Geodetic network
Encyclopedia
A geodetic network is a network of triangle
s 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 angle
s and of some spare distances; the precise orientation to the geographic north is achieved through methods of geodetic astronomy
. The principal instruments used are theodolite
s 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 satellite
s 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 continent
s by 2 to 20 cm per year. Therefore modern global networks like ETRF or ITRF
show not only coordinates of their "fixed points", but also their annual velocities
.
Triangle
A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three sides or edges which are line segments. A triangle with vertices A, B, and C is denoted ....
s which are measured exactly by techniques of terrestrial surveying or by satellite geodesy
Satellite geodesy
Satellite geodesy is the measurement of the form and dimensions of the Earth, the location of objects on its surface and the figure of the Earth's gravity field by means of artificial satellite techniques—geodesy by means of artificial satellites...
.
In "classical geodesy" (up to the sixties) this is done by triangulation
Triangulation
In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...
, based on measurements of angle
Angle
In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.Angles are usually presumed to be in a Euclidean plane with the circle taken for standard with regard to direction. In fact, an angle is frequently viewed as a measure of an circular arc...
s and of some spare distances; the precise orientation to the geographic north is achieved through methods of geodetic astronomy
Geodetic astronomy
Geodetic astronomy is the application of astronomical methods into networks and technical projects of geodesy.The most important topics are:* Establishment of geodetic datum systems Geodetic astronomy is the application of astronomical methods into networks and technical projects of geodesy.The...
. The principal instruments used are theodolite
Theodolite
A theodolite is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are mainly used for surveying applications, and have been adapted for specialized purposes in fields like metrology and rocket launch technology...
s and tacheometers, which nowadays are equipped with infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
distance measuring, data bases, communication systems and partly by satellite links.
Electronic
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
distance measurement (EDM) was introduced around 1960, when the prototype
Prototype
A prototype is an early sample or model built to test a concept or process or to act as a thing to be replicated or learned from.The word prototype derives from the Greek πρωτότυπον , "primitive form", neutral of πρωτότυπος , "original, primitive", from πρῶτος , "first" and τύπος ,...
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 satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s began around the same time. By using bright satellites like Echo I
Echo satellite
Project Echo was the first passive communications satellite experiment. Each of the two American spacecraft was a metalized balloon satellite acting as a passive reflector of microwave signals. Communication signals were bounced off of them from one point on Earth to another.-Echo 1:NASA's Echo 1...
, Echo II and Pageos
PAGEOS
PAGEOS was a balloon satellite which was launched by the NASA in June 1966. Pageos had a diameter of exactly , consisted of a thick mylar plastic film coated with vapour deposited aluminium enclosing a volume of and was used for the Weltnetz der Satellitentriangulation -- a global cooperation...
, global networks were determined, which later provided support for the theory of plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
.
Another important improvement was the introduction of radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
and electronic satellites like Geos A
GEOS
GEOS may refer to:Computer software*GEOS , an operating system originally designed for the Commodore 64*GEOS , a DOS-based graphical user interface and x86 operating system...
and B (1965-70), of the Transit
Transit (satellite)
The TRANSIT system, also known as NAVSAT , was the first satellite navigation system to be used operationally. The system was primarily used by the U.S...
system (Doppler effect
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
) 1967-1990 — which was the predecessor of GPS - and of laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
techniques like Lageos
LAGEOS
LAGEOS, or Laser Geodynamics Satellites, are a series of scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth...
(USA) or Starlette (F). Despite the use of spacecraft, small networks for cadastral and technical
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...
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
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
satellites and navigation
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...
systems like GPS and Glonass
GLONASS
GLONASS , acronym for Globalnaya navigatsionnaya sputnikovaya sistema or Global Navigation Satellite System, is a radio-based satellite navigation system operated for the Russian government by the Russian Space Forces...
, 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
Fixed point
"Fixed point" has many meanings in science, most of them mathematical.* Fixed point * Fixed-point combinator* Fixed-point arithmetic, a manner of doing arithmetic on computers* Benchmark , fixed points used by geodesists...
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
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
s by 2 to 20 cm per year. Therefore modern global networks like ETRF or ITRF
International Terrestrial Reference System
The International Terrestrial Reference System describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a physical standard might be described as a set of procedures for creating a realization of...
show not only coordinates of their "fixed points", but also their annual velocities
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
.
See also
- CadastreCadastreA cadastre , using a cadastral survey or cadastral map, is a comprehensive register of the metes-and-bounds real property of a country...
- SurveyingSurveyingSee Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...
- MapMapA map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....
s - ED50ED50ED 50 is a geodetic datum which was defined after World War II for the international connection of geodetic networks....
- GRS80
- History of geodesyHistory of geodesyGeodesy ,[1] also named geodetics, is the scientific discipline that deals with the measurement and representation of the Earth.Humanity has always been interested in the Earth...
- TrigonometryTrigonometryTrigonometry is a branch of mathematics that studies triangles and the relationships between their sides and the angles between these sides. Trigonometry defines the trigonometric functions, which describe those relationships and have applicability to cyclical phenomena, such as waves...