The great field for aerial photographic mapping in the near future lies in filling in detail on maps heretofore completed as to general outlines, or, as in the war, on maps far out of date. The war-time procedure in country largely unknown, such as Mesopotamia, was probably closely that which will be necessary in peace. Conspicuous points in the landscape were first triangulated from friendly territory, and from these the outline map was drawn, whose details were to be supplied by aerial photographs. Much of the “mapping” of cross country aerial routes so far done is frankly of a pictorial nature, showing conspicuous landmarks and good landing fields—extremely valuable and useful, but not to be confused with precision mapping. In assembling mosaics of this kind the elaborate procedure described above is not followed. The process is the simple one of juxtaposing adjacent prints as accurately as possible by visual examination. Errors are of course cumulative, but as long as exact distances are not in question this is no matter. CHAPTER XXVII OBLIQUE AERIAL PHOTOGRAPHY 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 portions 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.