Oblique views from the airplane are of very great value. While vertical views are more searching in many respects, they do nevertheless present an aspect of the earth with which ordinary human experience is unfamiliar. Consequently they are difficult to interpret without special training. They suffer, too, from the military standpoint, from the limitation that it is with vertical extension just as much as with horizontal that an army has to contend in its progress. Elevations and depressions of land show on an oblique view where they would be entirely missed in a vertical one. For illustration, study the picture of part of the outskirts of Arras (Fig. 144), presenting moat, walls and embankments, all of which would be serious obstacles, but would hardly be noticed on a vertical view. Pictures taken from directly overhead are eminently suited to artillery use, but oblique views of the territory to be attacked, taken from low altitudes, formed an essential part of the equipment of the infantry in the later stages of the war.
Pictorially, oblique views are undoubtedly the most satisfactory. The most revealing aspect of any object is not one side or face alone, but the view taken at an angle, showing portious of two or three sides. Best of all is that taken to show portions of front, side and top—the well-known but heretofore fictitious "bird's-eye view" (Fig. 145). This possibility is ordinarily denied the surface-of-the-earth photographer, but the proper vantage point is attained in the airplane.
Aerial obliques may be taken at any angle, although a distinction is sometimes made between obliques of high angle and panoramic or low angle views (Fig. 146). In addition to ordinary obliques, a very beautiful development is the stereo oblique. Both kinds of oblique photography call for special instrumental equipment and technique.
Fig. 144. - The outskirts of Arras. Low oblique showing contours.