Determining the flow fields of two glacier outlets of the Jostedalsbreen ice cap,
Norway, using digital photogrammetry
B. Wangensteen, T. Eiken, K. Melvold, O. M. Tønsberg and J. O. Hagen
Department of Physical Geography, University of Oslo, P.O.Box 1042 Blindern, 0316 Oslo, Norway
Abstract
During the last decades most glaciers in the world have retreated. However, some
of the glaciers in the western part of Norway have advanced during the last 10
years. This is related to increased winter precipitation in the late 1980s and the
beginning of the 1990s. As a result the glacier fronts have grown steeper, more
active and thereby more dangerous. The Jostedalsbreen ice cap (468 km2) is one
of the most visited glaciated areas in Norway, where hundreds of tourists visit
every day during the summer season. It is also popular for glacier climbing, both
as guided tours and in individual groups. The steep advancing glacier fronts have
made this activity more difficult over the last years. Several ice blocks have also
broken off at the front some of the outlets. Although large icefalls from hanging
glaciers happen very rarely, the consequences of such events can be dramatic.
In order to survey the outlets of this relatively large glacier, several aerial
photographs were taken in August 2001. The same strips were flown and
photographed at Jostedalsbreen both August 19th and 29th 2001.
The temporal resolution of ten days is feasible for mapping the expected
displacements of a few meters. The imagery covers ten of the outlets on
Jostedalsbreen. In this presentation we show how the flow field and extent of one
hanging glacier, Bergsetbreen, and one valley glacier, Nigardsbreen, can be
determined from orthophotos from the different dates (Fig.1).
Method
An orthophoto of both Nigardsbreen and Bergsetbreen was generated for
each day with a digital photogrammetric workstation (Z/I Imaging). This
was done by automatically constructing a DTM with a spatial resolution
of 2 m and then orthorectifying the aerial images. The aerial photos have
a scale of about 1:23,000 and a 14μm scanning resolution gives a
spatial resolution of approximately 30 cm for the aerial photos. For these
calculations an orthophoto resolution of 50 cm has been used. The
orthophotos was co-registered before the velocity calculations were
done by using IMCORR.
The cross-correlation software IMCORR, developed at National Snow
and Ice Data Center (US), was used for the displacement calculations.
IMCORR use a fast Fourier-transform version of the normalized crosscovariance method (see Berenstein, In Manual of Remote Sensing, 1983) to
cross correlate sub scenes of the two orthophotos of the same glacier.
This is done by searching for the point of best correlation between a
small reference chip of the orthophoto of the first date within a larger
search chip in the same area in the orthophoto of the second date. The
point of the best correlation,
Figure 2. Upper: Displacement field at the upper part of Bergsetbreen
shown as vectors (a). Lower: Magnitude of the horizontal displacement
at Bergsetbreen (b).
Acknowledgement: This work is partly financed by the EU founded
project GLACIORISK (Contract ECV1-2000-00512).
N
B
Figure 1: Location of the Jostedalsbreen ice cap in the
western part of South Norway, and the two outlets
Nigardsbreen (N) and Bergsetbreen (B).
given that it is greater than a certain threshold, is taken to be the new
position of the reference chip in the orthophoto of the second date. The
difference between these two positions is caused by the horizontal
displacement of the moving glaciers and it is the conservation of the
crevasse pattern that makes it possible to use this technique.
Results
Figure 2a shows the velocity field of the steep upper part of the hanging
Bergsetbreen displayed as velocity vectors. Figure 2b and 3 show the
magnitude of the displacement at Bergsetbreen and Nigardsbreen
respectively. The calculated horizontal velocities on Nigardsbreen are
in good agreement with measurements done by static GPS method
(Fig. 3). The GPS measurements were carried out on August 22nd and
September 19th.
Some shadows on the lower part of Nigardsbreen in the original air
photos introduce difficulties for the matching algorithm. Since the
shadowing is different in the two scenes it is more difficult to perform
the cross correlation and hence the density of succsessfully matched
points decrease in this area. The same pattern is seen on the lower part
of Bergsetbreen, but here it is due to less matching features (crevasses).
Figure 3. Magnitude of the calculated horizontal displacement at
Nigardsbreen. The black dots show measured velocity by static GPS
between 22.08.01 and 19.09.01.