The I?S system at UCL has 12 512 x 512 pixel 8 bit refresh 
memories. Each of these memory planes also has zoom, scroll, and 
split screen registers. These determine which pixel is to be sent to 
the display. For instance, if each pixel is read twice, and each line 
twice, this has the effect of magnifying the image (or zooming) by 2 
times. If the 100th pixel of the 100th line is the first pixel read in a 
cycle, the image is effectively shifted up and to the left (scroll). The 
split screen register allows the display to be segmented and different 
refresh memories displayed simultaneously. Each of these facilities 
has been utilised in the design of the digital system described here. 
The I?S System 575 software is basically a series of 
computer programs run on the VAX host computer and called from 
the Command Interpreter. A subroutine library is provided, so it is 
a relatively simple procedure to program the system to perform new 
functions. 
Design Implementation 
The digital stereoplotting system described here has been 
developed in parallel with a system on the Kern DSR1 analytical 
plotter at UCL using photographic images of simulated SPOT data. 
The system has three componenets. Firstly a digital stereo 
comparator program is used to record sub-pixel image coordinates of 
control points. The exterior orientations of the images are then 
calculated using ground control point coordinates, and conventional 
methods. The method of orientation of SPOT imagery is described by 
Dowman and Gugan (1985). Following which a stereo model can be 
formed on the I?S display monitor. 
This paper will describe the digital stereoplotting instrument 
design which is analogous to the hardware features of an analytical 
plotter, rather than the orientation software. 
Real-Time Model Formation 
A stereo model is formed in an analytical plotter by using 
the collinearity equations to find xy photo coordinates relating to a 
defined XYZ ground point. Hand/foot wheel encoder readings (XYZ) 
are transformed to photo coordinates by inner, relative and absolute 
orientation parameters determined during the orientation phase. The 
centres of the binocular trains are then mechanically positioned at 
these xy photo points. With digital imagery, the floating mark is 
digitally incorporated into the imagery, removing the necessity of 
high precision optical/mechanical componenets (and also conforming 
perfectly to Abbe's Comparator Principle. 
To maintain the stereo model, the optical system (floating 
mark) or the images must move relative to the defined ground point. 
This is achieved in the I?S system by scrolling the images and 
keeping the floating mark position fixed. One advantage of this 
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