOR AIRPLANE
PHOTOGRAPHY

There is, however, need of careful research to determine the conditions
under which the best results can be obtained. The height and time of day
for exposures with a certain lens, the emulsion and kind of ray filter
best suited under certain conditions, the effect of light as it enters
and emerges from the water, and the effect of polarization are subjects
demanding consideration. Chief among the experiments now under way is
the determination of the kind of emulsion and ray filter or color screen
that will give the best results. It is a demonstrated fact that, with an
emulsion sensitive to red light, objects in the air invisible to the eye
because of intervening haze can be photographed through a red filter. It
is possible that water can be penetrated in the same way and that
filters of other colors will prove advantageous.

Certainly, the air photograph is only in its infancy--but an infancy
full of promise. As a means of securing new and advantageous views of
subjects of interest, it is not only entertaining but scientifically
and commercially valuable. As an aid in mapping it can, even in its
present stage of development, serve an important purpose by supplying
accurate knowledge of otherwise inaccessible regions, by furnishing
details that are valuable but expensive to obtain, and by permitting the
frequent and inexpensive revision of existing maps.




INDEX


Accuracy, 1, 74

Acknowledgments, viii, xi

Aerial photo-hydrography, 99

Aerial photo-topography, 93

Air photographs, how to read, 4;
  improvements under way, 102;
  oblique and vertical, 1;
  value in coast charting, 93, 99, 101

Air photography, application, ix;
  development, ix;
  elements to be recorded, 2;
  outlook, 102

Air Services, co-operation, xi

Aircraft, x

Airplane photography. _See_ Air photography

Aliso Creek, California, 68

Amundsen, Roald, 71

Anacostia flats, D. C., 12

Anacostia River, 13, 22 (ill.);
  land along, mosaic photograph, opp. 32 (Fig. 13)

Animal trails, 29, 39

Annapolis, 7. _See also_ United States Naval Academy

Arabian coast, 101

Architects, 7, 11

Architecture, 11

Arizona, southern, 71

Arroyos, 61, 65 (ill.)

Atlantic City, N. J., 32 (ill.)

Atlantic Coastal Plain, 27;
  meandering streams, 50;
  salt marsh areas, 32 (ill.);
  submarine land forms, 45

Atlantic Ocean, waves and surf, 91 (ill.)


Back River, Virginia, 76

Bagley, J. W., xii, 74

Baltimore and Annapolis Railroad bridge, 9 (ill.), 10

Barnegat Bay, 85

Barnegat Inlet, 84 (ill.), 85

Barrier beaches, 41;
  between Brigantine and Little Egg Inlets, New Jersey, 86 (ill.);
  cities and surroundings, 32 (ill.);
  inlet formation through, 81 (ill.);
  Long Branch, N. J., 18 (ill.);
  tidal inlet through, Beach Haven, N. J., 85 (ill.).
  _See also_ Beaches

Beach Haven, N. J., cusps, 80 (ill.);
  inlet formation through barrier beach, 81 (ill.);
  tidal inlet, 85 (ill.)

Beaches, bluff and beach on shore of York River, 77 (ill.);
  cusps near Beach Haven, 80 (ill.);
  Far Rockaway, Long Island, 100 (ill.);
  sandy, with cusps--Sandy Hook, 78 (ill.).
  _See also_ Barrier beaches

Bear’s Cut, Florida coast, 96 (ill.)

Belmar, N. J., tidal delta, 82 (ill.)

Benning, Camp, opp. 26, opp. 50

Benning, D. C., 13, 22 (ill.)

Benning Road, 13

Benning Road Bridge, opp. 22 (ill. and map)

Bidwell, Lake, California, 62 (ill.), 63 (map)

Bluffs, 83;
  beach and bluff on shore of York River, 77 (ill.)

Board of Surveys and Maps, 72

Boundary disputes, 26;
  Texas-Oklahoma, 51 (ill.)

Braided channels, 50, 51 (ill.)

Brigantine Inlet, New Jersey, 86 (ill.), 87;
  recurved spit, 88 (ill.)

Bridges, 11;
  Baltimore and Annapolis Railroad, 9 (ill.), 10;
  Benning Road, opp. 22 (ill. and map);
  Hell Gate, 11, 15 (ill.);
  Pennsylvania Avenue, opp. 22 (ill. and map)

Bronx Borough, Port Morris section, 15 (ill.)

Bronx Kill, 15 (ill.)

Brooks, A. H., 69, 71

Buildings, construction records, 11;
  pictures from a new angle, 7


Calabasas, Cal., topographic sheet, 65

Cameras, adaptation, 2;
  automatic data records, 2;
  construction experiments, 35;
  Eastman mapping, recording symbols, 2, 3 (ill.);
  faithfulness, 23, 26;
  human eye and, 5, 101;
  panoramic, xii;
  stereoscopic, 6;
  use, x;
  use in map-making, 72;
  value in coast charting, 93, 99, 101

Camouflage, 5, 6

Camp Benning, Ga., opp. 26, opp. 50

Canyons, 57;
  Pecos River, 70 (ill.);
  Santa Monica Mountains, 66 (ill.), 67 (map)

Cape Charles, Virginia, 43;
  sand bars, 99 (ill.)

Capitol, National, frontispiece, 7

Catawba “Island,” Ohio, 72, 73 (ill. and map)

Channels, Far Rockaway, Long Island, 100 (ill.);
  Miami, Fla, 96 (ill.), 97 (ill.), 102;
  Potomac River, tributary, 101;
  underwater, 93 (ill.), 94. (ill.), 95 (ill.)

Chanute, Octave, ix

Charting, 72;
  coast, 83, 88

Chattahoochee River, opp. 50 (ill. and map)

Cherry Point, Virginia, 46 (ill.), 47 (map)

Chesapeake Bay, xi;
  Lambs Creek on, 48 (ill.);
  low lands along, 87;
  marshlands difficult to chart, 76 (ill.);
  wavy surface, 90 (ill.).

Cinder Cone, California, 57, 62 (ill.), 63 (map);
  top, 64 (ill.)

City geography, 17 (ill.), opp. 26, 54 (ill.), opp. 74, 79

City planners, 1, 11

City planning, 12 (ill.);
  Columbus, Ga., opp. 26 (ill. and map)

Clouds Rest, California, 60 (ill.)

Coast Charting, 83, 88

Coast surveys, experiments in mapping in United States, France, etc., 88

Coastal mud flats. _See_ Mud flats

Colonial Beach, Va., 83, 87

Color screen, 102

Columbus, Ga., opp. 26 (ill. and map);
  meanders near, 50, opp. 50 (ill. and map)

Columbus, Ga.-Ala., topographic sheet, opp. 50

Construction records, 11

Controls, 74

Coral islands, 93

Corona, Cal., topographic sheet, 68

Corsons Inlet, New Jersey, 28 (ill.)

Cousaic Marsh, Virginia, 34 (ill.), 35 (map);
  details, 31 (ill.)

Craters, 27, 57;
  Cinder Cone, California, 62 (ill.), 63 (map), 64 (ill.)

Cusps, Beach Haven, New Jersey, 80 (ill.);
  Sandy Hook, 78 (ill.)


Dayton, Ohio, 56

Deanewood, D. C., 13

Deltas, underwater, 33 (ill.), 45, 82 (ill.), 83 (ill.), 84 (ill.), 85 (ill.)

Depressions and elevations, interpreting, 4, 5

District of Columbia, Anacostia flats, 12

Drainage systems, 27;
  Lee Marsh, Virginia, 30 (ill.).
  _See also_ Marsh drainage

Drowned topography, 45;
  terraces at mouth of Piankatank River, 46 (ills.);
  valley--Lambs Creek, Virginia, 48 (ill.);
  valley--Roberts Creek, Virginia, 94 (ill.), 102


East Rockaway Inlet, Long Island, 100 (ill.)

Eastern Shore of Virginia, 41;
  mud-flat area, stream system, 42 (ill.), 43 (ill.)

Eastman mapping camera, opp. 74;
  recording symbols, 2, 3 (ill.)

El Capitan, 60 (ill.)

Elevations and depressions, interpreting, 4, 5

Ellipse, Washington, D. C., 16 (ill.)

Eltham Marsh, Virginia, 36 (ill.), 37 (map), 39

Emulsions, 102

Engineering, 11;
  projects covering large areas, 12, opp. 22 (ill. and map)

Erie, Lake, 72, 73 (ill. and map)

Erosion, headward, 57, 65 (ill.), 68 (ill.)

Everglades, 27

Exploration, 71

Exposure, 102

Eye versus camera, 5, 101


Far Rockaway, Long Island, 100 (ill.)

Filters, 102

Fishing Bay, Virginia, 46 (ill.), 47 (map)

Flats. _See_ Mud flats

Flood plain, 33 (ill.)

Florida coast, channels and shoals near Miami, 96 (ill.), 97 (ill.), 102;
  coral heads and pinnacle rocks, 93

Flowerfield, Mich., 52 (ill.), 53 (map)

Forests, 23


Gardens, 12

Genesee River, opp. 74 (ill. and map)

Geologic maps, 69

Geology, 69

Glacial drift plain, 52 (ill.) 55

Glaciers, 57;
  glacial gorge, Mt. Shasta, 59 (ill.);
  Mt. Shasta, 58 (ill.)

Gloucester Point, Va., 77, 83

Goddard, G. W., 72

Gorges, 57, 61;
  Genesee River, opp. 74. (ill. and map);
  glacial gorge on Mt. Shasta, 59 (ill.);
  San Joaquin Hills, California, 68 (ill.);
  Yosemite Valley, 60 (ill.)

Grand Trunk Railway, near Schoolcraft, Mich., 56 (ill.)

Great Plains, 50;
  river channel--Red River, 51 (ill.)

Gwynn Island, Virginia, 46 (ill.), 47 (map)


Half Dome, 60 (ill.)

Hampton, Va., small stream near, 33 (ill.)

Headward erosion, 57, 65 (ill.), 68 (ill.)

Hell Gate Bridge, 11, 15 (ill.)

Hereford Inlet, New Jersey, 98 (ill.)

Hill Marsh, Virginia, 34 (ill.), 35 (map), 38

Hooks. _See under_ Spits

Hotlum Glacier, Mt. Shasta, 58 (ill.), 59

Hudson River and West Point, 8 (ill.)


Ice cap, 57

Inclinometer, 2, 3 (ill.)

Index map showing areas photographed, 75

Inlets, formation through barrier beach, 81 (ill.);
  Hereford Inlet, New Jersey, 98 (ill.);
  River, New Jersey, 82 (ill.);
  tidal--Beach Haven, N. J., 85 (ill.)

Introduction, ix

Iroquois, Lake, opp. 74

Islands, tied, 77 (ill.);
  at Napatree Point, Rhode Island, 92 (ill.)

Ives, H. E., 6, 79


James River and Mulberry Island, 24 (ill.), 25 (map)

Jones, E. Lester, 74, 88

Jones, John Paul, 7;
  mausoleum, 9 (ill.)


Kenilworth, D. C., 13

Kettleholes, 52 (ill.), 53 (map), 55, 56 (ill.)

Kilmarnock, Va., topographic sheet, 47


Lambs Creek, Virginia, 48 (ill.)

Land forms, submerged, 45

Landscape gardeners, 1, 11

Landscape gardening, 11, 12 (ill.), 13 (ill.);
  Long Branch, N. J., 18 (ill.)

Langley, S. P., ix

Langley Field, xi

Lassen Peak, 57, 62 (ill.), 63 (map)

Lassen Peak, Cal., topographic sheet, 63

Lava, 62

Lee, Robert E., statue, 12 (ill.)

Lee Marsh, Virginia, 36 (ill.);
  details, 30 (ill.)

Library of Congress, 7

Little Egg Harbor, New Jersey, 81, 85

Little Egg Inlet, New Jersey, 86 (ill.), 87, 91 (ill.)

Little Hell Gate, 15 (ill.)

Long Beach, Long Island, 100 (ill.)

Long Beach, New Jersey, 84 (ill.), 85

Long Branch, N. J., part, showing barrier beach development, 18 (ill.)

Long Island, East Rockaway Inlet, 100 (ill.);
  Far Rockaway bars, channels, beaches, and marsh, 100 (ill.);
  Rockaway Beach, 11, 17 (ill.);
  Hell Gate Bridge to, 15 (ill.)

Los Angeles, 79;
  map of region between Santa Monica and, 67

Lower Cedar Point, Maryland, 87 (ill.)

Ludlam Beach, New Jersey, 28 (ill.)


Map, index of photographed areas, 75

Mapping, city, 79;
  from the air, 72

Mapping camera, opp. 22;
  experiment with, 53, 56

Maps, air mapping and, 74;
  air photograph adjustment, 35;
  use of air photographs in revising, 79

Marsh drainage, 27, 29

Marshes, 27;
  Chesapeake Bay, difficult to chart, 76 (ill.);
  details of drainage, 30 (ill.);
  details of frequently submerged, 31 (ill.);
  Far Rockaway, Long Island, 100 (ill.);
  salt marsh areas of coastal plain, 32 (ill.);
  salt marsh features, 92;
  stream development in tidal marsh, 28 (ill.)

Mathews, Va., topographic sheet, 47

Mattaponi River, 29, 36, 38

Meanders, 33 (ill.), 38;
  abandoned, 50;
  Chattahoochee River, opp. 50 (ill. and map);
  marsh, 27

Menoher, C. T., xi

Mexican border, 71

Miami, Fla, channels and shoals, 96 (ill.), 102;
  dredged channel, 97 (ill.), 102

Michigan, glacial drift plain, 52 (ill.), 53 (map)

Military Academy. _See_ United States Military Academy

Military observation, 4, 6

Mjöberg, Eric, 71

Monument Avenue, Richmond, Va., 12 (ill.)

Moraines, 52 (ill.), 53 (map), 55, 57;
  Mt. Shasta, 59 (ill.)

Mosaics, 12, 20, 22 (ill.);
  Anacostia River, D. C., land along, opp. 22 (Fig. 13);
  Columbus, Ga., opp. 26 (Fig. 17);
  Mulberry Island, Virginia, 24 (ill.);
  Rochester, N. Y., opp. 74 (ill. and map)

Mountains, 22, 27;
  features, 57;
  of volcanic origin--Cinder Cone, etc., California, 62 (ill.), 63 (map)

Mud flats, 36;
  coastal, 41;
  stream channels and, 42 (ill.), 43 (ill.)

Mulberry Island, Virginia, 24 (ill.), 25 (map)

Mumfort Islands, Virginia, 77

Muskrats, 29, 39


Napatree Point, Rhode Island, 92 (ill.)

Naval Academy. _See_ United States Naval Academy

New angles, 7

New Guinea, 7

New Kent, Va., topographic sheet, 27, 35, 37

New Point Comfort, spit, 90 (ill.)

New York Connecting Railroad Bridge. _See_ Hell Gate Bridge

New York Harbor, xi

Newport News, Va., xi, 11;
  shipyards, 14 (ill.)


Oblique photographs, 1

Ocean City, N. J., 32 (ill.);
  hook, 89 (ill.)

Oil, prospecting for, 71

Ores, 69

Orientation, 61

Oxbows, 33 (ill.), 50


Pamunkey River, 29, 36, 38;
  Cousaic Marsh, 31 (ill.);
  Eltham Marsh, 36 (ill.), 37 (map):
  marshes, 27;
  Sweet Hall Marsh, 34 (ill.), 35 (map)

Panchromatic film, opp. 74

Paris, mapping, 79

Pecos River, 70 (ill.)

Pennsylvania Avenue Bridge, 13, opp. 22 (ill. and map)

Perry, Commodore, 72

Photographs, mosaic. _See_ Mosaics

Photography, airplane. _See_ Air photography

Piankatank River, Virginia, drowned terraces at mouth, 46 (ills.)

Pictures from new angles, 7

Plains, glacial drift, 55;
  glacial drift, Michigan, 52 (ill.), 53 (map);
  photographing from the air, 50

Popes Creek, Virginia, 83 (ill.)

Poquoson River, Virginia, 76, 94 (ill.)

Port Clinton, Ohio, 72, 73 (ill. and map)

Potomac Park, Washington, D. C., 16 (ill.)

Potomac River, xi, 16 (ill.);
  channels, tributary, 101;
  Lower Cedar Point, spit, 87 (ill.);
  Popes Creek and, 83 (ill.);
  Powells Creek and, 93 (ill.);
  Quantico Creek and, 95

Potter, Lieutenant, opp. 74

Powells Creek, Virginia, 93 (ill.), 101

Put-in-Bay, 72, 73 (ill. and map)


Quantico Bay, 95 (ill.)

Quantico Creek, 95 (ill.)


Rahbeg, Arabia, 101

Railroads, 23

Randalls Island, 15 (ill.)

Ray filter, 102

Reconnaissance work, 69, 71

Red River, Texas-Oklahoma, 50, 51 (ill.)

Relief, means for showing, 5;
  representation on maps, 66

Revision of maps, 79

Richmond, Va., Monument Ave., etc., 12 (ill.)

Rivers, 22;
  Plains--Red River, 50;
  miniature system, 33 (ill.)

Roads, 23

Roberts Creek, Virginia, 94 (ill.)

Rochester, N. Y., 2, opp. 74 (ill. and map)

Rochester, N. Y., topographic sheet, opp. 74

Rockaway Beach, Long Island, 11;
  part, showing development, 17 (ill.)

Rocks, Florida coast experiment, 93;
  sedimentary, 69, 70 (ill.);
  study of, 69

Royal Arches, California, 60 (ill.)


San Joaquin Hills, California, 68 (ill.)

Sand bars, 45, 46 (ill.);
  Cape Charles, Virginia, 99 (ill.);
  Far Rockaway, Long Island, 100 (ill.)

Sandy Hook, N. J., 78 (ill.)

Santa Monica, Cal., 65, 66;
  map of region between Los Angeles and, 67

Santa Monica Mountains, 57, 66 (ill.), 67 (map)

Santa Monica, Cal., topographic sheet, 67, 79

Scale in vertical photographs, viii, 74

Schoolcraft, Mich., 54 (ill.), 55 (map), 56, 72;
  kettleholes near, 56 (ill.)

Schoolcraft, Mich., topographic sheet, 53, 55

Seashore, 22

Sedge grass, 29, 30, 38

Sentinel Rock, 60 (ill.)

Sepulveda Canyon, California, 66 (ill.), 67 (map)

Sevenmile Beach, New Jersey, spit and shoal, 98 (ill.)

Severn River, Virginia, 7, 9 (ill.)

Shadows, 5, 61

Shark River Inlet, New Jersey, 82 (ill.)

Shasta, Mt., 57, 58 (ill.);
  glacial gorge on, 59 (ill.)

Shipyards, 11;
  Newport News, 14 (ill.)

Shoals, 45, 84 (ill.), 85, 94 (ill.);
  Hereford Inlet, New Jersey, 98 (ill.);
  Miami, Fla., 96 (ill.), 97 (ill.), 102

Shore features, 83

Shrubbery, 13 (ill.)

Sierra Nevada Mountains, Cal., 62 (ill.), 63 (map)

Silt, 29, 36, 39

Simi Hills, California, 65 (ill.)

Simons, J. W., Jr., xi

Sky, cloudy and overcast, 49

Spa Creek, Maryland, 7, 9 (ill.)

Spits, lines of growth, Tucker Beach, New Jersey, 91 (ill.);
  recurved, 88 (ill.), 89 (ill.), 90 (ill.);
  Sevenmile Beach, New Jersey, 98 (ill.);
  simple spit, 87 (ill.)

State-War-Navy Building, Washington, D. C., 16 (ill.)

Stereoscopic camera, 6

Stevens, A. W., opp. 74, 89

Stone Canyon, California, 66 (ill.), 67 (map)

Stonington, Conn., 92

Stove Point Neck, Virginia, 46 (ill.), 47 (map)

Stream channels and mud flats, 41, 42 (ill.)

Streams, development in tidal marsh, 28 (ill.)
  _See also_ Rivers

Submarine geography, 45

Submarines, 101

Submerged land forms, 45

Submerged objects, detection, 101

Sunken mines, 101

Sunlight, 49

Surface, general aspects as seen from the air, 22

Swamps, 27

Sweet Hall Marsh, Virginia, 34 (ill.), 35 (map), 39

Swiss school of hill shading, 66


Terraces, underwater, 15, 94 (ill.)

Texas-Oklahoma boundary, 51 (ill.)

Thomas, H. H., 101

Thoroughfares, 34 (ill.), 35 (map), 39;
  Eltham Marsh, 36 (ill.)

Tidal Basin, Washington, D. C., 16 (ill.)

Tidal deltas, 82 (ill.), 83 (ill.);
  Barnegat Inlet, 84 (ill.), 85

Tidal inlet, Beach Haven, N. J., 85 (ill.)

Tidal marshes, stream development, 28 (ill.)

Tied island, 77 (ill.);
  development--Napatree Point, Rhode Island, 92 (ill.)

Tombolos, 92 (ill.)

Topographic mapping, 72

Treasury Building, Washington, D. C., 16 (ill.)

Trees, 13 (ill.)

Tucker Beach, New Jersey, spit, 91 (ill.)


Underwater topography, 45;
  best conditions for photographing, 47;
  channel in Quantico Bay, 95 (ill.);
  channels (natural) at Miami, Fla., 96 (ill.), 102;
  channels, shoals, terraces--Roberts Creek, Virginia, 94 (ill.);
  Chesapeake Bay, 90 (ill.)

United States Army Air Service, viii, xi, xii

United States Coast and Geodetic Survey, 74;
  coast line charting, 87, 88;
  Florida coast experiment, 93

United States Geological Survey, co-operation, xii

United States Land Office, 56

United States Military Academy, 8 (ill.)

United States Naval Academy, 7, 9 (ill.)

United States Naval Observatory, 13 (ill.)

United States Navy Air Service, viii, xi, xii


Vertical photographs, 1;
  scale, viii, 74;
  horizontal control, 74

Viewpoint, 1

Village, prototype, 54 (ill.)

Virginia, Eastern Shore mud-flat area, stream system,
     41, 42 (ill.), 43 (ill.);
  tidewater portion, 38

Visibility under water, 102

Volcanic craters. _See_ Craters

Volmat, J., 99, 101


War and Navy offices, new, Washington, D. C., 16 (ill.)

Wards Island, 15 (ill.)

Warwick Creek and Mulberry Island, Virginia, 24 (ill.), 25 (map)

Washington, D. C., Capitol, frontispiece, 7;
  Library of Congress, 7;
  mapping, 79;
  part showing White House, Treasury, and many familiar features, 16 (ill.);
  topographic map, part, opp. 22;
  United States Naval Observatory, 13 (ill.)

Washington Column, California, 60 (ill.)

Washington Monument, 16 (ill.)

Wash-overs, 81 (ill.), 86 (ill.)

Watch Hill, R. I., 92

Water, Visibility under, 102

West Point, N. Y., United States Military Academy at, 2, 8 (ill.)

West Point, Va., 36 (ill.), 38;
  marshes, 29, 30 (ill.)

White House, Washington, D. C., 16 (ill.)

Wichita Falls, Tex., 51

Wildwood, N. J., 98

Wintun Glacier, 58 (ill.)

Wright, Orville, ix

Wright, Wilbur, ix


York River, 38, 76, 83, 87;
  shoreline, 77 (ill.)

Yosemite and Mt. Lyell topographic sheets, 61

Yosemite Valley, 60 (ill.), 61 (map)


FOOTNOTES:

[1] Cf. his The Use of the Panoramic Camera in Topographic Surveying,
With Notes on the Application of Photogrammetry to Aerial Surveys, _U.
S. Geol. Surrey Bull. 657_, Washington, D. C., 1917.

[2] H. E. Ives: Airplane Photography, Philadelphia, 1920, pp. 328-350.

[3] The Pamunkey gets its name from a tribe of Indians famous in the
early days of Virginian history but now reduced to a few families
living on a reservation situated on the banks of the river near
Lester Manor. Mattaponi is a combination name. The Mat and the Ta
unite to form Matta Creek. The Matta and the Po unite, and Ny Creek
is a tributary to the Po. The waters of these streams unite to form
the river, and the names Mat, Ta, Po, and Ny unite to form its
name--Mattaponi.

[4] A. H. Brooks, personal communication.

[5] Eric Mjöberg: A Proposed Aërial Expedition for the Exploration of
the Unknown Interior of New Guinea, _Geogr. Rev._, Vol. 3, 1917, pp.
89-106.

[6] The Use of Aerial Photographs in Topographic Mapping: A Report of
the Committee on Photographic Surveying of the Board of Surveys and
Maps of the Federal Government, 1920, _Air Service Information Circular
(Aviation) No. 184_, War Department, Washington, D. C., 1921.

[7] What can be done, however, by photographing obliquely from a
high altitude, thereby increasing the area in the field of vision,
is illustrated by Figure 54, which encompasses Lake Erie from one
shore to the other and, in its representation of the main features
of the region, is akin to maps on a relatively small scale, such as
1:1,000,000.

[8] J. W. Bagley: The Use of the Panoramic Camera in Topographic
Surveying, With Notes on the Application of Photogrammetry to Aerial
Surveys, _U. S. Geol. Survey Bull. 657_, p. 84. “The scale of the
photograph is given by the relation _f_/H, _f_ being the focal length
of the lens and H the height of the camera above ground.” (_Ibid._)

[9] E. Lester Jones: The Aeroplane in Surveying and Mapping, _Flying_,
June, 1919, pp. 438-441, 472, and 476.

[10] H. E. Ives: Airplane Photography, 1920, pp. 407-408.

[11] E. Lester Jones: Surveying From the Air, _Science_, Vol. 52, 1920
(Oct. 17), pp. 574-575, and _Engineering News-Record_, Dec. 16, 1920,
pp. 1184-1186.

[12] E. Lester Jones, _op. cit._ (_Science_), p. 575.

[13] J. Volmat: Application de la photographie aérienne aux levés
hydrographiques, _Comptes Rendus de l’Acad. des Sci. [de Paris]_,
Vol. 169, 1919, Oct. 27, pp. 717-718; _idem_: Rapport sur la mission
photohydrographique de Brest (1919), _Annales Hydrogr._ (publ. by
Service Hydrographique de la Marine, Paris), 3rd Series, 1919-20, pp.
191-220, with seven air photographs and corresponding sections from
French coast charts.

[14] H. Hamshaw Thomas: Geographical Reconnaissance by Aeroplane
Photography, With Special Reference to the Work Done on the Palestine
Front, _Geogr. Journ._, Vol. 55, 1920, pp. 349-376; reference on p. 369.