Fig. 172.—Aviation field, showing hangars, planes, landing “T” and refuge trench.
Just as the detective of classic story makes full use of freshly fallen snow to identify the footprints of the criminal, so does the aerial photographer utilize a snowfall to pierce the enemy's attempts at deception. Tracks in the snow show which trenches or batteries are in actual use. Melting of the snow in certain places may mean fires in dugouts beneath. Black smudges in front of trench walls show where guns are active. Guns, wire and other objects, however carefully painted to match the gray-green earth, stand out in violent contrast to this new white background (Fig. [173]).
Fig. 173.—Trenches and barbed wire in the snow of an Alpine ridge. Italian Air Service photograph.
Fig. 174.—A fully interpreted aerial photograph.
After the aerial photograph has been interpreted the results of the interpretation must be made available to the artilleryman or the attacking infantryman. This may be done by legends marked directly on the photograph. Another method is to mount over the photograph a thin tissue paper or oilskin leaf, with the interpretation marked on it. A yet more elegant method consists in outlining all the chief features of the photograph in ink, writing in the points of importance in interpretation, and then bleaching out the photograph with potassium permanganate solution. Photographic copies of the resultant line drawing are then mounted side by side with the original photograph. Fig. [174], which shows a fully interpreted photograph, is an example of this kind of mounting.
CHAPTER XXX
NAVAL AERIAL PHOTOGRAPHY
The problems of naval aerial photography are quite different from those of military aerial work, and on the whole they are more simple. At the same time, photography has played a considerably less important part in naval aerial warfare than in land operations. Photography as a necessary preparation for attack has not figured in naval practice nearly so much as have the record and instruction aspects. To some extent this is due to the nature of the naval operations in the Great War, to some extent to the limitations of ceiling and cruising radius of the naval aircraft.