A geodetic control network (also geodetic network, reference network, control point network, or control network) is a network, often of triangles, which are measured precisely by techniques of terrestrial

surveying Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ca ...

or by satellite geodesy. A geodetic control network consists of stable, identifiable points with published datum values derived from observations that tie the points together. Classically, a control is divided into horizontal (X-Y) and vertical (Z) controls (components of the control), however with the advent of satellite navigation systems,

GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...

in particular, this division is becoming obsolete. Many organizations contribute information to the geodetic control network. The higher-order (high precision, usually millimeter-to-decimeter on a scale of continents) control points are normally defined in both space and time using global or space techniques, and are used for "lower-order" points to be tied into. The lower-order control points are normally used for engineering, construction and navigation. The scientific discipline that deals with the establishing of coordinates of points in a control network is called geomatics or

Cartography

After a cartographer registers key points in a digital map to the real world coordinates of those points on the ground, the map is then said to be "in control". Having a base map and other data in geodetic control means that they will overlay correctly. When map layers are not in control, it requires extra work to adjust them to line up, which introduces additional error. Those real world coordinates are generally in some particular map projection, unit, and geodetic datum. Minnesota Geospatial Information Office
"Plan for GIS implementation"
1997.

Triangulation

In "classical geodesy" (up to the sixties) control networks were established by

triangulation In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points. Applications In surveying Specifically in surveying, triangulation involves only angle me ...

using measurements of angles and of some spare distances. The precise orientation to the

geographic north True north (also called geodetic north or geographic north) is the direction along Earth's surface towards the geographic North Pole or True North Pole. Geodetic north differs from ''magnetic'' north (the direction a compass points toward the ...

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 A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and Software prototyping, software programming. A prototyp ...

instruments became small enough to be used in the field. Instead of using only sparse and much less accurate distance measurements some control networks was 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.

Satellite geodesy

The geodetic use of satellites began around the same time. By using bright satellites like Echo I,

Echo II Echo II or Echo 2 or ''variant'', may refer to: * ''Echo II''-class submarine of the Soviet Navy * Echo 2 (satellite), a 1964 NASA communications satellite * Echo II (expansion card), a speech synthesizer card for the Apple II See also * EchoSt ...

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 The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...

) 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.

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

and Glonass, which was followed by the European

Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...

satellites in 2020 and China's Beidou

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

. 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.

References

{{Authority control Civil engineering Geodetic surveys Surveying and geodesy markers