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vision for relative orientation by movement of
either projector alone. This would require motor
drives to all six projector axes and for Bx, By
and Bz motions also. Motors for common orient-
ation of both projectors may also be required.
The operator would normally select the axes or
motions used, although an automatic program
could probably be devised, :
Stereomat can also assist triangulation
by the semi-automatic locating and recording of
significant points in the photographs. For the
automatic location of control points, a small
supplementary photograph of the terrain is re-
quired with the control point at the centre. The
supplementary photograph could be cut from a
paper print, or preferably, printed from the
negative on to a small glass plate.
An automatic triangulation instrument would
be provided with a separate scanning means for
supplementary photographs. To set up control
points the procedure would be as follows:
1. Relative orientation would be completed.
2. The photo-cell of the separate scanner would
be connected to the correlation unit in place
of the photo-cell for either the left or right
plate.
3. The floating mark or pattern centre would be
set to the known coordinates of the control
point.
4. The X and Y parallax signals developed by
the correlation circuits would be used to
actuate a common orientation axis of both
projectors, or to adjust Bx, as selected by
the operator.
5. The procedure would be repeated for other
control points, the parallaxes being auto-
matically cleared in each case by an ap-
propriate common orientation or base adjust-
ment.
An alternative procedure would be to ad-
just only one plate to the control point, thus
avoiding the requirement for common orientation
motors. Normal scanning would then be resumed,
and the second plate again brought into relative
orientation.
Automatic recording of pass points is also
possible. Pass points selected by the operator
could be printed by a small C.R.T. similar to
that used for producing ortho-photography. The
resulting print can then be used to set up the
plotting instrument in a manner similar to that
just described for control points.
PERFORMANCE FACTORS
AUTOMATIC STEREO
be expressedas the fraction of the average model
area that the system will fail to plot automati-
cally.
Accuracy in turn can be divided into static
accuracy and dynamic accuracy. Static accuracy
refers to the system at rest, and is primarily a
function of the parallax sensing operation. Dy-
namic accuracy refers to the additional errors
that are introduced as the result of the motion
during profile or contour tracing.
Plotting speed and accuracy are inter-
dependent with Stereomat, and increased speed
can always be obtained at the expense of re-
cord smoothness andaccuracy. For a given accu-
racy, however, the plotting speed is limited
fundamentally by the rate at which information
is extracted from the stereo photographs. Scan-
ning the images at higher speed increases the
information rate, but also raises the frequency
of the image signals, thus increasing the diffi-
culty of separating them from the spurious
"quantum noise" signal also present in the out-
put from the photo-cells.
Quantum noise refers to irregularities in
the image signal owing to the corpuscular or
quantized nature of light. At very low-light levels
the rate at which light quanta or photons reach
the photo-cells may be low enough for them to
register individually. The resulting roughness
in the output signals tends to obscure the image
signals, and to introduce irregularities in the
data signals from the correlation circuits. These
irregularities are transferred to the profile or
contour records, and produce an erratic Jiggling
or oscillating of the servos.
Increasing the amount of light reaching
the photo-cells reduces quantum noise, and there-
fore allows a higher scanning velocity to be
usefully employed. There are physical limits to
the brilliance of the scanning spot however, and
Stereomat approaches these limits closely.
Optical aperture is, therefore, a factor governing
CIRCULAR SPIRAL. ©
RADIAL. ROSETTE
TV RANDOM cé
Three performance criteria for an automatic
plotting system are: speed, accuracy, and falli-
bility. Speed and accuracy are generally ex-
pressed in numerical terms, and fallibility could Figure 6. Scanning Patterns
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