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.
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.