b. Shadows also aid in the penetration of camouflage, which might otherwise
successfully conceal a military installation or activity. Even fake shadows employed in
camouflage can be easily detected on aerial photos, because camouflage shadows are
usually painted on and appear in a true position to the sun at only one time of the day.
Thus, shadows painted to appear true in the morning will be completely out of position in
the afternoon.
c. Shadows further aid in the orientation of a photo with a map, as the shadows
generally fall toward the north (between NW-NE) in the northern hemisphere, depending
on the time of day.
d. Furthermore, shadows can hide objects as well. A tank parked in the shadow of
a building or tree can be overlooked by the analyst who doesn't pay attention to such
details as tank tracks leading up to, but not away from, the building or tree.
4. Shade. The distinguishable variations of gray in which an object appears on a black
and white photo are known as its tone or shade. The shade of an object is almost entirely
due to the amount of light reflected from it to the camera. Light reflection, in turn,
depends on a number of contributing factors, such as the texture of the surface, position
of the sun, and wind velocity.
a. Texture is the predominant factor in both light reflection and the shade it creates.
Because of texture, objects will often appear much lighter on a photo than their color
would seem to warrant. Some colors, however, reflect more light than others; for example,
white reflects light and black absorbs it. A smooth surface is a good reflector and
appears lighter when the camera is in a position to catch the reflected rays. On the other
hand, if the reflected light does not reach the camera, a smooth surface will appear dark.
An example is water, which sometimes appears light and sometimes dark, depending on
the sun, position of the camera, and the velocity of the wind. Any change in texture of a
portion of an object is evident on an aerial photo as a difference in shade.
b. The wind may disturb the reflecting surface of a body of water, or it may expose
an entirely new surface to reflect light, as when the wind bends crops or vegetation.
Therefore, the shade of an object may vary even on two consecutive photos of the same
mission.
c. Thus when the earth, grass, or vegetation is crushed flat, say by a tank or truck,
the reflected light will be altered from the norm. Depending on weather conditions, tracks
made by one man walking across a grassy field can be detected up to 48 hours later on
aerial photos taken from 30,000 feet.
d. The center object of the separated squares in Figure 1-4 reflects only part of the
light shining on it; therefore, it is a medium shade. The rectangle on the right absorbs
almost all light shining on it; therefore, it is a dark grey or black. The rectangle on the left
reflects
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