70 REPORT OF COMMISSION VII
one must be able to clearly understand what is beyond doubt—perhaps con-
firmed by other sources of intelligence—and what is only probable.
As an example of the wealth of detail and the high quality of photographs
obtainable while on aerial reconnaissance at very great altitudes with modern
material and fine-grain emulsions, it may be mentioned that, in interpreting
verticals taken at an altitude of 36,700 ft. (about 11,200 m.) at an approximate
scale of 1:11,000 over lines of defense in mountainous country, it was possible
for the author to establish the exact positions of the barbed wire belts. The posts
| of these belts were made of slender trees, whose sawn-off shiny fibers reflected
|| the light, and revealed the tops as white dots in the photographs (see Figure
| 11). Moreover, when examined through a stereoscope with a fairly small magni-
Fic. 11. An enlarged detail of a vertical showing a strong point in mountainous country with
the barbed wire belts clearly visible in spite of the great reconnaissance altitude, 36,700 ft. (about
11,200 m.).
fying capacity, these tops showed up in relief. Pits, one meter across, blasted
in the rocks for the use of riflemen, were revealed by the fact that their rims
reflected the light and consequently appeared as white circles round black dots
(the pits), which otherwise would not have been detected. Corresponding cir-
cumstances may be pointed out in the case of sandy ground where trenches
and other field works are revealed by the luster of the grains of sand in the earth
that has been dug uparound the dark excavations (see Figures 12-13). But even
works in vegetable soil can usually be detected by the variations in the grey
tones of the photos; also it should be noted that mine strips can be localized
in the reconnaissance photos through the lighter tone of the ground where it
has been turned or perhaps only touched.
From a tactical point of view, the fact that tracks of different kinds are
often easily discernible in aerial photos is of extraordinary importance. Espe-
