Fig. 142.—Arranging prints for a mosaic map.
When the prints are made they must be mounted together on a large card or cloth background. For a very small mosaic they may be juxtaposed by simple examination, matching corresponding details in successive prints. For a mosaic of any size an accurate outline map must be drawn on the surface to which the prints are to be attached. The prints are then laid out on this outline, moved to their correct positions, and held down by pins (Fig. [142]). When they are all arranged the final mounting may be begun. The excess paper, beyond what is necessary for safe overlaps, may be trimmed off, exercising judgment as to which print of each adjacent pair is of the better quality, and utilizing it for the top one at the overlapping junction. If one print shows serious distortion it may be placed under its fellows on all four edges, thus minimizing its weight. The edges are best made irregular by tearing. Straight edges are apt to force themselves on one's attention in the final mosaic and give an erroneous impression of the existence of straight roads or other features. Both forms of edging are shown in Figs. [124] and [143].
An alternative method of securing the final print mosaic, where film negatives are used, is to trim successive film negatives so that the trimmed sections will exactly juxtapose, instead of overlap. The sections are then mounted, by stickers at their edges, on a large sheet of glass, and printed together. Captured German prints show that this was the method commonly used with the German film camera (Fig. [62]).
It will be noted that the procedure which has been described and illustrated by Figs. [142] and [143] assumes the previous existence of a map accurately placing at least the chief features of the country covered. This draws attention at once to the limitations and true sphere of aerial photographic mapping at the present time. With the cameras thus far it is not possible, nor is it attempted, to do primary mapping of unknown regions. Distortions due to lens, shutter, film warping and paper shrinkage considerably exceed the figures permitted in precision mapping. From the standpoint of geodetic accuracy the cumulative errors of deviations in direction, altitude and level, peculiar to flying, would soon become prohibitive.
Fig. 143.—A partly completed map. Prints mounted over an outline sketch map to proper scale.
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.