Air Fronts: FM 21-26, Advanced Map and Aerial Photograph Reading - Section 12.

SECTION XII: STEREOVISION

75. GENERAL. Stereovision is the three-dimensional effect seen by blending into one image two pictures of the same object taken from different points. It is used when making a thorough study of terrain. Aerial photographs for stereovision are obtained by making two successive overlapping exposures with the axis of the camera lens vertical and the aircraft at the same altitude. The area that overlaps is suitable for stereoscopic study. The two pictures are called a stereo-pair. Trees, buildings, hills, and other terrain features in the overlap area rise in exaggerated relief when viewed stereoscopically. This enables the map reader to see through thin foliage of wooded areas; to detect camouflage; to study terrain to be fought over; to identify planes, ships, and important military installations behind-the enemy's lines; and to determine the results of military action. Normally, stereo-pairs are examined with the aid of issue stereoscopes, but the naked eyes may be used if the individual has trained himself to see stereoscopically.

76. LEARNING TO USE A STEREOSCOPE. a. General. In teaching an individual to see stereoscopically it is often helpful to precede actual use of the stereoscope with preliminary eye exercises. The exercises and aids described in b and c below are included for this purpose.

b. Preliminary exercises. (1) Finger exercise. Finger exercises such as the "sausage" exercise shown in figure 79 (1), (2), and (3) are an aid in learning stereovision. To practice this exercise, focus the eyes on a

Figure 79. Sausage exercise.

window about 8 feet away. Hold both forefingers parallel to the ground and bring them into your line of vision from the sides without changing the focus of the eyes. As the tips of the fingers near each other, an image of a sausage appears suspended in mid-air between them.

Figure 80. Fusion exercise.

Figure 81. Depth perception exercise.

(2) Fusion exercise. The next exercise is to learn to fuse two drawings. Focus the eyes on a distant object and move the two dots in figure 80 into the line of sight, approximately 8 to 10 inches from the eyes. If four dots appear, move the manual away from or closer to the eyes until there are three dots. The center dot is in stereovision. If the field manual is turned slowly, the center dot appears to separate into two dots, one jumping out of the top of the other.

(3) Exercise to see relief. In figure 81, use the same method described above. The fused image in figure 81 is a pyramid protruding from the bottom of a square pit. In other words, instead of a flat line drawing, the reader sees the third dimension or relief.

c. Aids. Anaglyphs and vectographs may be used to help an individual to get the effect of stereovision for the first time. An anaglyph is two nearly superimposed aerial photographs each printed in a different color, red and blue-green, and is viewed through glasses with red and blue-green lenses. TM 5-230 gives a detailed discussion of anaglyphs. A vectograph uses the principle of polarized light to present a three-dimensional view.

77. STEREOSCOPES. The stereoscope consists of a pair of lenses set in a frame. The stereoscope gives a small amount of magnification. There are a number of different types but all are used similarly. The instrument is placed on overlapped photographs, as in figure 82.

78. STUDY OF STEREO-PAIRS. a. General. To see photographs stereoscopically, they must be placed in the same relative positions as when they were taken. Any pair of overlapping vertical photographs taken at approximately the same elevation, can be viewed stereoscopically if arranged properly.

Figure 82. Lens stereoscope set up over a stereo-pair.

b. Arrangement of stereo-pairs. (1) Place one photograph on top of the other so the detail common to both roughly matches.

(2) Turn the photographs as a unit until your eyes are parallel to an imaginary line which runs across the approximate centers of the photographs.

(3) Place the stereoscope over the photographs so the left photograph is under the left lens and the right photograph is under the right lens.

(4) Now separate the photographs along the line joining the centers, moving the right one toward the right and left toward the left. The photographs can be kept in proper relation by keeping parallel the details which appear in the overlap area, such as roads or streams. The two objects in the overlap area which are to be studied should be separated so each is directly under the left and right lens respectively. By slight movements the photographs can be adjusted so the center images fuse and the effect of relief or depth is obtained. For a detailed discussion see TM 5-230.

(5) The stereo-pair in figure 83 (1) and (2) shows the overlapping portions of two vertical photographs. They are placed in the proper position for stereovision to be obtained.

79. THE FLOATING LINE. a. The special application of stereovision described below is useful in determining if a line of sight is free of obstructions. In figure 83 there are three black lines. If these lines are viewed stereoscopically, the relative elevation of terrain on either side may be clearly seen. The short line that runs from the roof of the house up to the left and across the road to a tree seems to float in the air; moreover, the end of the line at the tree is lower than at the roof of the house. This line is lower than the tops of the trees around the house but is well above the ground. Therefore an observer at the house can see the tree across the road. Long lines run from the roof of the house in the center of the photograph to the roofs of houses above, and below it. These lines seem to float everywhere except where they move across wooded areas. The lines seem to cut furrows through the tops of the trees. This means that an observer sighting along these lines from the first house cannot see the roofs of the other houses.

b. To use this method, draw a fine ink line between the same two points on both the right and left pictures of a stereo-pair. The ends of the line appear to assume the elevation at their starting and stopping points. When stereo-pairs are reversed, moving the left photograph to the right of the right photograph, houses and trees will be depressed rather than in relief and that

(1) Left half of stereo-pair.                                                                                  (2) Right half stereo-pair. 
Figure 83.

portion of a line which did not float in the original position will float freely. This checks the line of sight for obstructions.

c. To avoid drawing numerous lines on the stereopair, narrow strips of acetate on which lines have been  scratched and inked in may be used. The strips are pivoted around pins stuck through the strips and fastened to the photographs at points from which lines of sight are being determined. By placing the other ends of the lines on the points in question on both photographs and viewing the pair stereoscopically, the presence or absence of obstructions to vision can be determined.

d. This method can not be used when the floating line is parallel, or nearly parallel to the line between centers of the photographs.