The relation between the shape of the shadow and the object casting it must be well learned. This is a part of the training of every architectural draftsman, but the appearance of shadows from above has not heretofore been a matter of importance. The difference between high and low trenches, between cuttings and embankments, between shell holes, occupied or unoccupied, and “pill boxes,” must be detected largely from the character of the shadows. Which elevations and depressions are of military and which of merely accessory nature, whether this black dot is a machine gun or a signaling device, whether that dark spot is an active gun port or an abandoned one—these are all matters of shadow and of light and shade study. Several illustrations of these points appear in Figs. 163, 164 and 165. [Illustration: FIG. 164.—Guide to interpretation of shell holes and other pits.] [Illustration: 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. [Illustration: 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! [Illustration: FIG. 167.—Length of shadow of object one meter high, at different times of the day and year, for latitude of Paris.] [Illustration: