150
The first two solutions are relative geoids, while the third
solution is absolute one. For the absolute orientation almost 90
GPS stations were used. Orthometric heights of these points
were determined by geometric levelling. Figure 2 shows the
latest solution from 1999.
The influence of of the actual relief of the Earth's surface on the
geoid forms is clearly visible on the figure; see for instance its
raising under the Alps or its decline in the Adriatic sea. The
Figure 2: Astrogeodetic geoid in Slovenia (solution from 1999)
Speaking about the geoid we would like to outline some major
facts about the situation of gravimetry in Slovenia
Till 1991 the most work on gravimetry was in the competency
of the Military Geographic Institute (MG1) in Belgrade.
Gravimetric surveys for the establishment of network were
performed by MGI and the Geological Survey of Slovenia. For
the last ten years they had no activities beacuse of out-of-date
equipment and no interest. In 1991 the situation in gravimetry in
Slovenia was as followed: about 50 gravity points with
unhomogenous disposition and rather low accuracy.
Since 1995 Surveying and Mapping Authority of Republic of
Slovenia (SMARS) put a lot of efforts in revitalizing
gravimetric activités in Slovenia. In 1997 SMARS prepared a
project of absolute gravity measurements in Slovenia. The
project of absolute gravity measurements in Slovenia was
organized by the International Gravity Commission, Slovenian
Union of Geodesy and Geophysics and Surveying and Mapping
Authority of Republic of Slovenia. Six stations were stabilized:
castle Bogensperk near town Litija, Gotenica near town
Ribnica, church Sv. Areh on mountain Pohorje, castle Sevniski
grad near town Sevnica, castle Socerb near Koper and fortress
Kluze near Bovec.
Because of the rather bad situation of the gravity investigations
in Slovenia, activités in this field should be started from the
begining. The network of absolute gravity points will be the
base for the further development of gravity network. There are
plans of establishment of a new fundamental gravity network
for the whole country.
SLOVENIA AND EUREF
The development of a new reference coordinate system in
Slovenia that should comply all the needs of the contemporary
geodesy regarding state and unified European coordinate
maximum varitation of the calculated geoid heights is 5.7m
(Figure 2)
For the time being there is no local gravimetric solution for the
geoid. But, gravity anomalies from territory of Slovenia were
used in the computation of European Gravimetric Geoid 1997
(EGG97), (Denker et al., 1997). Figure 3 shows the differences
between gravimetric geoid and aforementioned astrogeodetic
solution (gravimetric undulations were transformed to Slovenian
height system).
Figure 3: Differences between astrogeodetic solution and
EGG97
system, started with the connection of the first order horizontal
network of Slovenia with the EUREF. This connection is based
on GPS observation campaigns performed in 1994 and 1995.
Both campaigns were performed with the substantial
contribution of the Institut für Angewandte Geodäsie, Frankfurt
(now Bundesamt für Kartographie and Geodäsie) to the
Surveying and Mapping Authority of the Republic of Slovenia.
The EUREF-CRO-SLOV '94 GPS observation campaign was
performed at 8 first order points (among theese were 5 points
choosen as "real" EUREF points) during the 5 days of
permanent GPS observations. The results were published in
1995 (Altineret al., 1998).
The purpose of the second EUREF campaign at the territory of
Slovenia, under the name SLOVENIA '95 Campaign, was to
density the existing EUREF points at the territory of Slovenia.
Slovenia is placed in tectonical and seismological active part of
Europe and Surveying and Mapping Authority of Slovenia
wanted to connect the project of the establishment of new
reference coordinate system with the monitoring the strain
accumulation in south-eastern part of the Alps region and in the
northern part of the Adriatic Plate. The second observation
campaign in 1995 was performed at all 34 (35) first order
points, at 2 stations of the triangulation base lines network
(Bukovec and Radovljica) included in the first order network; at
1 second order point, and at 12 geodynamic points. GPS
observation were carried out for six days, in two blocks with 3
day of 24 hours of observations each. The final coordinates of
the Slovenia '95 Campaign are given in the ITRF93 epoch
1995.7, respectively in the ETRS 89 epoch 1989.0. Unweighted
RMS values with respect to the combined solutions are 2 mm in
north direction, 2 mm in east direction and 6 mm in height
direction. At the moment became availble also the common
solution of the three GPS observation Campaign performed at
the territory of Slovenia and the surrounding area (Y. Altiner et
al., 1998). Namely in 1996 there was another GPS observation
campaign CROREF-CRODYN '96, with the duration of 15