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Proceedings of the Symposium on Progress in Data Processing and Analysis

with Stuttgart University, developed a system on
the Planicomp C 100 analytical plotter (Giilch,
1984; Pertl, 1984), whilst Kern of Switzerland
introduced the Kern Correlator, based on the
DSR11 (Bethel, 1986).
The third major European manufacturer, Wild
Heerbrugg of Switzerland, did not attempt similar
efforts using any of their range of analytical
instruments. However, outside of the commercial
field, some work has been done at ETH Zürich on a
modified AC1, also mainly with a view to
automatic DTM generation (Baltsavias, 1988). The
experience gained with this instrument provided a
basis for much of the work done on the S9AP.
2.2 Aspects of the S9AP Construction
The S9AP is an integral part of System-9, the
geographic information system (GIS) launched by
Wild Heerbrugg in 1985. It is well documented in
the literature (eg Schneeberger and Bürgermeister,
1987), but some aspects of the hardware
construction must be outlined as they have
implications on the work of the project.
The viewing system of the instrument is of
particular interest. The design of the S9AP was
based on the existing BC range of instruments,
rather than the AC1 which was known to be of
slightly higher accuracy. The essential difference
between the two is the means of image
measurement: in the AC1, the entire optical train
remains fixed whilst the photo carriage moves in
two directions; in the BC range and in the S9AP,
the carriage moves only in the Y direction, whilst
measurement in the X direction is effected by
translation within the optical train. The major
advantage of this is a reduction in the size of the
instrument, at the cost of a small reduction in
accuracy in the case of measurements by manual
operator. The consequences for digital
measurements are somewhat more significant,
however, as will later be described.
Plate positioning is done via a feedback loop using
servo motors and linear encoders of 1 pm
resolution. Instrument calibration relates the linear
encoder scales to a stage coordinate system defined
by a grid of crosses engraved on each photo
carriage, the relative positions of which are
determined to high accuracy in the factory before
installation. The quality of this calibration can be
affected by the optical design as described above.
2.3 The S9AP with CCD Cameras
The S9AP at ETH Zürich has a modification
allowing the attachment of most types of CCD
camera. The optical and structural design and
implementation of this modification was carried
out by Wild engineers in Heerbrugg, with the
consequence that the instrument in its manual use
loses nothing in terms of optical quality. The
important features of this realisation are:
• The cameras are mounted externally at the rear
of the instrument via a C-mount fitting. Any
camera using such a standard thread can be
simply “screwed on”.
• The magnification is approximately unity. It can
be changed by a translation of the objective lens,
but only within a small range. The lens is,
however, not easily accessible.
• The distance of the camera from the stage plate
is rather long, having the disadvantage that
systematic effects arising from the opto
mechanical design are somewhat magnified.
On the other hand, the long focal length of the
objective lens (~ 30 cm) means that any effects
of distortion are minimised.
Figure 1 shows the relevant part of the S9AP optics
and the positioning of the camera (one side only).
Figure 1. Part of the S9AP optical train showing
positioning of the CCD camera mount.
The camera used for the investigations was the
Sony XC-77CE, an inter-line transfer device with
pixel size 13.6 x 11.1 pm on the stage, giving an
image size of 8.0 x 6.4 mm.
The image acquisition hardware used comes from
the Maxvideo family of products manufactured by
Datacube Inc in the USA. The boards operate on
the VMEbus, thereby allowing all system
components - host computer, S9AP and image
acquisition hardware - to be controlled as a single
system from one workstation.
3. System Calibration
3.1 Purpose of Calibration
The primary purpose of calibration in the context
of an analytical plotter fitted with CCD cameras is
to derive a relationship between the acquired
digital images and the analogue imagery being
digitised. Once this relationship is determined, it is
also necessary to know about its accuracy and
stability. Both are influenced by many factors, and
hence the whole procedure encompasses many