Jökull - 01.12.1994, Side 33

1959-1991 (Sigmundsson and Einarsson, 1992).
The present paper describes an experiment per-
formed in 1991 and 1992, designed to detect isostatic
uplift around Vatnajökull. Accurate positions were
determined for 10 geodetic points around the glacier
using GPS geodesy. Gravity measurements were also
made on the same points. We plan to remeasure this
network 5-10 years after these initial measurements.
MODEL OF GLACIAL ISOSTASY
The volume reduction of Vatnajökull has been es-
timated by Helgi Bjömsson (pers. comm. 1991) by
comparing a map of the glacier rnade in 1904 - 1938
to a Landsat image from 1978. Assuming that the ice
cap takes the shape of a perfectly plastic material, the
ice thickness has a function of distance r from the
center is a parabola (Paterson, 1981):
h = 4.0 (R-r)l/2
where R is the radius of the ice cap and dimensions
are in meters. The thickness curve of the glacier is
calibrated such that the current mean radius of 51 400
m gives a current maximum thickness of 910 m. The
average change in the radius is estimated to be 920 m
and the resulting thickness change as a function of
distance is shown in Fig. 2. The change in thickness is
about 8 m near the center and about 90 m near the pe-
riphery. We stress that these are estimated average
values. The length of valley glaciers changed consid-
erably more than this, and thickness changes are also
larger at their snouts. To simplify the model calcula-
tions the thickness change curve is approximated by a
step curve. The effect of removing the ice load, i.e. a
disc load and two annular ring loads, is calculated fol-
Table 1. Times of occupation of the GPS points
1991 Day of Session Common time Hours Höfn Stap Hoff Gild Flat Hein Djúp Upps Hrol Hval
July the year
6 187 0 14:19-19:00 5:19 X X
7 188 0 01:52-09:27 7:35 X X
8 189 0 00:50-08:35 7:45 X X
9 190 0 02:15-10:00 7:45 X X
10 191 0 01:50-09:00 7:10 X X
11 192 0 02:43-10:27 7:44 X X
12 193 0 01:30-10:18 8:48 X X
12 193 1 15:24-19:29 4:05 X X
13 194 0 02:09-09:56 7:47 X X X
13 194 1 14:36-19:31 4:55 X X X
14 195 0 01:03-09:14 8:11 X X X
14 195 1 15:08-19:35 4:27 X X X
15 196 0 00:40-11:33 10:53 X X
15 196 1 15:15-20:15 5:00 X X X
16 197 0 00:30-08:37 8:07 X X X
16 197 1 15:10-19:30 4:20 X X X
17 198 0 00:48-09:30 9:18 X X X
17 198 1 15:09-19:55 4:46 X X X
18 199 0 02:55-09:20 6:25 X X X
18 199 1 15:15-20:20 5:05 X X X
19 200 0 01:15-08:08 6:53 X X X
Total number of sessions station occupied: 16 3 7 4 6 5 4 3 3 3
Table 1. Times of occupation of the GPS points
!• tafla. Mœlingatími á GPS-punktum.
JÖKULL, No. 44
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