FIG Working Week 2000, 21-26 May, Prague

Modern Geodetic Network and Datum in Europe

by Jaroslav Šimek and Jan Kostelecký

Key words: geodetic network, geodetic datum, positioning, height system, GPS. 


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
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