Fig. 165.—Illustrating the importance of distinguishing between objects of similar appearance but different military importance.
Shadows may be used to get exact information as to directions and magnitudes. If we know the time of day at which a picture is taken, the direction of the shadows will give the points of the compass. A chart for doing this is shown in Fig. [166]. The length of a shadow is a measure of the height of the object casting it, and the exact relation between the two dimensions is determined by the day and hour. Fig. [167] embodies in chart form the values of this relationship for all times of the year and day, while Fig. [168] shows the kind of picture in which shadow data could be utilized to great profit.
Fig. 166.—Location of true north from direction of shadows. Place the dial on the photograph, the hour line corresponding to the time it was taken being pointed in the direction of the shadows. North lies between the two arrows, the exact direction being obtained by joining the center of the dial to the point on the figure of eight corresponding to the date on which the picture was taken. (Number on figure of eight represent the 1st of the month.)
Minute changes, both in light and shade and in position, must be watched for with great care. Naturally growing foliage and the cut branches used for camouflage differ in color progressively with the drying up of the leaves. Hence a mere spot of lighter tone in a picture of a forest, especially if the picture is taken through a deep filter, becomes instant object for suspicion. The complete study of any position calls for photographs of all kinds—verticals, obliques, and stereos. Stereoscopic views are the worst foe to camouflage. A bridge painted to look like the river beneath is labor thrown away if the stereo shows it to be a good ten feet above the real river!
Fig. 167.—Length of shadow of object one meter high, at different times of the day and year, for latitude of Paris.
Fig. 168.—Bethune, August, 1918, illustrative of interpretation by shadows.
