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Proceedings of the Symposium on Progress in Data Processing and Analysis

For manual image interpretation and extraction of the terrain
features, a digital system can hardly be justified. The gain is very
modest, whereas the loss in image quality and thus the reduced
information content can be great.
The measuring resolution and thus geometric accuracy is restricted
by the pixel size (10-50 pm). After digitizing images, there are no
further mechanical and optical sources of the geometric errors;
hence, no geometric calibrations are necessary. The AD image
conversion itself, however, is a part of the overall process; the
comparator-like scanner/ digitizer should be calibrated
periodically, as is an analytical stereoplotter.
Fair conclusions can be drawn only from an objective evaluation. The
evaluation criteria address the versatility, flexibility, degree of
automation, performance, operational ease, ease of integration,
support and cost. In the following, the main differences between the
analytical and digital techniques and systems are summarized.
For GI applications, the differences in practice in versatility and
flexibility of the analytical and digital systems are minor, though
digital systems have some potential advantages.
One of the major advantages of digital systems is their suitability
for automation of processes in the semantic domain. Great freedom in
modelling the processes potentially permits sophisticated
preprocessing, matching, analysis and understanding of images.
In the semantic domain, the performances of the analytical and
digital systems differ considerably. The reduced quality of digital
images impairs the economics of photogrammetrie GI surveys.
Geometric positioning is more accurate in analytical plotters
because the measuring resolution is higher. Exceptions might be
aerial triangulation, and in very favourable circumstances also
terrain relief modelling, i.e., when image matching is by the least
squares fit.
A major difference between the two system types seems to be in the
required storage capacity for digital images, and the corresponding
access and processing time. Digital systems, however, can be highly
automated and are therefore potentially more time-efficient in
The operational ease differs to some extent. Operation of a digital
system includes also management, processing and display of digital
images. The graph-image overlay is simpler than in analytical
systems. The main advantage of the analytical systems is high image
quality and a large field of view for observation.
Integration of a digital system into a given production environment
is at present more demanding than integration of an analytical
system. The major problems concern the compatibility of the