Geodetic network
A geodetic network (also: reference or controlnetwork) 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 the geographic north is achieved through methods of geodetic astronomy. The principal instruments used are theodolites and tacheometers, which nowadays are equipped with infrared distance measuring, data bases, communication systems and partly by satellite links.
Electronic distance measurement (EDM) was introduced around 1960, when the prototype instruments became small enough to be used in the field. EDM increased network accuracies up to 1:1 million (1 cm per 10 km; today at least 10 times better), and made surveying less costly. The geodetic use of satellites began around the same time. By using bright satellites like Echo I, Echo II and Pageos, global networks were determined, which later provided support for the theory of plate tectonics.
Another 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 the use of spacecraft, small networks for cadastral and technical projects are mainly measured terrestrially, but in many cases incorporated in national and global networks by satellite geodesy.
Nowadays, several hundred geodetic satellites are in orbit, supplemented by a large number of remote sensing satellites and navigation systems like GPS and Glonass, which will be followed by the European Galileo satellites in 2013. While these developments have made satellite-based geodetic network surveying more flexible and cost effective than its terrestrial equivalent, the continued existence of fixed point networks is still needed for administrative and legal purposes on local and regional scales. Global geodetic networks cannot be defined to be fixed, since geodynamics are continuously changing the position of all continents by 2 to 20 cm per year. Therefore modern global networks like ETRS89 or ITRF show not only coordinates of their "fixed points", but also their annual velocities.