The most difficult problem in automatic contour- 
ing (as in manual contouring) is control of the plot- 
ting motions in areas of very low terrain slope. The 
terrain-slope direction angle, ß, is given by 
Ss 
p — arc tangent 
Sx 
where Sy and Sy are the terrain slope components in 
the X and Y directions. As both Sy and Sy approach 
zero, the arc tangent becomes indeterminant. Fur- 
thermore, very small errors in Sy and S, tend to pro- 
duce large errors in the slope angle. This difficulty is 
solved in the automated analytical stereoplotter 
through the use of “parallax integral steering". Here 
B is computed as 
B arc tangent 5X + p' 
Sx 
where 
Kzkfo a 
If the slope angle is incorrect (due to error in Sy or 
Sy ), the forward plotting motion will cause the gen- 
eration of X-parallax error over a period of time as 
the operating point tends to move off the contour. 
This action causes g’, which is proportional to the 
time-integral of the x-parallax error, to assume a value 
such that direction is corrected. The time constant 
associated with the 6’ correction is such that it tends 
to predominate at low slopes, whereas the arc tangent 
term is effective at high slopes. At terrain slopes 
below a few degrees, the plotting direction is defined 
almost entirely by the x-parallax error and the system 
tends to plot in a straight line until a small parallax 
develops and the plotting direction is adjusted accord- 
ingly. 
The generation of plotting motions for profiling is 
simpler in several respects but otherwise analogous to 
automatic contouring. As shown in Figure 8, the plot- 
ting velocity is generated in the selected horizontal 
Plotting Velocity 
Component 
      
    
  
  
  
Model Point Terrain 
Surface 
    
Model 
Elevation 
Horizontal Distance 
Along Profile 
Vertical 
Parallax- Correction 
Component 
Figure 8 Profiling Steering 
AUTOMATION IN COMPILATION 
direction with a vertical component based on the 
measured terrain slope. Depending upon image qual- 
ity and terrain roughness, the plotting velocity is 
generally varied in the same manner as it is in con- 
touring. The parallax-correction is applied vertically, 
changing model elevation at a rate proportional to the 
measured x-parallax error. 
Overall Plotting Control 
In addition to generating plotting motions, essen- 
tially a continuous process during automatic plotting, 
the overall compilation process requires numerous 
intermittent actions, such as the following: 
(1) Controlling the plotting stylus. 
(2) Detecting the logical end of a plotting line and 
indexing to the next line. 
(3) Detecting situations where automatic plotting 
operation is not possible and taking suitable 
corrective action. 
The stored-program digital computer in an analytical 
stereoplotter is very well suited to performing inter- 
mittent functions such as these. The computer can be 
readily programmed to perform a complicated se- 
quence of operations, the sequence being varied de- 
pending upon actual conditions. The following para- 
graphs discuss how these plotting-control functions 
are performed. 
During automatic plotting, the plotting stylus 
must be raised temporarily, but very rapidly, to avoid 
plotting of transient errors. This action is controlled 
by the computer based on the measured X-parallax 
information. The computer can also be programmed 
to record locations where raising the stylus causes a 
gap of appreciable length to appear in the manuscript. 
This recorded information facilitates subsequent man- 
ual fill-in of the gaps. 
When the logical end of a plotting line is reached, 
it is necessary to stop plotting, index to the next line, 
and resume plotting. Normally, the indexing point is 
defined within the computer by specified model or 
photograph boundaries. The photograph boundaries 
are logically determined by the edges of the photo- 
graphs. The model boundaries may be established by 
the operator to define a rectangular area to be plot- 
ted. In addition, the initial point plotted on a contour 
may be recorded by the computer and used to deter- 
mine contour closure so that automatic indexing can 
be performed for closed contours. 
When a point is reached outside of the model or 
photo boundaries, or when contour closure occurs, 
the control computer stops plotting the contour, 
raises the output stylus, and indexes to the next line. 
Indexing in contouring involves changing the eleva-