Automatic Comparator
R. B. FORREST
W. H. MOORE
F. A. SCARANO*
This paper describes an automatic three-stage stereocomparator which uses a
general-purpose on-line computer and electronic image correlation equipment to
automate various operations. There are several unusual features in the new compara-
tor, e.g., an ability to perform coordinate transformations and corrections of system-
atic errors on-line, capability to process observations and analyze them for reliability, a
facility for self-calibration, and a keyboard that can be interpreted by the software.
Together, the unusual features offer unprecedented possibilities for photogrammetric
research.
INTRODUCTION
Since the development of the high-speed digital
computer made analytical photogrammetry practical,
computers continually have improved in speed and
power. Yet the photogrammetric comparators that
supply coordinate data to these computers still are
simple x,y measuring engines with rather rudimentary
optics. The measurement accuracy of these compara-
tors is quite adequate for most photogrammetric
work. However, much time is required to measure
image coordinates of selected points. This is true
whether stereocomparators are used to select and
measure the points, or whether single-stage compara-
tors are used in conjunction with stereoscopic point-
transfer devices. In addition to this direct time re-
quired for image measurement, errors detected in
subsequent computations often necessitate the re-
measurement of all points on a photograph.
The Automatic Comparator is an image-measure-
ment system designed to greatly increase stereocom-
parator productivity. The Automatic Comparator?
consists of a modified OMI TA3/P three-stage stereo-
comparator, a specially interfaced and programmed
PDP-9 digital computer, and special electronic image
scanning, correlation, display, and control equipment.
The Automatic Comparator combines the power
of a modern digital computer and automatic elec-
tronic correlation with the precision measurement
capability of a three-stage photogrammetric compara-
tor. This combination results in an extremely flexible
instrument capable of many operations and applica-
Y «Rome Air Development Center, Griffiss Air Force Base, New
ork.
1Bendix Research Laboratories, in collaboration with OMI, recently
completed development of the first Automatic Comparator for Rome
A Development Center, U. S. Air Force, under Contract AF 30(602)-
BENDIX TECHNICAL JOURNAL SUMMER 1968
tions. The major features of this instrument are char-
acterized below:
1. The Automatic Comparator is unique in provid-
ing a capability for true interior and relative
orientation. This capability makes it possible to
create a continuous stereo model before mea-
suring pass-points. Thus, the operator can main-
tain stereoscopy as he moves from one pass-
point measurement to the next in the overlap
area. He need only change the observed height
of a floating mark to keep it near the terrain
surface of the model.
2. Image coordinates can be recorded in photo-
graph coordinates rather than comparator stage
coordinates. Calibrated lens distortion or reseau
values can be included in the coordinate adjust-
ments.
3. Conjugate image points can be located auto-
matically by means of the electronic scanning
and correlation equipment. At the operator’s
request, the correlation circuitry automatically
adjusts one viewed photograph (the slave
photo) until the scanned area exactly matches
the area scanned on the other viewed photo-
graph (the master photo). The correlation
equipment uses the smallest achievable scanned
image area in making the conjugacy adjustment.
The quality of correlation determines the mini-
mum area, down to a minimum area 50 microns
square.
4. Three different viewing systems are provided
for the operator:
a. Direct-viewing optical train with automati-
cally controlled image rotation and zoom
magnification.
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