- Geodetic network
A geodetic network is a network of
triangle s which are measured exactly by techniques of terrestrial surveying or bysatellite geodesy .In "classical geodesy" (up to the sixties) this is done by
triangulation , based on measurements ofangle s and of some spare distances; the precise orientation to geographicNorth is done by methodes ofgeodetic astronomy . The mainly used instruments aretheodolite s andtacheometer s, which nowadays are equipped byinfrared distance measuring,data base s, communication systems and partly by satellite links.Beginning with ca 1960, the electronic distance measurement (
EDM ) was introduced, when the firstprototype s 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 ofsatellite s begun, e.g. the bright satellites ofEcho I and II andPageos . By means of these space probes, global networks were determined, which later proved the theory ofplate tectonics .An important improvement was the introduction of
radio and electronic satellites like Geos A and B (1965-70), of theTransit system (Doppler effect) 1967-1990 — which was the predecessor of GPS - and oflaser techniques likeLageos (USA) or Starlette (F). Despite of the space techniques, small networks forcadastre andtechnical 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 thenavigation systems ofGPS and Glonass, which will be followed by the EuropeanGalileo satellites in 2008. Nowadays thespace -based geodetic networks are more flexible and economic than terrestrial ones; the further existence offixed point networks is already discussed, but will survive al least for administrational and legal demands on local and regional scales. Whereas theworldwide networks can not be defined to be fixed, becauseGeodynamic s is chancing the position of allcontinent s by amounts of 2 cm up to 20 cm per year. Therefore modern global networks likeETRF orITRF show not onlycoordinate s of their "fixed points", but also their annual velocities.See also
*
Cadastre
*Surveying
*Map s
*ED50
*GRS80
*History of geodesy
*Trigonometry
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