(526) 
Specifications for commercial aerial photography generally require that 
the average tilt of the vertical aerial photographs for the entire mapping pro- 
ject be no more than 1 degree, that the maximum of any one photograph be 
no more than 3 degrees, and that the relative difference in tilt between two 
adjacent photographs not exceed 5 degrees. Therefore, except in certain unus- 
ual cases of maximum allowable tilt, the effect of tilt will generally be of little 
consequence, except possibly in cases of dips approaching the horizontal. 
7. Buckling and Swelling of Acetate Sheet. 
This will be appreciable only on paper-thin acetate, where it may signifi- 
cantly affect measurements on the transparency. This can be avoided by using 
an acetate sheet of medium thickness. 
8. Drafting Inaccuracies. 
This applies specifically to: 1) the permanent line ruled on the smooth 
underside of the transparency, 2) the two lines drawn on the transparency to 
the two positions of u from the two marked photo centers (sec. III B 2m), and 
3) the transfer of points from the photograph to the transparency. If careful 
work is done, errors due to drafting inaccuracies can be reduced to a negligible 
minimum. 
B. Time Required for Calculations. 
A dip calculation on either a slope or bedding trace using the transparent 
overlay method (sec. III B) and the nomogram can be made in about 10 min- 
utes by a worker familiar with the procedure. If the height finder is used to 
measure the difference in parallax, a slightly longer time will be needed. 
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