Contour interval 
(1) 
(2) Boundaries of the area to be contoured 
(3) Plotting scale 
(4) Desired accuracy 
Automatic contouring may be initiated at any point in the model 
selected by the operator. Contouring speed is controlled by the com 
puter so as to maintain system errors within the desired level of 
accuracy under the varying conditions of terrain slope and photography. 
Values of the slope magnitude and rate-of-change of slope direction, as 
computed from the correlator outputs, are used to slow the contouring 
when steep or rapidly changing slopes make accurate contouring 
impossible at normal speeds. Also, the contouring velocity is reduced 
under poor photographic conditions or when the measured parallax 
errors exceed the desired system accuracy. 
The computer is programmed to detect the absence of corre 
ctable detail and, if necessary, to execute a search routine in order 
to regain acceptable correlation. If a contour line is lost, the com 
puter halts the plotting motion, raises the plotting head on the coordi- 
natograph, and initiates a search routine. If the search is successful, 
the plotting operation is resumed without operator assistance. 
The computer also controls the indexing from one contour level 
to another. When a given contour line reaches the edge of the photo 
graph, the boundary of the desired area, or the starting point of the 
same contour, tracing of the contour line is halted and the plotting 
head is raised. The computer then changes the model elevation to 
the next contour level to be plotted, and X-parallax information from 
the correlator controls the model location so as to determine the 
initial point on the new contour line. After the contour has been 
located, the plotting head is lowered and contouring along the new 
line is started automatically. 
AP-2 COMPUTER MODIFICATIONS 
Many of the additional computer functions in the automated 
system -- terrain-slope computation, generation of the plotting 
motions, scan size computation, and dynamic error compensation 
involve real-time control operations. It is desirable that they be