e 
where K, (is a constant and Sw is the terrain slope magnitude, 
  
For small values of terrain slope, the value of Sy is limited to 
avoid division by a near-zero quantity. 
n. The most difficult problem in automatic contouring (as 
in manual contouring) is the control of the plotting motions in 
areas of very low terrain slope. Referring to equation (1), it 
will be noted that as both Sy and Sy approach zero, the arctangent 
function in the first term becomes indeterminate. Further, very 
small errors in Sx and Sy tend to cause large errors in p. This 
difficulty is solved in the AS-11C by use of "parallax integral 
steering". The effect of parallax integral steering is indicated 
in equation (1) by the correction term, 8', which is defined in 
terms of a rate, dB'/dt, which is proportional to the measured 
elevation error. The time constants associated with the arctangent 
term and the correction term are scaled according to terrain slope 
such that the correction term tends to predominate at low slope, 
whereas the arctangent term is effective at high slopes. Thus, 
at terrain slopes below a few degrees the plotting direction is 
defined almost entirely by the parallax error. In areas of small 
terrain slope, the system tends to plot in a straight line until a 
small parallax develops and then adjusts the plotting direction 
accordingly. 
o. In automatic profiling, the plotting velocity is con- 
trolled as a function of correlation and parallax in the same manner 
as for contouring. Profiling is performed at an arbitrary angle 
y in the X-Y model-coordinate plane. (In the orthophoto mode, 
profiling is performed in the Y model-coordinate direction.) 
The general equations for profiling operation are: 
Vy = V cos vy 
Vy = V sin y 
Vg ^ V (Sp + Sp') + K, Px 
where Sp is the terrain slope in the profiling direction and Sp' is a 
parallax integral correction analogous to B' for contouring, The use 
of parallax integral steering is not as essential for profiling as 
for contouring; however, it does result in smoother operation in areas 
where terrain-slope information is poor. Two printing carriages are 
provided, one for printing the orthophotograph and the second for 
the formline contour chart. In printing each image is compiled as 
an assembly of incremental images which are generated in scaled 
model coordinates on the faces of the respective printing CRTs and 
are transferred optically to the film. Both printer carriages are 
driven in Y and indexed in X by model-coordinate commands trans- 
formed to the orthophoto scale by the computer. The operation of 
the printer in a scaled X-Y model-coordinate system with video 
signals generated by scanning in the photocoordinate system, provides 
the basic image transformation required for orthographic compilation. 
In this process, the elements of imagery appearing in the orthophoto 
are rectified to compensate for camera geometry and orientation, and 
shaped to eliminate effects of terrain slope and elevation change 
within the model. 23