the building for recording on the photographic plates. 
Third is to have a known angle, generally a right angle, in the plane of the 
horizon. One dimension is now essential for scale, and two dimensions, in the 
directions of the two horizontal coordinates, serve with the horizontal angle to reveal 
elusive errors of the convergence of camera axes. Two vertical dimensions, at 
greatly differing depth in the optical model can serve the same purpose, and additional 
measurements allow determinations of accuracy and error of the entire photogram- 
metric procedure. 
A choice of camera station which best satisfies the requirements of photographic 
coverage and the system of survey control described above is shown in Figure2, a 
stereopair of the north and west facades of Congress Hall, Independence National 
Park, Philadelphia, Pa. The parallel camera axes intersect the facades of the 
building to each side of the vertical corner at approximately the same depth in the 
optical model. The cameras are inclined upwards 10.4 grades, the maximum 
possible on the Wild T 30 phototheodolite with the base levelled. 
The procedure for orienting such a stereopair in the A 7 Autograph is as follows: 
a. Set measured elements of inclination w' and w'' in the autograph 
b. Measure distances within the optical model with change of base - 
bx - to choose approximate scale for greatest possible range of plotting 
c. Make absolute orientations on single photographic plates of the elements 
of rotation K' and «'' along vertical lines intersected or nearly intersected 
  
  
  
  
  
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