Jökull - 01.01.2004, Blaðsíða 17

Reviewed research article
Glaciological application of InSAR topography data of western
Vatnajökull acquired in 1998
Eyjólfur Magnússon1, Helgi Björnsson1, Finnur Pálsson1 and Jørgen Dall2
1Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, 101 Reykjavik, Iceland
2 Ørsted-DTU, Electromagnetic Systems, Technical University of Denmark, Building 348, DK-2800 Kgs. Lyngby
email: eyjolfm@raunvis.hi.is
Abstract – A new digital elevation model (DEM) of western Vatnajökull has been derived from interferomet-
ric SAR (InSAR) data acquired in August 1998 with the airborne EMISAR system. Glaciological applications
of the new DEM included definition of ice divides of glacier outlets and water divides of rivers draining W-
Vatnajökull as well as predicting main subglacial watercourses. Measurements of river runoff were used to
adjust ambiguous predictions of water divides. We note significant changes in the location of the water divides,
since the 1980s, caused by recent surges and the eruption in Gjálp in 1996. Moreover, the accumulation area
percentage of the total area for the recently surged outlets has been close to 5% lower in the years after the
surges than it would have been if they had not occurred. Finally, the DEM reveals a previously unrecognised
depression in the snout of Köldukvíslarjökull, most likely generated by geothermal activity.
INTRODUCTION
Since the beginning of cartographical surveying in
Iceland, glacier maps have been the ones of lowest
quality. Maps prior to the 20th century generally in-
dicated glacier outlines only roughly. The first two
comprehensive maps of Icelandic glaciers were com-
pleted by the Danish General Staff surveys in 1902–
1939 and the Army Map Service in the late 1940s and
early 1950s. The glacier outlines on these maps are
quite accurate but the elevation is only accurate for
some glacier areas. The white glacier surface on the
aerial or oblique photographs, which these maps were
based on, made stereocompilation difficult or impos-
sible. Fairly accurate maps based on photogramme-
try have been made of Icelandic glaciers in the recent
decades, but they still contain inaccurate parts within
accumulation zones of the glaciers.
Other methods have been used successfully to
map the surface of Icelandic glaciers. Since 1980
the Science Institute, University of Iceland, has map-
ped large areas of Vatnajökull, the entire Hofsjökull,
Langjökull and most of Mýrdalsjökull (e.g. Björnsson
and Pálsson, 1991). During these surveys, the surface
elevation was measured along profiles or fixed points
on the glacier surface initially using barometric preci-
sion altimeter and later differential GPS. The advan-
tage of these survey techniques is that their accuracy
is independent of the surface characteristics as long as
the site is accessible. The disadvantage is, however,
the discontinuity of the point measurements and con-
siderable distance between the survey profiles. Some
surface features are therefore poorly mapped or miss-
ing.
Glacier surface maps provide fundamental infor-
mation for various glaciological investigations, such
as mass balance studies, glacier dynamics and glacier
hydrology. A digital elevation model (DEM) derived
from data collected during Science Institute surveys
and recent photogrammetric maps have been used pre-
viously for definition of ice divides for major outlet
glaciers and water divides of the glacial rivers drain-
ing them (Björnsson, 1988).
JÖKULL No. 54 17