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.