e 
an 
ELECTRONIC CORRELATOR 
for the 
PLANIMAT 
INTRODUCTION 
The Planimat, a stereo plotting instrument of second order, 
has recently been introduced by Carl Zeiss. Provision has been 
made in this instrument for the installation of an electronic 
image correlator to allow automation of the stereo profiling 
operation. This paper describes the design and performance 
goals of this automation equipment, which is designated the 
Itek EC-5 Electronic Correlator. 
Even with the most modern instruments, manual stereo 
plotting is a tedious and time consuming operation. The time 
required to manually profile a stereo model is between 2 and 4 
hours, depending on the roughness of the terrain. In the basic 
profiling operation, using a stereo plotter such as the Planimat, 
the operator is required to view the diapositives stereoscopically, 
traverse over the stereo model in a fixed pattern at a fixed or 
variable speed, and adjust the apparent height of a floating mark 
by means of a handwheel or foot wheel to follow the terrain 
profile. When functioning in this way, a human operator be- 
comes part of a closed loop feedback system and is subject to 
some basic limitations. For example, his response time, i.e., the 
time delay between the perception of an error in the height of 
the floating mark and its subsequent correction by means of 
the handwheel, has a definite minimum value making it neces- 
sary to slow down the traversing speed in rough terrain. 
It is evident that an electrical servosystem, given a suitable 
input, can be made to respond in a much shorter time than a 
human operator and will have superior ability in following 
rapid fluctuations of terrain height. 
The performance of an electronic correlation system can be 
improved over that of a human operator in two ways: 
1. The accuracy of terrain following can be maximized by 
the use of a sufficiently high loop gain. 
2. The speed at which terrain fluctuations can be followed 
can be maximized by the use of a wide servo bandwidth. 
The exploitation of these possibilities requires an image 
correlator that is capable of rapidly and accurately detecting 
small variations in the x parallax between conjugate stereo 
images while accommodating residual distortions caused by ter- 
rain slope and camera attitude variations. 
In addition to these qualities, a successful automation sys- 
tem must have adaptive features that allow it to adjust to vary- 
ing image quality and other anomalies, such as clouds and water 
areas, to minimize the necessity for human intervention. 
Many adaptive features have been included in the design of 
the EC-5 Electronic Correlator, with the aim of minimizing the 
necessity for operator intervention. 
The Itek EC-5 Electronic Correlator can be integrated into 
a complete orthophoto production system based on the Zeiss 
Planimat and GZ-1 Orthophotoscope. Off-line operation can be 
secured by means of the SG-1 storage unit in which profile data 
from the Planimat is stored in analog form on scribed plates. 
These plates are then placed in the LG-1 scanning unit which 
drives the Orthophotoscope. The time required to profile an 
average stereo model is reduced from 2 to 4 hours with manual 
operation to 1/2 to 1 hour with the Electronic Correlator. This 
increase in profiling speed brings the throughput time of the 
Automated Planimat into line with the time required to print 
the orthophoto in the GZ-1 Orthophotoscope, resulting in better 
utilization of equipment. 
Fig. 1 shows the Automated Planimat with the electronics 
rack and control box on the left and the SG-1 storage unit on 
the right.