26 
outer one-third of the spread of the cross-type target, and each leg of the 
diamond-centered target, is white or yellow. The materials that are used for 
constructing targets are muslin and canvas of appropriate colors; appropriately 
painted plastics, plywood, masonite, cardboard, rock, smooth road and street 
surfaces; and blackened and white lime, especially in dry climate regions. 
vmm mue 
     
    
  
Aerial view of cross-type target. Bordering ground area has dark tone. Consequently 
there is considerable contrast between this area and the white legs, and little con- 
trast between it and the dark red central square of the target. 
  
Ground view of cross-type target. Materials used are dark red and white muslin. 
  
mu : : A ; E ; Ec. mu NS up 
Aerial view of cross-type target with legs of dark red muslin and exposed ground in central 
portion. Light color tone of ground makes dark legs easily identifiable, permits them to 
retain their symmetry, and eliminates need for central portion. A target of all dark legs 
should not be used wherever dark shadows might be cast by vegetation or buildings. 
In these aerial views, target and other images were too small at contact print size for ease 
in identification on reproduced copies at that scale. Consequently, they were enlarged two 
diameters in preparation of the stereograms. Ground dimensions of dark targets are the same 
as recommended for light targets. Overriding of light color tone over dark reduces image 
size of dark targets greater than the direct scale differential between the photograph and 
ground, but this reduction is not functionally detrimental. Its effect is noticeable when 
central portion of target is dark and outer portions are white. 
The necessary horizontal clearance to prevent obscuration of the targets 
by tall trees, buildings, and so forth is given in table 3. 
Table 3. — Necessary target clearance for 
photography of various focal lengths 
  
  
  
  
  
  
Photography focal lengths Distance necessary from target to possible 
obscuring object equals number of times h¥* 
inches millimeters minimum desirable 
6 152 0.75 h 1.0 h 
8 1/4 210 0.55 h O7 h 
12 305 0.4 h 0.5 h 
  
  
  
  
  
*h = height of top of possible obscuring object above placement point of target