Geodetic network
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A geodetic network is a network of triangles which are measured exactly by techniques of terrestrial surveying or by satellite geodesy.
In "classical geodesy" (up to the sixties) this is done by triangulation, based on measurements of angles and of some spare distances; the precise orientation to geographic North is done by methodes of geodetic astronomy. The mainly used instruments are theodolites and tacheometers, which nowadays are equipped by infrared distance measuring, data bases, communication systems and partly by satellite links.
Beginning with ca 1960, the electronic distance measurement (EDM) was introduced, when the first prototypes became small enough to work in the field. EDM increased the network accuracies up to 1:1 million (1 cm per 10 km; today at least 10 times better), and also the economy of surveying. At the same time the geodetic use of satellites begun, e.g. the bright satellites of Echo I and II and Pageos. By means of these space probes, global networks were determined, which later proved the theory of plate tectonics.
An important improvement was the introduction of radio and electronic satellites like Geos A and B (1965-70), of the Transit system (Doppler effect) 1967-1990 — which was the predecessor of GPS - and of laser techniques like Lageos (USA) or Starlette (F). Despite of the space techniques, small networks for cadastre and technical projects are mainly measured terrestrially, but in many cases closed together to national and global networks by satellite geodesy.
In the meantime, several hundred geodetic satellites are orbiting, supplemented by a huge number of remote sensing satellites - and last but not least by the navigation systems of GPS and Glonass, which will be followed by the European Galileo satellites in 2008. Nowadays the space-based geodetic networks are more flexible and economic than terrestrial ones; the further existence of fixed point networks is already discussed, but will survive al least for administrational and legal demands on local and regional scales. Whereas the worldwide networks can not be defined to be fixed, because Geodynamics is chancing the position of all continents by amounts of 2 cm up to 20 cm per year. Therefore modern global networks like ETRF or ITRF show not only coordinates of their "fixed points", but also their annual velocities.
