Jökull - 01.12.1994, Page 37

the instrument drift and uses every repeated reading at
any station during the whole survey. Linear drift, non-
linear drift, and the reading accuracy are taken into
account. The results are given in Table 4 for LCR G
445 in 1992. The instrument was recalibrated shortly
before the survey by the manufacturer. A comparison
with the Sodin gravity meter, and in 1993 with anoth-
er LaCoste-Romberg gravimeter D 187, showed no
systematic scale differences. Note that the gravity dif-
ferences and absolute values pertain to the locations
and heights of measurement, not reduced to bench-
mark heights because the true free air gravity gradient
is unknown and the assumption of a standard value
may lead to errors. For example, a local -10% devia-
tion of the free air gradient could, in the worst case (Pt
1000), lead to a 30 - 40 uGal error.
In one case a temporal gravity change has already
been detected, between point No. 5273 (Djúpivogur
Hotel, -50 km from the ice edge) and point No. 5219
(Homafjörður Airport, old, near Höfn, -20 km from
the ice edge). In 1968, 24 years earlier, the gravity dif-
ference (in the Icelandic base gravity network) was -
13.94 (±.024) mGal ('5219' - '5273') (Pálmason etal.,
1973; Thorbergsson et al., 1993). In 1992 it was -
13.969 (+.010) mGal; i.e. a change of -29 (±26) uGal
in the gravity difference is discemible. If Djúpivogur
is assumed to be fixed, this is a gravity decrease near
Höfn. It would, at face value, imply a land rise of
about 0.17 (±0.14) m in 24 years or nearly 7 (±6)
mm/year near Höfn, if interpreted as a change in the
Bouguer anomaly with mantle mass inflow below the
rising surface (£ = 3.3 g/cm3). If interpreted as a "Free
Air effect", i.e., purely elastic plate expansion with no
lateral mass movement, the implied land rise would
be only 0.1 (±0.08) m or 5 (±4) mm/year. The two in-
terpretations probably represent upper and lower
bounds.
OTHER MEASUREMENTS
Other methods capable of detecting the expected
changes in elevation include long-term sea-level mea-
surements, VLBI (Very Long Baseline Interferome-
try), and repeated levelling. A sea-level gauge was in-
stalled at the Skinneyjarhöfði promontory in June
1994 (Fig. 1). A drill hole was drilled into the rock
near the cliff's edge, with its bottom well below the
low tide level. A small explosion was set off at the
bottom of the hole to provide connection to the sea.
The sea-level gauge consists of a pressure transducer
at the bottom of the hole, 3 independent water level
detectors for automatic calibration of the pressure
transducer, and a digital data logger installed at the
top of the hole. Pressure values are read every 5-10
minutes, and every time the water level passes a water
level detector.
The VLBI technology is based on simultaneous
recording of radio signals from distant astronomical
objects at a set of radio telescopes. The relative posi-
tions of the observation points can be determined to
within several millimeters. A VLBI experiment was
carried out in the town of Höfn in the summer of 1992
Table 3. GPS lo-
cations, geocen-
tric coordinates
(WGS84), and
repeatability
# Station n network solution and weighted rms around them (cm)
X rx y ry Z rz m re ru
1 DJUP 3 265395952.38 0.23 -675676656.12 1.05 574102029.31 0.24 0.08 1.10 0.24
2 FLAT 11 267915860.36 2.33 -74264417.26 1.81 572113993.91 2.00 0.38 0.15 0.65
3 GILD 7 266889653.62 1.30 -73248203.31 0.39 572721422.07 2.01 0.80 0.27 2.36
4 HEIN 6 267027688.62 3.69 -74834583.68 2.52 572462019.39 1.33 0.53 0.10 1.67
5 HOFF 9 266257723.70 1.50 -73311888.96 1.44 573009899.70 3.77 1.83 1.40 2.95
6 HOFN 20 267971550.23 0.00 -72797065.40 0.00 572276434.63 0.00 0.00 0.00 0.00
7 HROL 2 268258664.64 0.24 -77305278.87 0.49 571556354.02 0.32 0.24 0.39 0.46
8 HVAL 5 267460130.43 0.76 -69378033.92 0.21 572935863.96 2.49 0.92 0.39 2.71
9 STAP 5 267317514.50 0.49 -72882277.35 0.21 572572524.31 0.73 0.21 0.24 0.83
10 UPPS 4 267861207.98 0.42 -75407892.39 0.37 571994582.67 1.17 0.58 0.28 1.49
3. tafla. Staðsetn-
ing GPS-punkta í
jarðmiðjuhnitum
(WGS84), ásamt
endurtekningar-
nákvœmni.
JOKULL, No. 44
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