European network

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As a European network in the are land surveying and satellite geodesy some international measurement network referred incurred since 1947 and either Western Europe cover or the entire continent. Most of them were calculated by merging national first-order networks . The average distance between the survey points is around 50 km.

The first of these networks are based exclusively on terrestrial measurements, above all on high-precision angle measurements ( triangulation ) and individual distance or base measurements . From around 1965 on, astrogeodetic observations and satellite geodesy methods were increasingly integrated. Since the 1980s, they no longer refer to a centrally located fundamental point , but to an international reference system, the European Terrestrial Reference System .

Central European network and European network 1950

The first cross-border operation was the Central European Network (ZEN) from 1947 - a large frame-like network over Central Europe and large parts of Eastern Europe . It covered a rectangle of around 1000 × 1500 km and was based on preliminary work by the German Army Survey , which from around 1940 aimed for a uniform coordinate system for the conquered areas. On the initiative of the USA , the standardized network that had begun in 1945-47 was completed by the Bamberg “Institute for Earth Measurement ” (predecessor of the Institute for Applied Geodesy ) under Erwin Gigas , but based on the North American model, not nationwide, but as a framework or node network . Its meshes, about 200 to 300 km wide, consisted of double triangular chains.

From the ZEN , the world's first triangulation network over more than two countries, further initiatives developed, including the first Europe-wide geoid determination (around 1948/50) by Helmut Wolf . Although it was only a few meters accurate (while Austria and Switzerland, for example, soon reached 60 to 90 cm), it resulted in a number of development and research projects .

The European Datum 1950 ( European Datum 1950 ) was a definite success story. It represents a transnational geodetic datum , based on the Hayford ellipsoid derived in the USA in 1924 and on the measurement data of the Réseau Européen des Triangulations (RETrig) standardized for all of Western Europe . Although this first mathematically strictly balanced European network still had to struggle with unequal accuracies and the extensive lack of perpendicular deviation reduction, it was still a big step compared to the ZEN. However, the Cold War got in the way of expansion to the east . A certain disadvantage was the choice of the Hayford ellipsoid as a reference ellipsoid , which adapts to the geoid in Europe less well than the (older) Bessel ellipsoid .

Refinements from around 1960

Further refinements were already being worked on in the late 1950s. In the first place, these were

  1. more precise distance measurements , as the lengthy basic measurements could be replaced from around 1965 with the electronic EDM process (see also geodimeters and tellurometers )
  2. the measurement of hundreds of vertical deviations on trigonometric points in the basic first-order network in order to be able to eliminate the network distortions due to the geoid undulations
  3. better calculation methods, especially for network adjustment and solving the huge systems of equations for over 10,000 TPs .

The ED77 was developed as an interim solution in 1977 , but despite international agreements not all states had yet provided the necessary data (1, 2). The final solution of the ED79 (European date 1979) was published around 1980 and represents the first terrestrial surveying network over Europe, which was calculated strictly according to the theory of astronomical-geodetic network adjustment (see Karl Ledersteger's "natural network").

Satellite geodesy

During the efforts to further develop the ED 50 into the ED 79, the rapid development of satellite geodesy gradually led to the establishment of a space-based network. The first thing to do in the early 1970s was the WEST ( West European Satellite Triangulation ), with which the stellar triangulation method, invented in Finland in 1959, was extended over several thousand kilometers. After disappointing accuracy (a few tens of meters), which was mainly due to the very uneven distribution of the satellite cameras , a long terrestrial baseline across the continent from Malvern (southern England) to Graz (Austria) was jointly measured and evaluated.

At about the same time - initiated by the US-Swiss geodesist Hellmut Schmid - the world network of satellite triangulation was measured. It consisted of 45 ground stations distributed around the world (excluding Russia and the Eastern Bloc ) with thousands of photo plates , each of which was taken simultaneously on two to four stations by the PAGEOS balloon satellite . In 1974, the group around HH Schmis published the result, which showed an astonishing accuracy of an average of 4 to 5 meters - in relation to the earth's radius , an accuracy of 1: 1.5 million.

This “world network” initially only had two stations in Europe , namely Catania (Sicily) and Tromsø (northern Norway). It was therefore decided during the first measurement campaigns to integrate a third network point in southern Germany as No. 46, the TP Hohenpeißenberg . The Malvern – Graz satellite base ran over it, so that all satellite geodesy measurements that had been made up to that point could be integrated into the global network.

The accuracy of this “new European network” reached about two meters and was further compressed in the 1970s and 1980s and supplemented by new measurement methods such as SLR lasers and Doppler measurements . Overall, the advances in measurement and computing technology resulted in an increase in accuracy of 1:10 per decade , so that by 1990 the range of a few centimeters was reached. Around this time, radio interferometry and its long-distance application VLBI were added, which also achieves centimeter accuracies (and today mm) by measuring the signals from quasars .

Current status

When the Global Positioning System became practically applicable from around 1990 , terrestrial measurements in the first-order network lost their importance and were increasingly replaced by differential GPS (DGPS). The European network, which is now supported by satellite measurements that are accurate to the centimeter at a distance of about 50 km, was named EUREF ( European Reference Frame ).

Its superordinate coordinate frame is the ETRS89 ( European Terrestrial Reference System 1989 ). This three-dimensional geodetic reference system is based on combined measurements from a few dozen satellite stations on the stable part of the European continental plate and is there identical to the points of the international IERS Terrestrial Reference System .

See also

literature