Читать книгу The Face of the Earth as Seen from the Air - Willis T. Lee - Страница 3
ОглавлениеINTRODUCTION
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.
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.
