Terrestrial techniques Triangulation In "classical geodesy" (up to the sixties) control networks were established by
triangulation using 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.
Trilateration Electronic distance measurement (EDM) was introduced around 1960, when the
prototype instruments became small enough to be used in the field. Instead of using only sparse and much less accurate distance measurements some control networks were established or updated by using
trilateration more accurate distance measurements than was previously possible and no angle measurements. 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.
Levelling Optical levelling, which determines relative height difference between two points, can be used to determine the vertical component of control networks, particularly relative to fixed datums (such as that employed for
Ordnance Datum Newlyn). Double run level techniques may be employed to minimise error.
Satellite geodesy 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, Italy) or
Starlette (France). 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.
Global navigation satellite systems (GNSS) Nowadays, several hundred geospatial satellites are in orbit, including a large number of
remote sensing satellites and
navigation systems like
GPS and
Glonass, which was followed by the European
Galileo satellites in 2020 and China's
Beidou constellation. While these developments have made satellite-based geodetic network surveying more flexible and cost effective than its terrestrial equivalent for areas free of tree canopy or urban canyons, 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. == See also ==