6 
Although fiducial marks are not available on 35-mm 
cameras, the principal point may be located at the intersec- 
tion of the diagonals. 
The long dimension of the 35-mm negative format is 
either parallel (longitudinal—Figure 5a) or perpendicular 
(transverse—Figure 5b) to the flight line. With the longi- 
tudinal position and & given overlap, the base to height 
ratio will be 1.5X greater than in the transverse position, 
thus giving a greater accuracy in height measurements; the 
other position provides a greater depth of fusion. However, 
there are other factors that will determine the choice of 
position: : 
l. The transverse position requires the same spacing of 
flight lines as in conventional photography, there- 
fore requiring less flying time than with the closer 
spacing required by the longitudinal position. 
2. The transverse position also provides a greater leeway 
in navigation by maintaining lateral overlap. 
3. The longitudinal position makes it possible to view 
the uncut film with the stereoscope, provided the 
film moves in the same direction as the flight. 
Either way requires 33% more photographs than con- 
ventional square negatives if the S.A.P. is done with & 2h- 
mm lens from the same altitude as the conventional photogra- 
phy. 
  
  
       
  
  
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a 
  
Figure 5,—These diagrams illustrate the 24 x 36 mm format in 
both parallel (longitudinal) and perpendicular (transverse) 
positions to the flight line. 
The type of shutter used in aerial photography must 
be considered with respect to: (a) adequate speed, and (b) 
low-temperature operation. The shutter speed required in a 
specific situation may be calculated according to established