INSTALLATION OF THE SYSTEM 
INPUT 
  
  
Digitising of photogrammetric plotters in order that the X, Y, Z coordin-. 
ates of points may be recorded and processed digitally is very commonplace 
and has been so for many years. An optional extra for the Thompson-Watts 
Mark II plotter was an automatic digital recording system (Hilger and 
Watts, 1964) but this item had been out of production for over ten years 
when funds became available to digitise one of the Photogrammetric Unit's 
plotters. Thus the commonest option for digitising a plotting machine - 
the instrument manufacturers own 'package' - was not available. The next 
option was to have a complete customised system prepared and installed on 
the machine. However the amount of money available was very limited, and 
it was decided that considerable savings could be effected by buying the 
necessary items 'off the shelf' at the most competitive prices from 
individual suppliers. 
Fitting encoding devices to the plotter 
Rotary incremental encoders were selected, the model used being made by 
'Datatech', type RS23 giving 1000 cycles per turn. The main reason for the 
choice of rotary encoders was that the plotting machine was already fitted 
with brackets and suitably located take-off gears for the original Hilger 
and Watts system (Fig 2 and 3). A circular mounting plate and a 'Tufnol' 
gear giving a 5:1 ratio was fabricated for each of the three axes, and the 
installation was thus extremely straightforward. The possibility of 
installing linear encoders was considered, but with brackets already in 
place for rotary encoders, it would clearly have been more expensive. 
The lead screw pitch on all three axes is 2 mm, and with the 5:1 gearing 
and 1000 pulse per revolution encoder, thus gives one pulse per 10 micron 
movement in the model for each axis. 
Microcomputer 
A North Star Horizon microcomputer was chosen, with a Hazeltine 1420 VDU. 
This microcomputer utilises a Z80 chip, uses an S100 bus, and may be run 
with a CP/M operating system. Initially it had been intended to use one 
of the 'home' computers such as an 'Apple' or a Commodore 'Pet', and 
although the microcomputer would have been cheaper, the problems and 
cost of interfacing to the plotter would at the time have made the whole 
more expensive. A microcomputer with the combination of Z80, S100 bus 
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