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handled in the incremental section of the computer, as are the photo 
graph position computations in the standard AP-Z. In order to make 
integrators available for the additional computations, several elements 
of the photo position computations are modified or shifted to the whole- 
number section. Specifically, the separate corrections available in the 
basic instrument for systematic effects and model distortion are com 
bined, and the air refraction correction is simplified. The compu 
tations for optical magnification and image rotation, which are slowly 
changing parameters used to control zoom lenses and Dove prisms in 
the viewing optics, are moved to the whole-number section. 
The functions of detecting loss of correlation, searching to re 
cover correlation, and indexing between plotting lines, are performed 
in the whole-number section of the computer. The whole-number 
program also controls the computation of the partial derivatives re 
quired for the scan shaping process; in fact, these same derivatives 
are used in the optical magnification and rotation computations. The 
calculation of the basic partial derivatives is executed at a relatively 
low rate since these parameters change rather slowly with motion 
through the model. However, the computer output information which 
controls the scan pattern shaping in the correlation system is updated 
in the incremental section to reflect changes in terrain slope at the 
normal computation rate of 100 times per second. 
In the basic AP-2 instrument, the whole-number section of the 
computer is fully utilized only during the set-up and orientation pro 
cedures. Therefore, the above functions, which are required mainly 
during contouring and profiling, can be handled without addition to the 
basic computing capability. The only modifications required in the 
computer are an expansion of the whole-number memory to permit 
storage of the larger programs and an expansion of the input-output 
section to provide communication with the correlator electronics. 
CONCLUSION 
The addition of an image correlation system to the AP-2 
analytical stereoplotter extends the capabilities of the basic instru 
ment for faster and more automatic production of contour and profile 
charts from any of several types of photography. The expanded instru 
ment makes effective use of the control, computational, and decision 
making abilities of the AP-2 computer to provide not only a highly