Mapping Glacier Velocities at Spitsbergen Using ERS Tandem SAR Data
Bjørn Wangensteen*, Dan Johan Weydahl**, Jon Ove Hagen*
*Department of Physical Geography, University of Oslo, PO Box 1042 Blindern, 0316 Oslo, Norway,
**Norwegian Defence Research Establishment, Division of Electronics, PO Box 25, 2007 Kjeller, Norway,
E-mail: bjorn.wangensteen@geografi.uio.no, Dan-Johan.Weydahl@ffi.no, j.o.hagen@geografi.uio.no
Abstract: The results presented here show how glacier
velocities can be measured and calculated from ERS tandem
INSAR data. A semi-automatic algorithm using existing and
new implemented moduls in the GIS package Arc/INFO has
been developed for the calculation of glacier velocity. The
velocities are decomposed into the flow direction of the
glacier using an external DEM. Velocity fields for the glaciers
Isachsenfonna, Akademikerbreen and Nordbreen at
Spitsbergen have been calculated.
INTRODUCTION
Surface velocities were firts calculated from intreferometric
SAR data in 1993 [1]. When ERS-2 was launched in 1995 it
was possible to get tandem scenes from ERS-1 and ERS-2
taken only one day appart. The scenes used in this study are
from the time period of the tandem mode in 1995 and 1996.
The short time interval between the acquisition of the scenes
increases the possibility for detection of glacier movement
due to less influence of factors like snowdrift and melting.
The method presented here was developed during the
work with the first authors cand.scient thesis (equivalent to
masters degree) in remote sensing. In the study several hight
differentiated INSAR (DINSAR) scenes from the area
between Kongsfjorden and Wodfjorden at Spitsbergen were
used. Spitsbergen is the largest island of the artic archipelago
Svalbard. Svalbard is situated in the North Atlantic between
74° and 81°N and 10° and 35°E. Svalbard is manly covered
with ice caps and tundra vegetation. The INSAR scenes were
processed at the Norwegian Defense Research Establishment
(FFI) with SAR data from an ESA AO Tandem project [2].
The topographic influence in the INSAR scenes was removed
during the processing at FFI using a DEM delivered by the
Norwgian Polar Institute. These DEMs were also used in the
decomposing of the glacier velocities.
METHOD
The method for calculating glacier velocities was
developed using images of Isachsenfonna and parts of
Holtedahlsfonna and Kronebreen (79°N, 13°E). These
glaciers are situated close to Kongsfjorden and Ny Ålesund at
the northwestern part of Spitsbergen. Since the topographical
effect are removed, the DINSAR scenes only contain
information about the movement between the acquisition of
the two scenes. The distance between the satellites and a
target could be described as a number of wavelengths and a
phase. The DINSAR scenes show the phase difference
between the two acquisitions. Since one only knows the phase
difference, the number of wavelengths constitutes an
ambiguity. One of the main problems with velocity
calculations is therefore to transform the discrete fashion of
the fringe pattern in the DINSAR scenes to continious values.
This is called unwrapping. The DINSAR scene of
Akademikerbreen is shown in figure 2. The method that has
been developed is described in figure 1.
All of the steps have been done in the GIS package
ARC/INFO. The phase difference is measured in intervals
from 0 to 2 PI. This is represented by graylevels from 0 to
255 in the images. The phase difference increases from 0 to 2
PI, before one gets a discrete transition to 0 again. The
discrete fringe transissions must be identified before the
INSAR image can be unwrapped to continous values and
eventually to glacier velocities.
The median filter was used twice. This removes a lot of
the speckle noise that is characteristic for SAR and INSARimages without any substansial smoothing of the phase
transitions.The gradient filter is applied after the median
filtering. It is a derivating filter that uses the Prewitt-operator
to calculate the gradient in the directions of azimuth and
range. The total gradient is then calculated. A value is added
to increase the contrast in the image. There is still some noise
in the image after the gradient filtering and it is therefore
thresholded. The user can define the threshold after looking
at the histogram of the images. All pixels with values less
than the threshold is set to 0, and the others are kept as they
are. A threshold around 150 has shown to be suitable. The
selection of a threshold is a choice between the amount of
noise reduction and the preservation of the edges between the
fringes.
Figure 1: Flow scheme of the method for calculation of
glacier velocities.
Since the Norwgian Polar Institute had terrain models
available, the velocities were decomposed in the flow
direction of the glacier. Surface parallel flow was assumed.
The slope is calculated together with the aspect or slope
direction. This is done automatically with a gradient filter.
Before the terrain parameters could be calculated, substantial
smoothing of the terrain model had to be done. In ground
scale the size of the moving mean filters were several
kilometers. The look angle was also calculated. It varies
between 20 and 26 degrees from top to bottom in the image.
When these three parameters have been calculated,
together with the glacier velocity towards the satellite, it is
possible to calculate the velocity in the flow direction of the
glacier (pers. com, Knut Eldhuset, FFI):
Vglac = Uw/(cos(s)cos(a)sin(i)+cos(i)sin(s))
Uw are the values in the unwrapped image, s is the slope
of the glacier, a is the angle between the aspect of the slope
and the range direction of the satellite and i is the look angle.
Alle the parameters are calculated for every pixel at the
glacier surface resulting in a complete velocity field for the
whole glacier.
Figure 2: The DINSAR scene of the glacier Akademikerbreen
acquiered 20. and 21. of October 1995.
The thresholded image is then vectorized with an excisting
alogrithm in ARC/INFO. Points that are closer than a certain
distance are connected with vectors. The resulting data have
to be edited to some extent. This is done partially automatic
and partially manuel. The method used in the unwrapping is
quite simular to the edge segment linking described in [3].
It is important for the calculations to chose the right
borders for the glacier. The border of the glacier is digitized
manually after the median filtering to limit the amount of
calculations that has to be done. It is best to use one of the
amplitude images, the DEM and the DINSAR scene are more
difficult to apply.
When the lines are edited, one can create the topology for
the polygons that mark the transitions between the fringes.
The polygons are numbered after their relative position. The
outermost polygon gets the value 0, the next, 1, and so on.
This information is used in the unwrapping of the INSAR
image. This could ideally have been done automatic, but since
some interpretation is needed, it is done manually. During the
unwrapping of the DINSAR scene, half a wavelength (2.83
cm) is multiplied with the value of the polygons and added to
the values in the DINSAR scene.
If the results from the unwrapping is divided by the sine of
the look angle, i, (Vsat = UW/ sin(i)) one gets the component of
the glacier movement horisontally towards the satellite. This
is often referred to as the line in sight component. For a
glaciologist a velocity calculated towards a satellite is of
limited use.
RESULTS
The described method has been used to calculate the
surface velocities for three glaciers at Spitsbergen. Some data
from the results is presented i table 1. Figure 3 shows the
calculated velocity field of Akademikerbreen.
Isachsenfonna is situated east of Kongsfjorden.
Iscachsenfonna is an ice cap with several outlets. The
velocity is calculated for one of them. The maximum
velocity is 42.3 cm/day from the images acquired in January
96. The average velocity is 22.5 cm/day. For the images
acquired in April 96 the maximum velocity is 42.0 cm/day
and the average velocity is 18.2 cm/day. In the transition
between Holtedahlsfonna and Iscahsenfonna the calculated
velocities are in agreement with GPS measurements done by
the Norwegian Polar Institute. These shows movement of 10
cm/day.
Akademikerbreen is situated on Olav V Land on western
Spistbergen. The maximum velocity is 40.9 cm/day and the
average velocity is 7.25 cm/day. There is a significant
Table 1: Data from the velocity calculations .
Glacier
Location Date
Isachsenfonna
79°N13°E 10/11.10.96
Isachsenfonna
79°N13°E 05/06.04.96
Akademikerbreen 79°N19°E 20/21.10.95
Nordbreen
80°N16°E 27/28.09.95
Nordbreen
80°N16°E 05/06.04.96
Nordbreen
80°N16°E 10/11.05.96
Vmax
42,3
42,0
40,9
36,3
35,8
32,5
Vmean
22.5
18.2
7.25
16.7
12.5
10.2
CONCLUSIONS
The results from this work show how glacier velocities at
Spitsbergen can be calculated with terrain models and height
differentiated tandem INSAR data. A semi-automatic
algorithm is developed in order to calculate glacier velocities
in the flow direction , resulting in maps of the velocity fields
for the glaciers. Velocity fields for the three glaciers
Isachsenfonna, Akademikerbreen and Nordbreen have been
calculated.
ACKNOWLEDGEMENT
The SAR raw data are part of an ESA AO Tandem project
carried out at FFI. The final part of this work was in part
funded by the EC Environmental Climate Research Program
through the project "The Response of Artic Ice Masses to
Climate Change - ICEMASS", contract no. ENV4-CT970490.
REFERENCES
[1] Goldstein, R.M., Engelhardt, H., Kamb, B. and Frolich,
R.M. "Satellite Radar Interferometry for Monitoring Ice
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Figure 3: The velocity field of Akademikerbreen obtained
from a INSAR scene from September 1995. The arrows
indicate the orientaion of the map and the direction of glacier
flow.
increas in the velocity in the narrow passage between two
nunataks. In this passage the surface slope is also steeper than
for the rest of the glacier.
Nordbreen is a smaller glacier in the nortwestern part of
Spitsbergen, by Wijdeforden. This is an outlet of the ice cap
Åsgårsfonna. The maximum velocities varies between 32.5
and 36.3 cm/day and the average velocities varies between
10.2 and 16.7 cm/day. The maximum velocities are found in
the areas of the greatest slope.
The maps show velocity patterns like one would expect
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found along the edges of the glacier, since the edges excerts a
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