A geodetic control network (also geodetic network, reference network, control point network, or control network) is a network, often of

triangle A triangle is a polygon with three edges and three vertices. It is one of the basic shapes in geometry. A triangle with vertices ''A'', ''B'', and ''C'' is denoted \triangle ABC. In Euclidean geometry, any three points, when non- colline ...

s, 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 ...

or by

satellite geodesy Satellite geodesy is geodesy by means of artificial satellites—the measurement of the form and dimensions of Earth, the location of objects on its surface and the figure of the Earth's gravity field by means of artificial satellite techniques ...

. 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 A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location ( longitude, latitude, and altitude/ elevation) to hig ...

systems, GPS 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 Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...

construction Construction is a general term meaning the art and science to form objects, systems, or organizations,"Construction" def. 1.a. 1.b. and 1.c. ''Oxford English Dictionary'' Second Edition on CD-ROM (v. 4.0) Oxford University Press 2009 and ...

and

navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation ...

. The scientific discipline that deals with the establishing of coordinates of points in a control network is called

geomatics Geomatics is defined in the ISO/TC 211 series of standards as the "discipline concerned with the collection, distribution, storage, analysis, processing, presentation of geographic data or geographic information". Under another definition, it ...

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 In cartography, map projection is the term used to describe a broad set of transformations employed to represent the two-dimensional curved surface of a globe on a plane. In a map projection, coordinates, often expressed as latitude and l ...

, unit, and

geodetic datum A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame) is a global datum reference or reference frame for precisely representing the position of locations on Earth or other pla ...

. 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

angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles ...

s and of some spare distances. The precise orientation to the geographic north is achieved through methods of

geodetic astronomy Geodetic astronomy or astronomical geodesy (astro-geodesy) is the application of astronomical methods into geodetic networks and other technical projects of geodesy. Applications The most important applications are: * Establishment of geodetic d ...

. The principal instruments used are

theodolite A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and ...

s and

tacheometer Tacheometry (; from Greek for "quick measure") is a system of rapid surveying, by which the horizontal and vertical positions of points on the earth's surface relative to one another are determined without using a chain or tape, or a separate l ...

s, which nowadays are equipped with

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...

distance measuring,

data base In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases s ...

s, communication systems and partly by satellite links.

Trilateration

Electronic distance measurement Length measurement, distance measurement, or range measurement (ranging) refers to the many ways in which length, distance, or range can be measured. The most commonly used approaches are the rulers, followed by transit-time methods and the int ...

(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 programming. A prototype is generally used to ...

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

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...

s 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) The Echo II was a plug-in expansion card, speech synthesize ...

and Pageos, global networks were determined, which later provided support for the theory of

plate tectonics Plate tectonics (from the la, label= Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of larg ...

. Another important improvement was the introduction of

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a tr ...

and electronic satellites like Geos A and B (1965–70), of the

Transit Transit may refer to: Arts and entertainment Film * ''Transit'' (1979 film), a 1979 Israeli film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world * ''Transit'' (2006 film), a 2006 ...

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 ...

) 1967-1990 — which was the predecessor of GPS - and of

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...

techniques like

Lageos LAGEOS, Laser Geodynamics Satellite or Laser Geometric Environmental Observation Survey, are a series of two scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth. Each satel ...

(USA) or Starlette (F). Despite the use of spacecraft, small networks for

cadastral A cadastre or cadaster is a comprehensive recording of the real estate or real property's metes-and-bounds of a country.Jo Henssen, ''Basic Principles of the Main Cadastral Systems in the World,'/ref> Often it is represented graphically in a cad ...

and

technical Technical may refer to: * Technical (vehicle), an improvised fighting vehicle * Technical analysis, a discipline for forecasting the future direction of prices through the study of past market data * Technical drawing, showing how something is co ...

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 Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Ear ...

satellites and

systems like GPS and

Glonass GLONASS (russian: ГЛОНАСС, label=none, ; rus, links=no, Глобальная навигационная спутниковая система, r=Global'naya Navigatsionnaya Sputnikovaya Sistema, t=Global Navigation Satellite System) is ...

, 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 The BeiDou Navigation Satellite System (BDS; ) is a Chinese satellite navigation system. It consists of two separate satellite constellations. The first BeiDou system, officially called the BeiDou Satellite Navigation Experimental System a ...

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

continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...

s by 2 to 20 cm per year. Therefore, modern global networks like ETRS89 or ITRF show not only

coordinate In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is sign ...

s of their "fixed points", but also their annual velocities.

References