Greinar (Vísindafélag Íslendinga) - 01.01.1951, Síða 44

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stations were used so that control of the instrument drift
was more than sufficient.
We have thus obtained 134 new gravity points with, in
most cases, relative errors less than 0.2 mgal. As to the
absolute values, the station Skólavarda has been com-
pared repeatedly with Paris by the French expedition with
the same instrument. A gravity of 982268.5 ± 0.8 mgal.
was obtained by Martin in 1949.
The calculation of the Bouguer anomalies was made
with an accuracy of 0.1 mgal. for the points between
Reykjavik and Selfoss. The maximum terrain correction
here was 1.1 mgal. but mostly it was less than 0.5 mgal.
For the other points, an accuracy of only 0.5 mgal. is
intended, and here terrain correction was dropped except
in a few cases where it would clearly amount to 0.5 mgal.
or more.
If the altitude of a point is incorrect by 2 m. there is
an error 0.6 mgal. in the free air correction. For points
below an altitude of 200 m., which is true for most points,
we have a further reduction, less than 0.0419XpX200 =
8.38 Xp, where P is the specific gravity of the underlying
rocks. If we suppose an error of 0.2 in the specific gra-
vity of the underlying rocks, then this corresponds to
1.7 mgal. at most.
We see that the uncertainty of specific gravity will,
in a great number of cases, be more serious than that
of the altitude.
The uncertainty in the Bouguer anomaly for the points
at higher places may thus be 1 to 2 mgal. and in the few
points at 500 to 700 m. the error may be as large as 5
mgal. Yet, owing to the monotony of the rock material,
these errors should not be relative ones from one point
to another in the same region, but only absolute and rela-
tive between highland and lowland.
These possible errors are, however, quite insignificant
in view of the result that the Bouguer anomalies vary
regularly from about +50 mgal. at the coast, to about