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4 . Automatic Height Measurements
Automatic image correlation and automatic height measurements are other
important elements in the new conception of photogrammetric image restitution.
In these fields, developments are still going on. The first operational system for
automatic height measurements and production of orthophotos was developed by
Hobrough in the seventies. Some image correlators had already appeared on the
market earlier, but their possible applications were very limited. Most of these
early processors were based on analog procedures which did not provide suffi
cient flexibility.
More favourable conditions are offered by the various digital procedures and most
of the research in photogrammetry is currently concentrated in this field.
Apparently, operational procedures might already be available for industrial
applications (cf. [2]). Interesting procedures are also available for small scale
plotting and automatic height measurements from satellite images (cf. [3]).
Large scale mapping, however, entails much more difficult conditions.
In recent years, the Institute of Photogrammetry of the EPF-Lausanne, in collab
oration with Kern & Co., has worked intensively on this problem and all tests
indicate that the procedure being developed will satisfy the requirements in prac
tice. The most important difficulties for the automatic derivation of a digital ter
rain model are the development of robust algorithms for image matching, the
assurance that the measurements are effectively carried out on the natural ter
rain and the development of procedures allowing for thorough control of the
results with appropriate possibilities for editing.
The image correlation elaborated at the Institute of Photogrammetry uses Multi-
Templet Matching, an algorithm developed from dynamic programming. This
process is based on the computation of height parallaxes of image segments of
varying size. The parallaxes are introduced into an adjustment algorithm which
approximates the terrain with the help of finite elements. According to this ter
rain approximation, the aerial photographs are resampled and the height corre
lation is iteratively refined (cf. fig. 3). In order to accelerate the computations, all
image operations are done on an array of Transputers. In a subsequent phase,
the terrain obstacles are eliminated by a special filtering process and the data are
finally controlled by image injection in an analytical plotter. Various tests have
shown that a precision of the order of 0.1 %o of the flying height can be obtained,
even with photographs of rather poor contrast and a terrain cover by houses or
trees of up to 30% can be automatically eliminated. With the current technology,
about 12 hours are still necessary for a model, but the new generation of
Transputers should considerably reduce the processing time. This procedure
would allow one to significantly reduce the cost of the establishment of digital ter
rain models and that of the the production of orthophotos, which could then serve
as a base for other tasks in photogrammetric production.
5 . Conception of Future Mapping Systems
These various new elements for image measurements and image analysis enable
us to foresee new and more versatile systems for photogrammetric production.
The most spectacular development is the transition from analog to digital ortho
photos. Along the same lines, it can be expected that purely digital plotters be
introduced into the practice, but a much more interesting development will be the