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tonic units that comprise the Hengill triple junction
thus affect the seismicity, volcanism and spreading
and consequently the deformation, but are not yet
fully understood.
GPSSURVEYING
The GPS system, operation and its use as a geode-
tic tool is described fully by Wells (19S7), King et al.
(1985) and Leick (1991). For maximum precision
surveying, both P-code and phase measurements are
recorded on both frequencies. Combination of the two
frequencies enables correction for the variable effect
of ionospheric refraction, a major problem in Iceland
because of its high latitude. The effect of the tropo-
sphere may be corrected for using a standard tropo-
sphere model at the data processing stage.
Field measurements are conducted by erecting a
GPS receiver antenna over a ground control point and
recording data. Receivers are deployed simultaneously
at more than one point and data recorded from com-
mon satellites. The data are subsequently processed in-
terferometrically and, knowing the absolute position of
one point, the three-dimensional vector connecting it
to other points may be calculated. From these the abso-
lute coordinates of the other points may be derived.
EARTH TIDES AND OCEAN LOADING
Ocean tides cause changes in the gravitational po-
tential on land by loading and deformation of the sea
floor by the redistribution of water. In coastal areas
the deformation due to ocean loading will greatly ex-
ceed the effect of the Earth tides (Kuo et al., 1970).
Deformation as a result of ocean tidal loading causes
periodic variations in the relative positions of points
on the Earth's surface by a few centimetres vertically
and a few millimetres horizontally. Line lengths may
therefore vary on a diumal basis and this represents
an additional error source in GPS surveying hitherto
generally ignored. To capture the ocean loading effect
one must compare coordinates at particular instances
throughout the cycle since the effect would tend to av-
erage out if a session which covered most of the tidal
cycle were used.
The magnitude of the loading is difficult to esti-
mate as tides in the open ocean are poorly known and
often can only be inferred from data at coastal sta-
tions. Wu-Ling and Morgan (unpublished computer
program) compute vertical and horizontal movements
over the surface of the Earth resulting from ocean
loading by convolving Schwiderski's tidal model with
Farrel's load Green's function (Schwiderski, 1980;
Farrel, 1972). Schwiderski's model divides the open
oceans into 1 x 1 degree squares. For each square
there are 11 different partial wave functions, the
largest of which is the Principal Lunar Semi-diumal
Tide (M2). Schwiderski's model yields 66 deforma-
tion constants for each node on the grid. These are six
deformation constants for each of the 11 partial wave
functions, a sine and cosine term representing defor-
mation in the north, east, and up components. By
summing these, theoretical vertical and horizontal de-
formations at a point may be calculated.
Figure 2 shows the maximum vertical displace-
ments over Iceland with respect to reference stations
at Reykjavík, Akureyri, Búrfell and Borgarfjörður.
The variation in the height difference between two
points increases almost linearly with distance. The
variation could be as much as 140 mm if the baseline
spanned the island, e.g. Reykjavík to Bakkagerði
(Figure 2a), and 130 mm between points in the interi-
or and on the coast, (Figure 2c).
DESIGN AND MEASUREMENT OF THE
HENGILL GPS NETWORK
In August 1991 a 23-point GPS network was es-
tablished and measured in an area 750 km2 in and
around the Hengill triple junction. The point positions
were selected to achieve fairly uniform coverage of
the Hengill triple junction including all three volcanic
systems and the transform branch in Ölfus. Twenty of
the points were newly installed by cementing brass
pegs into stable bedrock. Five points were preexisting
GPS points and two were old triangulation monu-
ments (Fig. 3, Table 1). The network was measured
using three C/A code Ashtech dual frequency re-
ceivers. The receivers recorded the C/A code, full
wavelength phase measurements on L1 and half-
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