Modern Geodetic Network and Datum in Europe
by Jaroslav Šimek and Jan Kostelecký
Key words: geodetic network, geodetic datum, positioning,
height system, GPS.
Abstract
In space geodetic positioning, where the observation
techniques provide absolute positions with respect to a consistent
terrestrial reference frame, the corresponding precise definition and
realization of terrestrial and inertial reference systems is of
fundamental importance. Thanks to significant improvements in receiver
technology, to extension and densification of the global tracking network
along with more accurate determination of positions and velocities of the
tracking stations and to dramatically improved satellite orbits, GPS is
today approaching one ppb precision for longer baselines and it can
be considered to be a global geodetic positioning system providing nearly
instantaneous three-dimensional position at the 1 - 2 cm accuracy level.
With respect to this the reference system is one of the primary limiting
error sources.
The Terrestrial Reference System is geocentric and
is realized through a reference frame, i.e. a set of coordinates for a
network of stations. The Conventional Terrestrial Reference System,
which is monitored by the International Earth Rotation Service (IERS),
is called the International Terrestrial Reference System (ITRS).
Realizations of the ITRS are produced under the name International
Terrestrial Reference Frame. From each annual frame labelled ITRF-YY a
corresponding frame ETRF-YY can be derived which is represented by
coordinates of sites belonging to ITRF and located in Europe. The
terrestrial reference system used by the U.S. DoD for GPS positioning
is WGS84. This global geocentric system was originally based on
TRANSIT Doppler observations and realized with an accuracy of 1 - 2 m,
but after several improvements coincides with the ITRF96 within a few
centimeters (WGS-84(G873)).
In the first phase of its employment GPS was only
used as a new method of measurement in classical horizontal control
networks. The request of GPS users in Europe for a new 3D reference
frame became urgent to the end of eighties. Therefore in 1987 the IAG
Section I (Positioning) had decided to form the new Subcommission for
the European Reference Frame (EUREF) the goal of which was to develop
a GPS-based new European Reference System. At the same time the CERCO
(Comité Européen des Responsables de la Cartographie Officielle)
Working Group VIII on Geodesy, independently of the IAG, came to the
same decision. As at that time the IERS combined SLR/VLBI solutions (ITRS/ITRF)
were by far the best global realizations of a geocentric reference
system, it was decided to base EUREF on the ITRF by selecting about 35
European SLR/VLBI sites for the epoch 1989. The main campaign of the
project was carried out in 1989 for Western Europe and starting from
1991 the new reference frame was extended through a rapid sequence of
post-campaigns to the territories of most European countries and
adjacent areas. At present EUREF covers almost all European countries
except Russia and Belarus and in most countries its intensive
densification and update is in progress or has been accomplished.
The densification and maintenance of EUREF is
mainly being done by GPS and it benefits from growing number of
permanent GPS stations in Europe. EUREF permanent network is an
European densification of the global IGS network. In this connection
EUREF guaranteed a routine analysis of the data from all permanent
European sites on the basis of distributed processing. The work is
ensured by 10 EUREF Local Analysis Centers which submit weekly
solutions for a particular EUREF subnetwork to the EUREF Regional
Analysis Center (CODE, Bern) which is responsible for combining these
weekly free-subnetwork solutions into one European solution. Today
nearly 90 European permanent GPS stations are regularly contributing
to European combined solution.
Along with the unification of the GPS reference
frame the CERCO urgently requires the realization of a uniform
European Height System for unified Digital Cartographic Data Bases.
The new system will be based on normal heights and will be referenced
to the Amsterdam Tidal Gauge. Since 1994 also this task has been
solved within the EUREF Subcommission. The approach follows two
aspects. The first is the „extensive" and „intensive"
enlargement of the UELN (Unified European Levelling Network), which
means the incorporation of both primary levelling networks of the new
(Central and East European) countries and lower order levelling
networks of all European countries. The second is the realization of
an extensive GPS project aimed at the interconnection of all European
height datums. The project, called EUVN97, was carried out in
1997/1998 as a joint venture of all European EUREF countries. The
total of 217 EUVN sites, distributed over almost all European
countries, comprises 37 permanent stations, 66 EUREF sites, 54
first-order levelling bench marks and 63 tide gauges. All EUVN sites
were tied to first-order national levelling networks. The network was
processed by distributed processing carried out by 8 analysis centers,
the final combined solution was performed by two combination centers (BKG
Leipzig and CODE Bern).
The objectives of the EUVN project are basically to
contribute to the unification of European height systems on the
accuracy level of a few centimeters, to provide fiducial points for
the European geoid determination, to connect the European tide gauges
for investigations of sea level variations and to prepare an European
geokinematic height reference system. Simultaneously to the GPS and
levelling interconnection of different European height datums the
investigations towards construction of a very precise high resolution
(quasi)geoid for Europe are being done. The presently available
models, the global EGM96 and the regional (European) EGG97 can be
considered as the basis for the future work.
This presentation brings brief characteristics of
geocentric reference systems and frames and their interrelations,
describes the evolution of the EUREF project from the beginning up to
now, informs about the organization and development of the EUREF
permanent GPS network and about the objectives and evolution of the
European height system for GIS and geodynamics along with current
achievements in precise geoid modelling in Europe. A feasibility of
EUREF based improvement of classical terrestrial networks and of
nation/continent-wide GPS heighting is briefly discussed.
Ing. J. Šimek and Prof. Jan Kostelecký
GO Pecný
CZ-251 65 Ondřejov
E-mail: gope@asu.cas.cz
or kost@fsv.cvut.cz
Web site: http://www.vugtk.cz
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