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.]