ve 
cal 
Perspective Center 
Photo 2 
    
       
        
      
  
  
Perspective Center 
Photo 1 Shaped, Conjugate 
Scanning Patterns 
Photo 1 Photo 2 
Surface 
Z Model wy Model 
Reference 
Model - Coordinate 
Scan Pattern 
X Model 
Figure 4 Scan Pattern Transformations, 
Model-to-Photo Coordinates 
tion effects in the photographs due to photo-model 
geometry and to terrain slope measured by the corre- 
lator. The transformation is based on a plane-surface 
approximation to the local terrain. 
The purpose of the scan-size control is to provide 
the minimum model-coordinate pattern consistent 
with the photographic imagery, based on the cross- 
correlation signal. Minimum scan size is desirable to 
minimize the error in elevation measurement associ- 
ated with the plane surface approximation in scan 
shaping. 
Another special feature, used in the analytical 
stereoplotters to improve elevation accuracy, is an 
electronic gate in the x-parallax channel. As indicated 
in Figure 5, the gate discards information from the 
outer portions of the scanning pattern, reducing the 
effective scan-size for elevation measurement. Use of 
the x-parallax gate improves the elevation measure- 
ment accuracy without decreasing the sensitivity of 
the y-parallax and terrain-slope measurements. 
The major performance limitations of present 
  
Full Area Scanned, 
N Used for Y Parallax 
La&— and Terrain Slope 
Measurements 
  
  
  
pu Gated Area, Used 
NN | — — for X Parallax 
Y Model Measurement 
  
  
  
  
X Model 
Figure 5 Scan Gating for the X-Parallax 
Error Channel 
AUTOMATION IN COMPILATION 
automatic stereoperception systems are related to the 
deficiencies of the flying-spot scanner. As noted 
previously, the low level of illumination available 
from the CRT limits both the signal-to-noise ratio in 
the video system and the life of the CRT. Also, the 
spatial-frequency performance of the scanning system 
is limited by the finite CRT spot size and by the CRT 
phosphor persistence. A final important limitation is 
the serial method inherent in a flying-spot scanner; 
i.e., a CRT spot is used to scan a finite area by tracing 
a prescribed pattern. Thus, the maximum speed that 
information can be extracted from a photographic 
image is limited by the frame time, that is, the time 
required to scan the entire area once. These limita- 
tions are important reasons for continuing research 
on alternative methods for extracting information 
from the input photographs. 
AUTOMATIC PLOTTING 
Referring to Figure 1, the automatic stereopercep- 
tion system for the analytical stereoplotters measures 
terrain elevation and slope, using a closed-loop con- 
trol system composed of the computer, the viewer, 
and the scanning and correlation equipment. The 
same functional block diagram can be expanded to 
that of an automatic stereo-compilation system by 
simply adding computer programs for automatic plot- 
ting control. For automatic plotting, the computer 
performs these additional functions: 
(1) Generating the stereomodel motions required 
to trace out a contour (line of constant eleva- 
tion) or profile (elevation along a planimetric 
line). 
(2) Providing the overall control required to auto- 
matically plot, on a plotting table, all contours 
or profiles in a given area. 
The principal use of stereocompilation instruments 
is the generation of contour manuscripts for use as a 
basic component of conventional topographic maps. 
Automatic contour generation is thus an extremely 
important capability. On the other hand, as will be 
discussed, model motion generation and plotting 
control are more difficult for contouring than for 
profiling. Because of these difficulties, some auto- 
matic compilation systems avoid automatic contour- 
ing and restrict automatic plotting-motion generation 
to profiling. In such systems, a secondary processing 
of the profile data is used to obtain a contour line 
manuscript. Generally this is a rather inefficient 
process: either a significant amount of stereomodel 
information is lost in interpolating between plotted 
profiles, or profiling is performed at such a small 
spacing that the procedure is slow. Automatic con- 
tour-following is thus an important capability and has