Semi-controlled interferometric mosaic of the largest european glacier
Publication from Digital
Sharov Aleksey I., Dimitry B. Nikolskiy
ESA ENVISAT Symposium in Montreux, Switzerland, April 2006 , 2006
North Island (48,100 km2, 1952) in the Novaya Zemlya archipelago is
the second largest island (after Iceland) in the Eurasian Arctic,
and the Main Ice Sheet occupying 47.5 % of its surface is reputed
to be the largest mass of glacier ice in the "Old World". The last
extensive aerial and geodetic surveys in Novaya Zemlya were carried
out in 1952 and available topographic maps of the North Island are
obsolete and unreliable. There are very few instrumental records
documenting the rate of glacier ice flow in Novaya Zemlya, and present
mass-balance estimates for the Main Ice Sheet are very approximate.
This paper presents the main outcomes of the INTEGRAL (EC FP 6) and
SIGMA (ESA AO No. 2611) research projects related to the enhanced
geocoding, interpreting and mosaicking of spaceborne ERS-1/2 and
JERS-1 SAR interferograms of north Novaya Zemlya. A 50 m posting
DEM of the study region was generated from available 1:200 000 topographic
maps and upgraded using the ICESat-GLAS altimetry data obtained in
2003 – 2005. Six ERS-1/2 and one JERS-1 SAR overlapping interferometric
models, all obtained from different descending orbits, were orthorectified
and assembled into a semi-controlled seamless mosaic. Interferometric
phase distortions at specific target points characterized by the
locally highest coherence values were reliably estimated, and actual
topographic heights of main ice divides and ice coasts were precisely
measured and controlled from the ICESat transects and the phase information
in between. Significant height errors ranging from -225 m to +313
m were detected in old 1:200 000 topographic maps at specific locations
on the glacier surface and corrected in the mosaic. The resultant
mosaic covers a land area of approx. 18,000 km² (45,000 km² in total)
and contains 8 information layers: intensity orthoimage, coherence
orthoimage, fringe orthoimage, phase-gradient orthoimage, composite
orthoimage, 2-pass differential interferogram (DEM 1952), upgraded
differential interferogram, and cross-differential ERS-JERS interferogram.
These layers provided the basis for the interferometric map series
showing structural morphology, ice-flow pattern and mass-balance
characteristics of the Main Ice Sheet at 1:500 000 scale in the UTM
projection, Zone 41N, WGS84. The map series is presented in the form
of a 5-min animation.
Practical mapping work revealed essential changes in glacier elevations
and termini positions. It was determined that the main ice sheet
lost at least 200 cub.km of ice in the period between 1952 and 2003,
i.e. nearly 4 cub.km/a. The comparison with previous estimates made
by other researchers showed that land-ice-loss processes in north
Novaya Zemlya have accelerated by approx. 10 %. Frontal velocities
of tidewater outlets and corresponding values of the ice flux at
seaward glacier margins were estimated by analyzing the horizontal
shift of the coastal sea ice forced by glacial flow. Quantitative
integral estimations of ice flux were provided for 5 glacial provinces
of the Main Ice Sheet. Spatial asymmetry in present glacier regime
was deduced and explained.
The analysis of the cross-differential interferogram provided no evidence
for the geometric impact of radar penetration into dry snow on the
ice sheet. Our interferometric determinations were validated during
the field campaigns undertaken in September 2001 and 2003. It was
concluded that the combination of satellite interferometry and altimetry
offers a particularly potent solution for the topographic modeling
of large glacier complexes in the case of insufficient ground control.