INTRODUCTION
The Photogrammetric Unit of the Institute of Advanced Architectural
Studies is engaged in a programme of recording Ancient Monuments in
England in the care of the Department of the Environment. The principal
product of the Unit is 1:50 scale line drawings showing all architectural
detail and stonework of the facades of those buildings. An increasing
requirement has been to provide horizontal and vertical cross-sections
through parts of the facades. In some cases, what has amounted to a
complete and highly accurate plan of the building has been prepared.
This has been possible due to the system of theodolite control which is
used to establish a coordinate framework for the whole building (Fig 1).
This cross-section work has so far been carried out on one of the Unit's
Thompson-Watts plotters which is fitted with rather a complicated
mechanical interchange. In order to provide a more flexible system for
the plotting of horizontal and vertical cross-sections,and to eventually
lead to the possibility of complete digital manipulation of data, it was
decided that a 'digital' approach to the modification of the second
Thompson-Watts plotter would be made. Obtaining and storing the
coordinates of points in numerical form is of course very commonplace, and
relatively straightforward. However, the coordinates in themselves would
be of limited value, since the final product required is a graphical
output. Purchasing an automatic drafting table would clearly be a very
expensive solution, so attention was given to the possibility of electri-
cally driving the existing coordinatograph table of the Thompson-Watts
plotter,.and it was this solution which was finally implemented. For the
graphical plotting of the facade drawings, the table is connected and used
normally. For the drawing of the digitally derived cross-sections, the
table is disconnected from the plotter, and driven electrically through
stepper motors.
Concerning the approach to digitising the plotter, the various methods
have been thoroughly reviewed by Petrie (1980). The method adopted is
Petrie's 'software' approach, ie linking the plotter to a microcomputer,
into which various software options can be fed, both to control the basic
data acquisition, and also to provide other facilities such as orientation
procedures. The microcomputer chosen was a North Star 'Horizon' with 64K
store and twin floppy 5i" disks.
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