Geoid study

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A geoid study is a geodetic research project in which various methods of geoid determination are tested for an area of ​​around 1000 to 10,000 km² and their achievable accuracy is analyzed.

The first geoid study was proposed by Gauß around 1820, when it was possible to assume a great influence of vertical deviations on extensive surveying networks in the mountains, but also in geologically complex plains. The observation methods of astrogeodesy were only suitable for this around 1880, when Friedrich Robert Helmert , the President of the International Earth Survey , initiated such a test area in the Central German Harz Mountains . To about 10 measurement points of the first order was the astronomical width , in addition, the length or an azimuth is measured and the average level surface of the area by astronomical leveling calculated. The accuracy of the geoid was found locally to be about 10-30 centimeters.

To a large extent, vertical deviations were not measured in the state basic networks until the 1960s, when the emerging satellite geodesy made it possible to expect significantly higher positional accuracies in the future. During this time, the preparatory work for the European RETrig network began in order to be able to connect the national surveys of Western Europe at the respective borders with approximately decimeter accuracy. When the use of the NAVSTAR - GPS satellites became practical at the end of the 1980s , the international earth surveying organizations even declared reaching the centimeter geoid to be a medium-term goal; IAG President Wolfgang Torge (Hanover) proposed this as early as 1983.

Looking ahead, two astro-geodetic geoid studies had already been carried out in Central Europe between 1975 and 1985 , which yielded essential methodological knowledge:

They made it possible to expect centimeter accuracy - regardless of the roughness of the terrain - at point distances of about 10 km, which meant an increase in the existing astro points by five to ten times.

Other university institutes investigated the extent to which gravimetry could replace astronomical measurements, at least in flat areas, since gravity data for raw material research were already available in many places. In order to also achieve centimeter accuracy with gravimetric methods, point spacings of around 2-3 km would have to exist, but any additional measurements would be easier to carry out than night measurements of the vertical deviation.

Since around 1990, many countries have therefore been calculating their regional geoid using a geometrical-physical combination of the two methods, supplemented by satellite geodesy. In Germany, Austria and Switzerland an accuracy of 2–5 cm is achieved (in the Vienna and Harz test areas 0.5 cm), in other countries about 10 cm and worldwide about 20 cm. The local geoid undulations can be  "placed" on it with a good digital terrain model (DGM) to about ± 2–5 cm.

literature

  • Karl Ledersteger : Astronomical and physical geodesy (= handbook of surveying. Volume 5). 10th edition. Metzler, Stuttgart 1969.
  • Kurt Bretterbauer , G. Gerstbach: The astro-geodetic work of the Vienna University of Technology (geoid study Vienna, vertical disturbance research in the Vienna basin ). ÖKIE special volume III: The geoid in Austria. Graz 1983, pp. 61-73.
  • Wolfgang Torge : The centimeter geoid as a challenge. IAG conference paper, around 1985.
  • Heiner Denker , W. Torge: Present state and future developments of the European geoid. In: Surveys in Geophysics. Volume 14. Springer 1993, pp. 433-447 ( doi: 10.1007 / BF00690570 ).
  • Gottfried Gerstbach : How to get an European centimeter geoid (“astro-geological geoid”). In: Physics and Chemistry of the Earth. Volume 21/4. Elsevier, 1996, pp. 343-346 ( online ).
  • H. Denker, Jürgen Müller et al .: A new Combines Height Reference Surface for Germany ( GCG05 ) . EUREF Conference, Riga 2006 ( poster ; PDF; 414 kB).
  • Hans Sünkel , I. Marson (Ed.): Gravity and Geoid: Joint Symposium of the International Gravity Commission and the International Geoid Commission. Conference proceedings September 1995 Graz (Austria) Springer 1996.
  • various IAG conference reports 1995–2009.