rier, and
Fig. 6).
ht
ction
and
imposed
e, which
oriented)
At steep
intersect
layover
ce floes.
\ttachm.)
of small
| and ice
iding the
, can be
al layer-
cause a
ation are
dditional
can be
dvantage
yrmation,
ombined
e content
such an
accuracy
ation. It
between
nd those
animetric
measurements in the radar composite showed that significant
positional differences 0X, 0 Y are especially perceptible in high
objects such as ridges, peaks, nunataks and tops of ice caps
represented with low contrast or affected by shadows in optical
imagery. For example, a system of ice divides within the
accumulation zones of the largest ice caps and domes is often
reproduced with better contrast in radar images than in
stereophotographs (Fig. 7, Sharov 1997, a).
A A A A. A
10 20 30. 40 50
Location
Fig. 7. Representation of an ice-divides system in a radar image
(a) and transverse radiometric profile AB (b; location unit: 40m)
Therefore, all inaccuracies of stereoplotting in homogeneous
snow-covered areas at glacier tops become visible in radar
composite and, thus, can be iteratively compensated by
introducing appropriate corrections into the results of
cartographic vectorization. This approach provides an effective
solution for detecting and measuring the highest positions and
saddle points at ice domes. Moreover, the quality of ground
control may be decreased to some extent, which is very
advantageous in the severe environment of the High Arctic.
3.3. Interferometric analysis of complex SAR images
Satellite radar interferometry (INSAR) is a novel and powerful
tool for precise topographic modelling of glacial terrain and
monitoring of rapid environmental changes in the High Arctic
(Lefauconnier et al. 1993). The INSAR method is based on
generation of an interferogram by combining two complex radar
images, which contain the information on amplitude and phase
of radio signals reflected from the Earth's surface. Each fringe in
the resultant interferogram corresponds to a certain phase
difference between radio signals, which in its turn depends on
the elevation of terrain. The vertical accuracy of spaceborne
interferometric determinations reaches up the sub-meter range if
the orbital parameters and meteorological conditions are reliably
known. The impact of atmospheric effects and variations in
backscatter brought about by rapid physical changes of the
glacier surface may, however, limit to some extent the
applicability of INSAR in the High Arctic.
H \ b oue WO. Y
Datum
plane
p
Fig. 8. Geometric disposition of INSAR survey
The geometric disposition of a typical INSAR survey is shown
in Figure 8. Radar images must be taken from two neighbouring
orbital points S1 and S2, the length of a baseline preferably
living in the range of 50 to several hundreds of meters. The real
difference in altitude positions of satellites S1 and S2 may be
taken into account by introducing a virtual baseline B, as offered
in (Hartl and Thiel, 1993)
B, = B, * B, tan. (9)
where B, and B, are horizontal and vertical components of a
baseline, respectively. The value of the local incidence angle 0
is defined as follows
0 z 0.5- (0, 4 0,). (10)
The glacial topography of numerous ice caps and domes in FJL
is mostly homogeneous and the height monotonously increases
from the sea level to the glacier top. In this case, the elevation of
an ice cap with respect to the current sea level can be simply
determined by using a deterministic approach as follows
Ahzk-.e, (11)
where k is the fractional number of interferometric fringes
enclosed between two points of the interferogram corresponding
to the glacier top and the shore line; e is the height interval
corresponding to one interferometric fringe. The highest position
on the glacier is recognized by detecting extreme pixel values in
the interferogram in the vicinity of the presumed summit. The
height interval e depends on the terrestrial slope € and may be
derived from the following formula
International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 207
