Interior orientation is then performed by measurement of the 
fiducials, and the computer automatically computes and stores 
the photo principal-point locations, A control-program tape 
is then entered to establish operation in the desired operating 
mode. 
a. Orientation Mode - The orientation mode provides 
relative and absolute orientation of the stereo model. This 
mode is employed when a given stereo pair is being processed 
for the first time. If the model has been previously oriented, 
it can be recovered by reading a tape which contains the 
orientation parameters. 
In relative orientation, the values of the camera focal 
length and the correction coefficients are first entered from 
the viewer panel. The orientation errors are then determined 
from measurements of y parallax made automatically at from 5 
to 12 points in the model. At each point, the correlator outputs 
are used by the computer to automatically eliminate elevation 
error and y parallax. The y parallax is eliminated by inserting 
a differential y displacement of the two photographs. During 
this process, the amount of motion required is recorded as the 
y parallax, After these measurements have been made at the 
desired points, the computer calculates and stores in appropriate 
locations the necessary changes in the values of the orientation 
elements. 
Absolute orientation is based on the known model coordinates 
of three or more control points. The known coordinates of each 
point are sequentially entered through the viewer panel while 
the corresponding coordinates in the unoriented model are being 
measured, using the correlator outputs to clear the elevation 
error. When this procedure is completed, the computer determines 
the required changes in the orientation elements to level and 
scale the model. If more than three control points are used, the 
leveling and scaling computations minimize the mean-square errors 
at all control points. If desired, the residual errors at the 
control points can then be used to define the coefficients of the 
model-deformation correction. 
b. Contouring Mode - The contouring mode produces con- 
ventional contour manuscripts that are automatically plotted on the 
coordinatograph. To initiate operation, the contouring mode con- 
trol program tape is entered, and quantities defining the desired 
contour interval, plotting boundaries, and output scale are entered 
from the viewer panel. 
  
Automatic plotting may be ínitiated at any desired elevation. 
Plotting proceeds automatically along a contour line until the 
contour closes upon itself or intercepts a model or photo boundary. 
When one of these occurrences is detected by the computer, the 
plotting stylus is raised and the model elevation is automatically 
adjusted to the next contour elevation value. The plotting stylus 
is then lowered and plotting is resumed. 
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