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.
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.
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.
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
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
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.
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.
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.
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.
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.
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.
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
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.
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).
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.
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.)
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.
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.
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.
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.
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.
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.
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.
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.
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
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]
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.
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.
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.
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.
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.
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.