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Paper for Presentation at 
9th International Conference on Photogrammetry, London 
September 1960 
Control System Design Problems in 
Automatic Stereoplotting Machines 
Jamea M. Ham * 
Introduction: 
In stereoplotting machines, the basic information contained in 
the two-dimensional photographic fields of two plates must be examined, 
correlated and transformed into useful forms such as contour, steepest 
descent (drainage) or profile maps. It is important to emphasize that 
the central problem is one of data processing. In current machines, 
information is collected from the photographic fields by means of the 
transmitted component of the light of fine beams incident simultaneously 
on selected points on the plates. To plot, for example, a contour of 
constant elevation in the topography associated with the photographs, 
it is necessary to cause the scanning light beams to trace on the plates 
the two paths which together represent the contour. Every procedure of 
information extraction calls for controlled relative motion between the 
plates and the scanning beams. The many possible ways of achieving 
photogrammetrically useful relative motions correspond to a variety of 
machine designs of which the Projection plotters and the Helava 
analytical plotter are examples +. 
To obtain the accuracy of positioning required over the whole 
fields of the photographs, all existing plotting machines employ control- 
led mechanical motions of the scanning light sources and or of the photo- 
graphic plates relative to one another. It must be appreciated that at 
the present time such controlled relative mechanical motions are necessary 
only because there is no practical alternative that has the desired 
accuracy. In the future, it may be possible to do by electronic means 
not only local electronic scanning as is now done in Hobrough's ^ machine 
but also topographical tracking. What is required is a luminiscent spot 
that can be located with the desired accuracy over an area as large as 
a stereo photograph. With modern developments in luminescent screens 
and electron beams tubes, such an electronic positioning device is at 
least a possibility. However, existing and projected machines call for 
the precise positioning of mechanical masses. This paper will therefore 
discuss the limitations imposed on the automatic control of such motions 
by available positioning devices such as electric servomotors. Some 
basic difficulties in designing complete automatic plotting systems will 
then be considered. 
Performance Limitations Imposed by Servomotors: 
It is readily shown that the characteristics of any practical 
servomotor used to drive the moving parts of an automatic plotter place 
upper limits on its performance. 
* Professor of Electrical Engineering, University of Toronto, 
Canada. 
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