Jökull - 01.12.1977, Page 67

I he adjustment (f = 130) resulted in the
gravity values, given in Table 2, and in linear
drift factors for the three instruments. The root
mean square errors (r.m.s.e.) of the gravity
values vary between ± 3 and ± 16 figal (aver-
age ± 9 and refer to a mean gravity level
•n the main profile. For one observed gravity
difference; the r.m.s.e. (m) are
LCR 79, LCR 85: m = ± 14 Mgal
LCR D 14: m = ± 31 ^gal.
Linear clrift factors have been introduced as
unknowns in the adjustment, giving values of
LCR 79: d = - 0.8 ± 1.3 /rgal/li
LCR 85: d = - 0.4 ± 1.3 jtlgal/h
LCR D 14: d = - 1.4 ± 3.5 /xgal/h.
Tlnis no significant drift has been found.
For height control, geometric levelling has
been carried out, using a ZEISS Ni2 automatic
level. Readings were controlled by double turn-
ing points. The precision obtained is about
± 0.05 m, referring to the initial station no.
110. The height datum refers to the height
system introduced by Spickernagel (1966). The
results of the levelling 1976 are given in Table 2
too.
In the region between Mývatn/Grimsstadir
and Axarfjördur, about 50 stations established
between 1965 and 1970 could be reoccupied,
using one LaCoste-Romberg gravity meter. The
observations were carried out in loops, starting
and terminating at a main profile station. The
loop misclosures vary between 3 and 54 /rgal,
so we may estimate the accuracy of the gravity
values at ± 0.02 mgal. Results are given in
Table 3, the distribution of the gravity stations
can be seen from Fig. 4.
The gravity and height systems used in our
investigations, have been connected to the Ice-
landic control systems in this area by measur-
ing gravity and height differences to four Ice-
landic control points situated between Mývatn
and Krafla. The results are given in Table 4.
COMPARISON OF THE 1976 OBSERVA-
TIONS WITH THE 1975 AND 1965/70
RESULTS
The 1975 and 1976 gravity and lieight values
for the main profile and the gravity and lieight
variations are given in Table 2, íigure 3 showing
the variations over the profile. As the r.m.s.e.
of the gravity values in 1975 and 1976 is about
± 0.01 mgal, the differences of gravity 1976—
1975 liave an accuracy of about ± 0.01 . . .
± 0.02 mgal.
We find a significant gravity increase (0.04
mgal) of limited extension (5 km) at Náma-
skard, decreasing very rapidly to the west and
to the east. The lieight variations 1976—1975
are clearly correlated with this gravity increase,
giving a subsidence of 0.3 m at Námaskard. As
the indicated sinking of the extreme east (0.1 m
at Jökulsá) is not confirmed by the gravity
Fig. 2.
Base network 1976.
Mynd 2.
Grunnstöðvar þyngdar-
mœlinga 1976.
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JÖKULL 27. ÁR 65