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5.4 Terrain features modelling
The terrain features specified for a GI base should be modelled in
3D space, though 3D representation is not required for all
applications. Hence, basically the stereo methods of restitution
should be used; the mono methods can be regarded as a supplement.
Both methods can be implemented in analytical as veil as digital
systems. The procedures for preparation, manual restitution and
post-processing are similar, and both system types can have the
graph-image overlay capability for single images or stereopairs.
Analytical systems can be equipped with high-quality observation
optics, with a resolution greater than 150 lp/mm, and a field of
view up to 40 mm diameter. High-quality optics and a large field of
view allow effective manual image interpretation. Interpretation
requires a simultaneous perception and analysis of the specified
terrain features and their context. Both the features and their
context have their specific structures of the primitives and their
composites, which have to be analyzed and understood for correct
interpretation.
The geometric accuracy of analytical stereoplotters is usually very
high (s=l-10 ym). The positioning and measuring devices have to be
periodically tested and calibrated to update the parameter values of
the corresponding correction models. The calibration process can be
semi- or fully automatic.
Digi tal systems have the inherent potential to carry out processes
also in the semantic domain, i.e., preprocessing, analysis and
understanding of images, and selective feature extraction. There is
an on-going transition towards more intelligent systems but
automatic image interpretation is today still in the stage of
infancy.
At present, the major limitations are low quality of digital images,
caused by the relatively large pixel sizes (10-50 ym), and small
display windows (512 x 512 - 1024 x 1024 pixels). To preserve the
information content of a high quality image (e.g. 150 lp/mm), a
pixel size of 2 ym is needed. The image window required for display,
which would correspond to the field of view of 40 mm diameter in an
analytical stereoplotter, is represented by a matrix of about 18000
x 18000 pixels. This is, however, beyond the state of the art.
For the commonly used photogrammetric images and an average
stereoplotter, the requirements are less stringent. In this case (50
lp/mm; 30 mm diameter), a pixel size of 7 ym and thus a display
matrix of about 3800 x 3800 pixels suffice. This can be attained at
present, though the price is high.
The corresponding intensity value (8 bit) or the colour (24 bit) is
assigned to each pixel; this results in a huge amount of image
information to be stored, retrieved, transferred, processed and
displayed.
