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In section 4.2 some of the benefits of SAR. stereopsis
are discussed.
y, SELECTED EXAMPLES OF GEOSCIENTIFIC APPLICATIONS
4,1 Systematic Investigations on Thematic Image Contents
Variations of flight and imaging parameters have an
essential impact on geomorphological terrain feature
appearance and on the percentage of null-information in the
image. It is mainly the antenna elevation angle, defined as
the angle between a vector from the sensor position to the
nadir and the connection line between sensor and terrain
target, and the illumination direction, which influence
these two parameters.
In order to investigate the influence of varying
antenna elevation angles, simulated data may be chosen,
assuming the other system and target parameters to be
constant. Comparisons between the layover, foreshortening
and shadow statistics in slant range and in geometrically
rectified images can then give indications of the usefulness
of different elevation angles for non-corrected and geocoded
imagery. Obviously, the loss of information by radar shadow
("no-shows"), and by layover and foreshortening, where the
terrain structure can still be viewed, should be differen-
tiated. Images with large elevation angles, which display
much shadow, on the other hand even enhance slight geo-
morphologic structures (cf. Bodechtel 1983), which is
particularly: helpful "dn flat terrain. A conclusion drawn
from a study using slant range presentation is that for snow
and ice mapping in hilly and mountainous terrain optimum
antenna elevation angles in the range of 40 to 60 degrees
should be adequate, depending on the percentage of shadow
and, hence, on the relief. For areas with moderate relief
40 degrees may be useful, in mountain areas angles around 50
degrees are preferable (Rott et al. 1985).
The second geometric parameter, which strongly
influences the geomorphological evaluation of radar images,
is the flight/orbit direction relative to the ground.
In SAR images linear geomorphological structures
oriented perpendicular to the illumination direction, i.e.
parallel to the flight path, are strongly enhanced, whereas
they hardly can be viewed when they strike parallel to this
direction. Therefore, at least two flights with ‘clearly
intersecting directions (see below) are needed to provide
sufficient data for the extraction of all major linear topo-
graphic features (lineaments; Jaskolla et al. 1985).
To find out the biases of multi-directional illumina-
tions, a study in a test area comprising coastal lowlands,
hilly terrain and mountain ranges has been carried out.
Under the assumption of ERS-1 imaging parameters (orbit
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