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. 
6. CONCLUSIONS 
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 
production. 
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