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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 planetary bodies by means of geodetic coordinates.[1] Datums[note 1] are crucial to any technology or technique based on spatial location, including geodesy, navigation, surveying, geographic information systems, remote sensing, and cartography. A horizontal datum is used to measure a location across the Earth's surface, in latitude and longitude or another coordinate system; a vertical datum is used to measure the elevation or depth relative to a standard origin, such as mean sea level (MSL). Since the rise of the global positioning system (GPS), the ellipsoid and datum WGS 84 it uses has supplanted most others in many applications. The WGS 84 is intended for global use, unlike most earlier datums.

Before GPS, there was no precise way to measure the position of a location that was far from universal reference points, such as from the Prime Meridian at the Greenwich Observatory for longitude, from the Equator for latitude, or from the nearest coast for sea level. Astronomical and chronological methods have limited precision and accuracy, especially over long distances. Even GPS requires a predefined framework on which to base its measurements, so WGS 84 essentially functions as a datum, even though it is different in some particulars from a traditional standard horizontal or vertical datum.

A standard datum specification (whether horizontal or vertical) consists of several parts: a model for Earth's shape and dimensions, such as a reference ellipsoid or a geoid; an origin at which the ellipsoid/geoid is tied to a known (often monumented) location on or inside Earth (not necessarily at 0 latitude 0 longitude); and multiple control points that have been precisely measured from the origin and monumented. Then the coordinates of other places are measured from the nearest control point through surveying. Because the ellipsoid or geoid differs between datums, along with their origins and orientation in space, the relationship between coordinates referred to one datum and coordinates referred to another datum is undefined and can only be approximated. Using local datums, the disparity on the ground between a point having the same horizontal coordinates in two different datums could reach kilometers if the point is far from the origin of one or both datums. This phenomenon is called datum shift.

Because Earth is an imperfect ellipsoid, local datums can give a more accurate representation of some specific area of coverage than WGS 84 can. OSGB36, for example, is a better approximation to the geoid covering the British Isles than the global WGS 84 ellipsoid.[2] However, as the benefits of a global system outweigh the greater accuracy, the global WGS 84 datum has become widely adopted.[3]

City of Chicago Datum Benchmark

History edit

 
The Great Trigonometrical Survey of India, one of the first surveys comprehensive enough to establish a geodetic datum.

The spherical nature of Earth was known by the ancient Greeks, who also developed the concepts of latitude and longitude, and the first astronomical methods for measuring them. These methods, preserved and further developed by Muslim and Indian astronomers, were sufficient for the global explorations of the 15th and 16th Centuries.

However, the scientific advances of the Age of Enlightenment brought a recognition of errors in these measurements, and a demand for greater precision. This led to technological innovations such as the 1735 Marine chronometer by John Harrison, but also to a reconsideration of the underlying assumptions about the shape of Earth itself. Isaac Newton postulated that the conservation of momentum should make Earth oblate (wider at the equator), while the early surveys of Jacques Cassini (1720) led him to believe Earth was prolate (wider at the poles). The subsequent French geodesic missions (1735-1739) to Lapland and Peru corroborated Newton, but also discovered variations in gravity that would eventually lead to the geoid model.

A contemporary development was the use of the trigonometric survey to accurately measure distance and location over great distances. Starting with the surveys of Jacques Cassini (1718) and the Anglo-French Survey (1784–1790), by the end of the 18th century, survey control networks covered France and the United Kingdom. More ambitious undertakings such as the Struve Geodetic Arc across Eastern Europe (1816-1855) and the Great Trigonometrical Survey of India (1802-1871) took much longer, but resulted in more accurate estimations of the shape of the Earth ellipsoid. The first triangulation across the United States was not completed until 1899.

The U.S. survey resulted in the North American Datum (horizontal) of 1927 (NAD27) and the Vertical Datum of 1929 (NAVD29), the first standard datums available for public use. This was followed by the release of national and regional datums over the next several decades. Improving measurements, including the use of early satellites, enabled more accurate datums in the later 20th century, such as NAD83 in North America, ETRS89 in Europe, and GDA94 in Australia. At this time global datums were also first developed for use in satellite navigation systems, especially the World Geodetic System (WGS 84) used in the U.S. global positioning system (GPS), and the International Terrestrial Reference System and Frame (ITRF) used in the European Galileo system.

Dimensions edit

Horizontal datum edit

The horizontal datum is the model used to measure positions on Earth. A specific point can have substantially different coordinates, depending on the datum used to make the measurement. There are hundreds of local horizontal datums around the world, usually referenced to some convenient local reference point. Contemporary datums, based on increasingly accurate measurements of the shape of Earth, are intended to cover larger areas. The WGS 84 datum, which is almost identical to the NAD83 datum used in North America and the ETRS89 datum used in Europe, is a common standard datum.[citation needed]

Vertical datum edit

A vertical datum is a reference surface for vertical positions, such as the elevations of Earth features including terrain, bathymetry, water level, and human-made structures.

An approximate definition of sea level is the datum WGS 84, an ellipsoid, whereas a more accurate definition is Earth Gravitational Model 2008 (EGM2008), using at least 2,159 spherical harmonics. Other datums are defined for other areas or at other times; ED50 was defined in 1950 over Europe and differs from WGS 84 by a few hundred meters depending on where in Europe you look. Mars has no oceans and so no sea level, but at least two martian datums have been used to locate places there.

Geodetic coordinates edit

 
The same position on a spheroid has a different angle for latitude depending on whether the angle is measured from the normal line segment CP of the ellipsoid (angle α) or the line segment OP from the center (angle β). The "flatness" of the spheroid (orange) in the image is greater than that of Earth; as a result, the corresponding difference between the "geodetic" and "geocentric" latitudes is also exaggerated.

In geodetic coordinates, Earth's surface is approximated by an ellipsoid, and locations near the surface are described in terms of geodetic latitude ( ), longitude ( ), and ellipsoidal height ( ).[note 2]

Earth reference ellipsoid edit

Defining and derived parameters edit

The ellipsoid is completely parameterised by the semi-major axis   and the flattening  .

Parameter Symbol
Semi-major axis  
Reciprocal of flattening  

From   and   it is possible to derive the semi-minor axis  , first eccentricity   and second eccentricity   of the ellipsoid

Parameter Value
Semi-minor axis  
First eccentricity squared  
Second eccentricity squared  

Parameters for some geodetic systems edit

The two main reference ellipsoids used worldwide are the GRS80[4] and the WGS 84.[5]

A more comprehensive list of geodetic systems can be found here.

Geodetic Reference System 1980 (GRS80) edit

GRS80 parameters
Parameter Notation Value
Semi-major axis   6378137 m
Reciprocal of flattening   298.257222101

World Geodetic System 1984 (WGS 84) edit

The Global Positioning System (GPS) uses the World Geodetic System 1984 (WGS 84) to determine the location of a point near the surface of Earth.

WGS 84 defining parameters
Parameter Notation Value
Semi-major axis   6378137.0 m
Reciprocal of flattening   298.257223563
WGS 84 derived geometric constants
Constant Notation Value
Semi-minor axis   6356752.3142 m
First eccentricity squared   6.69437999014×10−3
Second eccentricity squared   6.73949674228×10−3

Datum transformation edit

The difference in co-ordinates between datums is commonly referred to as datum shift. The datum shift between two particular datums can vary from one place to another within one country or region, and can be anything from zero to hundreds of meters (or several kilometers for some remote islands). The North Pole, South Pole and Equator will be in different positions on different datums, so True North will be slightly different. Different datums use different interpolations for the precise shape and size of Earth (reference ellipsoids). For example, in Sydney there is a 200 metres (700 feet) difference between GPS coordinates configured in GDA (based on global standard WGS 84) and AGD (used for most local maps), which is an unacceptably large error for some applications, such as surveying or site location for scuba diving.[6]

Datum conversion is the process of converting the coordinates of a point from one datum system to another. Because the survey networks upon which datums were traditionally based are irregular, and the error in early surveys is not evenly distributed, datum conversion cannot be performed using a simple parametric function. For example, converting from NAD27 to NAD83 is performed using NADCON (later improved as HARN), a raster grid covering North America, with the value of each cell being the average adjustment distance for that area in latitude and longitude. Datum conversion may frequently be accompanied by a change of map projection.

Discussion and examples edit

A geodetic reference datum is a known and constant surface which is used to describe the location of unknown points on Earth. Since reference datums can have different radii and different center points, a specific point on Earth can have substantially different coordinates depending on the datum used to make the measurement. There are hundreds of locally developed reference datums around the world, usually referenced to some convenient local reference point. Contemporary datums, based on increasingly accurate measurements of the shape of Earth, are intended to cover larger areas. The most common reference Datums in use in North America are NAD27, NAD83, and WGS 84.

The North American Datum of 1927 (NAD 27) is "the horizontal control datum for the United States that was defined by a location and azimuth on the Clarke spheroid of 1866, with origin at (the survey station) Meades Ranch (Kansas)." ... The geoidal height at Meades Ranch was assumed to be zero, as sufficient gravity data was not available, and this was needed to relate surface measurements to the datum. "Geodetic positions on the North American Datum of 1927 were derived from the (coordinates of and an azimuth at Meades Ranch) through a readjustment of the triangulation of the entire network in which Laplace azimuths were introduced, and the Bowie method was used." (http://www.ngs.noaa.gov/faq.shtml#WhatDatum ) NAD27 is a local referencing system covering North America.

The North American Datum of 1983 (NAD 83) is "The horizontal control datum for the United States, Canada, Mexico, and Central America, based on a geocentric origin and the Geodetic Reference System 1980 (GRS80). "This datum, designated as NAD 83 ...is based on the adjustment of 250,000 points including 600 satellite Doppler stations which constrain the system to a geocentric origin." NAD83 may be considered a local referencing system.

WGS 84 is the World Geodetic System of 1984. It is the reference frame used by the U.S. Department of Defense (DoD) and is defined by the National Geospatial-Intelligence Agency (NGA) (formerly the Defense Mapping Agency, then the National Imagery and Mapping Agency). WGS 84 is used by the DoD for all its mapping, charting, surveying, and navigation needs, including its GPS "broadcast" and "precise" orbits. WGS 84 was defined in January 1987 using Doppler satellite surveying techniques. It was used as the reference frame for broadcast GPS Ephemerides (orbits) beginning January 23, 1987. At 0000 GMT January 2, 1994, WGS 84 was upgraded in accuracy using GPS measurements. The formal name then became WGS 84 (G730), since the upgrade date coincided with the start of GPS Week 730. It became the reference frame for broadcast orbits on June 28, 1994. At 0000 GMT September 30, 1996 (the start of GPS Week 873), WGS 84 was redefined again and was more closely aligned with International Earth Rotation Service (IERS) frame ITRF 94. It was then formally called WGS 84 (G873). WGS 84 (G873) was adopted as the reference frame for broadcast orbits on January 29, 1997.[7] Another update brought it to WGS 84 (G1674).

The WGS 84 datum, within two meters of the NAD83 datum used in North America, is the only world referencing system in place today. WGS 84 is the default standard datum for coordinates stored in recreational and commercial GPS units.

Users of GPS are cautioned that they must always check the datum of the maps they are using. To correctly enter, display, and to store map related map coordinates, the datum of the map must be entered into the GPS map datum field.

Examples edit

Examples of map datums are:

Plate movement edit

The Earth's tectonic plates move relative to one another in different directions at speeds on the order of 50 to 100 mm (2.0 to 3.9 in) per year.[22] Therefore, locations on different plates are in motion relative to one another. For example, the longitudinal difference between a point on the equator in Uganda, on the African Plate, and a point on the equator in Ecuador, on the South American Plate, increases by about 0.0014 arcseconds per year.[citation needed] These tectonic movements likewise affect latitude.

If a global reference frame (such as WGS84) is used, the coordinates of a place on the surface generally will change from year to year. Most mapping, such as within a single country, does not span plates. To minimize coordinate changes for that case, a different reference frame can be used, one whose coordinates are fixed to that particular plate. Examples of these reference frames are "NAD83" for North America and "ETRS89" for Europe.

See also edit

Footnotes edit

  1. ^ The plural is not "data" in this case
  2. ^ About the right/left-handed order of the coordinates, i.e.,   or  , see Spherical coordinate system#Conventions.

References edit

  1. ^ Jensen, John R.; Jensen, Ryan R. (2013). Introductory Geographic Information Systems. Pearson. p. 25.
  2. ^ "Geoid—Help". ArcGIS for Desktop. from the original on 2017-02-02. Retrieved 2017-01-23.
  3. ^ "Datums—Help". ArcGIS for Desktop. from the original on 2017-02-02. Retrieved 2017-01-23.
  4. ^ (PDF). Intergovernmental Committee on Surveying and Mapping. 2 December 2014. Archived from the original (PDF) on 2018-03-20. Retrieved 2017-02-20.
  5. ^ . Archived from the original on 2017-07-04. Retrieved 2007-03-01.
  6. ^ McFadyen. "GPS - An Explanation of How it Works". Michael McFadyen's Scuba Diving Web Site. from the original on 2006-08-19.
  7. ^ "Frequently Asked Questions". National Geodetic Survey. from the original on 2011-10-19.
  8. ^ Craven, Alex. "GDA94 : Frequently Asked Questions". Geoproject Solutions. from the original on 2016-08-15.
  9. ^ "日本測地系2011(JGD2011)とは? - 空間情報クラブ". club.informatix.co.jp. 2015-08-20. from the original on 2016-08-20.
  10. ^ "座標変換ソフトウェア TKY2JGD|国土地理院". www.gsi.go.jp. from the original on 2017-11-05.
  11. ^ Yang, H.; Lee, Y.; Choi, Y.; Kwon, J.; Lee, H.; Jeong, K. (2007). "The Korean Datum Change to a World Geodetic System". AGU Spring Meeting Abstracts. 2007: G33B–03. Bibcode:2007AGUSM.G33B..03Y.
  12. ^ 台灣地圖夢想家-SunRiver. "大地座標系統與二度分帶座標解讀 - 上河文化". www.sunriver.com.tw. from the original on 2016-08-20.
  13. ^ Analysis of Conversion Method and Map Merging from BJS54 XA80 Surveying and Mapping Results to CGCS2000 2016-09-18 at the Wayback Machine
  14. ^ "The transition to using the terrestrial geocentric coordinate system "Parametry Zemli 1990" (PZ-90.11) in operating the GLObal NAvigation Satellite System (GLONASS) has been implemented". www.glonass-iac.ru. from the original on 2015-09-07.
  15. ^ a b "Use of international references for GNSS operations and applications" (PDF). unoosa.org. (PDF) from the original on 2017-12-22.
  16. ^ Handbook of Satellite Orbits: From Kepler to GPS, Table 14.2
  17. ^ BeiDou Navigation Satellite System Signal In Space Interface Control Document, Open Service Signal (Version 2.0) 2016-07-08 at the Wayback Machine section 3.2
  18. ^ "Archived copy" (PDF). (PDF) from the original on 2017-01-26. Retrieved 2016-08-19.{{cite web}}: CS1 maint: archived copy as title (link)
  19. ^ "General concepts". itrf.ensg.ign.fr. from the original on 2008-12-04.
  20. ^ "Vertical Datum used in China – Hong Kong – onshore". from the original on 2012-11-13.
  21. ^ (PDF). geodetic.gov.hk. Archived from the original (PDF) on 2016-11-09. Retrieved 2016-08-19.
  22. ^ Read HH, Watson Janet (1975). Introduction to Geology. New York: Halsted. pp. 13–15.

Further reading edit

  1. from University of Colorado
  2. Gaposchkin, E. M. and Kołaczek, Barbara (1981) Reference Coordinate Systems for Earth Dynamics Taylor & Francis ISBN 9789027712608
  3. Kaplan, Understanding GPS: principles and applications, 1 ed. Norwood, MA 02062, USA: Artech House, Inc, 1996.
  4. P. Misra and P. Enge, Global Positioning System Signals, Measurements, and Performance. Lincoln, Massachusetts: Ganga-Jamuna Press, 2001.
  5. – Large amount of technical information and discussion.
  6. US National Geodetic Survey

External links edit

  • GeographicLib includes a utility CartConvert which converts between geodetic and geocentric (ECEF) or local Cartesian (ENU) coordinates. This provides accurate results for all inputs including points close to the center of Earth.
  • A collection of geodetic functions that solve a variety of problems in geodesy in Matlab.
  • NGS FAQ – What is a geodetic datum?
  • About the surface of the Earth on kartoweb.itc.nl

geodetic, datum, geodetic, datum, geodetic, system, also, geodetic, reference, datum, geodetic, reference, system, geodetic, reference, frame, global, datum, reference, reference, frame, precisely, representing, position, locations, earth, other, planetary, bo. 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 planetary bodies by means of geodetic coordinates 1 Datums note 1 are crucial to any technology or technique based on spatial location including geodesy navigation surveying geographic information systems remote sensing and cartography A horizontal datum is used to measure a location across the Earth s surface in latitude and longitude or another coordinate system a vertical datum is used to measure the elevation or depth relative to a standard origin such as mean sea level MSL Since the rise of the global positioning system GPS the ellipsoid and datum WGS 84 it uses has supplanted most others in many applications The WGS 84 is intended for global use unlike most earlier datums Before GPS there was no precise way to measure the position of a location that was far from universal reference points such as from the Prime Meridian at the Greenwich Observatory for longitude from the Equator for latitude or from the nearest coast for sea level Astronomical and chronological methods have limited precision and accuracy especially over long distances Even GPS requires a predefined framework on which to base its measurements so WGS 84 essentially functions as a datum even though it is different in some particulars from a traditional standard horizontal or vertical datum A standard datum specification whether horizontal or vertical consists of several parts a model for Earth s shape and dimensions such as a reference ellipsoid or a geoid an origin at which the ellipsoid geoid is tied to a known often monumented location on or inside Earth not necessarily at 0 latitude 0 longitude and multiple control points that have been precisely measured from the origin and monumented Then the coordinates of other places are measured from the nearest control point through surveying Because the ellipsoid or geoid differs between datums along with their origins and orientation in space the relationship between coordinates referred to one datum and coordinates referred to another datum is undefined and can only be approximated Using local datums the disparity on the ground between a point having the same horizontal coordinates in two different datums could reach kilometers if the point is far from the origin of one or both datums This phenomenon is called datum shift Because Earth is an imperfect ellipsoid local datums can give a more accurate representation of some specific area of coverage than WGS 84 can OSGB36 for example is a better approximation to the geoid covering the British Isles than the global WGS 84 ellipsoid 2 However as the benefits of a global system outweigh the greater accuracy the global WGS 84 datum has become widely adopted 3 City of Chicago Datum BenchmarkContents 1 History 2 Dimensions 2 1 Horizontal datum 2 2 Vertical datum 3 Geodetic coordinates 4 Earth reference ellipsoid 4 1 Defining and derived parameters 4 2 Parameters for some geodetic systems 4 2 1 Geodetic Reference System 1980 GRS80 4 2 2 World Geodetic System 1984 WGS 84 5 Datum transformation 6 Discussion and examples 6 1 Examples 7 Plate movement 8 See also 9 Footnotes 10 References 11 Further reading 12 External linksHistory editMain articles History of geodesy Spherical Earth History and Earth s circumference History See also History of navigation History of longitude and History of latitude nbsp The Great Trigonometrical Survey of India one of the first surveys comprehensive enough to establish a geodetic datum The spherical nature of Earth was known by the ancient Greeks who also developed the concepts of latitude and longitude and the first astronomical methods for measuring them These methods preserved and further developed by Muslim and Indian astronomers were sufficient for the global explorations of the 15th and 16th Centuries However the scientific advances of the Age of Enlightenment brought a recognition of errors in these measurements and a demand for greater precision This led to technological innovations such as the 1735 Marine chronometer by John Harrison but also to a reconsideration of the underlying assumptions about the shape of Earth itself Isaac Newton postulated that the conservation of momentum should make Earth oblate wider at the equator while the early surveys of Jacques Cassini 1720 led him to believe Earth was prolate wider at the poles The subsequent French geodesic missions 1735 1739 to Lapland and Peru corroborated Newton but also discovered variations in gravity that would eventually lead to the geoid model A contemporary development was the use of the trigonometric survey to accurately measure distance and location over great distances Starting with the surveys of Jacques Cassini 1718 and the Anglo French Survey 1784 1790 by the end of the 18th century survey control networks covered France and the United Kingdom More ambitious undertakings such as the Struve Geodetic Arc across Eastern Europe 1816 1855 and the Great Trigonometrical Survey of India 1802 1871 took much longer but resulted in more accurate estimations of the shape of the Earth ellipsoid The first triangulation across the United States was not completed until 1899 The U S survey resulted in the North American Datum horizontal of 1927 NAD27 and the Vertical Datum of 1929 NAVD29 the first standard datums available for public use This was followed by the release of national and regional datums over the next several decades Improving measurements including the use of early satellites enabled more accurate datums in the later 20th century such as NAD83 in North America ETRS89 in Europe and GDA94 in Australia At this time global datums were also first developed for use in satellite navigation systems especially the World Geodetic System WGS 84 used in the U S global positioning system GPS and the International Terrestrial Reference System and Frame ITRF used in the European Galileo system Dimensions editHorizontal datum edit The horizontal datum is the model used to measure positions on Earth A specific point can have substantially different coordinates depending on the datum used to make the measurement There are hundreds of local horizontal datums around the world usually referenced to some convenient local reference point Contemporary datums based on increasingly accurate measurements of the shape of Earth are intended to cover larger areas The WGS 84 datum which is almost identical to the NAD83 datum used in North America and the ETRS89 datum used in Europe is a common standard datum citation needed Vertical datum edit Main article Vertical datum Further information Chart datum A vertical datum is a reference surface for vertical positions such as the elevations of Earth features including terrain bathymetry water level and human made structures An approximate definition of sea level is the datum WGS 84 an ellipsoid whereas a more accurate definition is Earth Gravitational Model 2008 EGM2008 using at least 2 159 spherical harmonics Other datums are defined for other areas or at other times ED50 was defined in 1950 over Europe and differs from WGS 84 by a few hundred meters depending on where in Europe you look Mars has no oceans and so no sea level but at least two martian datums have been used to locate places there Geodetic coordinates editMain article Geodetic coordinates Further information Geographic coordinate system nbsp The same position on a spheroid has a different angle for latitude depending on whether the angle is measured from the normal line segment CP of the ellipsoid angle a or the line segment OP from the center angle b The flatness of the spheroid orange in the image is greater than that of Earth as a result the corresponding difference between the geodetic and geocentric latitudes is also exaggerated In geodetic coordinates Earth s surface is approximated by an ellipsoid and locations near the surface are described in terms of geodetic latitude ϕ displaystyle phi nbsp longitude l displaystyle lambda nbsp and ellipsoidal height h displaystyle h nbsp note 2 Earth reference ellipsoid editMain article Reference ellipsoid Defining and derived parameters edit Further information Reference ellipsoid Ellipsoid parameters The ellipsoid is completely parameterised by the semi major axis a displaystyle a nbsp and the flattening f displaystyle f nbsp Parameter SymbolSemi major axis a displaystyle a nbsp Reciprocal of flattening 1f displaystyle frac 1 f nbsp From a displaystyle a nbsp and f displaystyle f nbsp it is possible to derive the semi minor axis b displaystyle b nbsp first eccentricity e displaystyle e nbsp and second eccentricity e displaystyle e nbsp of the ellipsoid Parameter ValueSemi minor axis b a 1 f displaystyle b a 1 f nbsp First eccentricity squared e2 1 b2a2 f 2 f displaystyle e 2 1 frac b 2 a 2 f 2 f nbsp Second eccentricity squared e 2 a2b2 1 f 2 f 1 f 2 displaystyle e 2 frac a 2 b 2 1 frac f 2 f 1 f 2 nbsp Parameters for some geodetic systems edit Main article Earth ellipsoid Historical Earth ellipsoids The two main reference ellipsoids used worldwide are the GRS80 4 and the WGS 84 5 A more comprehensive list of geodetic systems can be found here Geodetic Reference System 1980 GRS80 edit GRS80 parameters Parameter Notation ValueSemi major axis a displaystyle a nbsp 6378 137 mReciprocal of flattening 1f displaystyle frac 1 f nbsp 298 257222 101World Geodetic System 1984 WGS 84 edit The Global Positioning System GPS uses the World Geodetic System 1984 WGS 84 to determine the location of a point near the surface of Earth WGS 84 defining parameters Parameter Notation ValueSemi major axis a displaystyle a nbsp 6378 137 0 mReciprocal of flattening 1f displaystyle frac 1 f nbsp 298 257223 563WGS 84 derived geometric constants Constant Notation ValueSemi minor axis b displaystyle b nbsp 6356 752 3142 mFirst eccentricity squared e2 displaystyle e 2 nbsp 6 694379 990 14 10 3Second eccentricity squared e 2 displaystyle e 2 nbsp 6 739496 742 28 10 3Datum transformation editMain article Datum transformation The difference in co ordinates between datums is commonly referred to as datum shift The datum shift between two particular datums can vary from one place to another within one country or region and can be anything from zero to hundreds of meters or several kilometers for some remote islands The North Pole South Pole and Equator will be in different positions on different datums so True North will be slightly different Different datums use different interpolations for the precise shape and size of Earth reference ellipsoids For example in Sydney there is a 200 metres 700 feet difference between GPS coordinates configured in GDA based on global standard WGS 84 and AGD used for most local maps which is an unacceptably large error for some applications such as surveying or site location for scuba diving 6 Datum conversion is the process of converting the coordinates of a point from one datum system to another Because the survey networks upon which datums were traditionally based are irregular and the error in early surveys is not evenly distributed datum conversion cannot be performed using a simple parametric function For example converting from NAD27 to NAD83 is performed using NADCON later improved as HARN a raster grid covering North America with the value of each cell being the average adjustment distance for that area in latitude and longitude Datum conversion may frequently be accompanied by a change of map projection Discussion and examples editSee also Reference ellipsoid A geodetic reference datum is a known and constant surface which is used to describe the location of unknown points on Earth Since reference datums can have different radii and different center points a specific point on Earth can have substantially different coordinates depending on the datum used to make the measurement There are hundreds of locally developed reference datums around the world usually referenced to some convenient local reference point Contemporary datums based on increasingly accurate measurements of the shape of Earth are intended to cover larger areas The most common reference Datums in use in North America are NAD27 NAD83 and WGS 84 The North American Datum of 1927 NAD 27 is the horizontal control datum for the United States that was defined by a location and azimuth on the Clarke spheroid of 1866 with origin at the survey station Meades Ranch Kansas The geoidal height at Meades Ranch was assumed to be zero as sufficient gravity data was not available and this was needed to relate surface measurements to the datum Geodetic positions on the North American Datum of 1927 were derived from the coordinates of and an azimuth at Meades Ranch through a readjustment of the triangulation of the entire network in which Laplace azimuths were introduced and the Bowie method was used http www ngs noaa gov faq shtml WhatDatum NAD27 is a local referencing system covering North America The North American Datum of 1983 NAD 83 is The horizontal control datum for the United States Canada Mexico and Central America based on a geocentric origin and the Geodetic Reference System 1980 GRS80 This datum designated as NAD 83 is based on the adjustment of 250 000 points including 600 satellite Doppler stations which constrain the system to a geocentric origin NAD83 may be considered a local referencing system WGS 84 is the World Geodetic System of 1984 It is the reference frame used by the U S Department of Defense DoD and is defined by the National Geospatial Intelligence Agency NGA formerly the Defense Mapping Agency then the National Imagery and Mapping Agency WGS 84 is used by the DoD for all its mapping charting surveying and navigation needs including its GPS broadcast and precise orbits WGS 84 was defined in January 1987 using Doppler satellite surveying techniques It was used as the reference frame for broadcast GPS Ephemerides orbits beginning January 23 1987 At 0000 GMT January 2 1994 WGS 84 was upgraded in accuracy using GPS measurements The formal name then became WGS 84 G730 since the upgrade date coincided with the start of GPS Week 730 It became the reference frame for broadcast orbits on June 28 1994 At 0000 GMT September 30 1996 the start of GPS Week 873 WGS 84 was redefined again and was more closely aligned with International Earth Rotation Service IERS frame ITRF 94 It was then formally called WGS 84 G873 WGS 84 G873 was adopted as the reference frame for broadcast orbits on January 29 1997 7 Another update brought it to WGS 84 G1674 The WGS 84 datum within two meters of the NAD83 datum used in North America is the only world referencing system in place today WGS 84 is the default standard datum for coordinates stored in recreational and commercial GPS units Users of GPS are cautioned that they must always check the datum of the maps they are using To correctly enter display and to store map related map coordinates the datum of the map must be entered into the GPS map datum field Examples edit Examples of map datums are WGS 84 72 66 and 60 of the World Geodetic System NAD83 the North American Datum which is very similar to WGS 84 NAD27 the older North American Datum of which NAD83 was basically a readjustment 1 OSGB36 of the Ordnance Survey of Great Britain ETRS89 the European Datum related to ITRS ED50 the older European Datum GDA94 the Australian Datum 8 JGD2011 the Japanese Datum adjusted for changes caused by 2011 Tōhoku earthquake and tsunami 9 Tokyo97 the older Japanese Datum 10 KGD2002 the Korean Datum 11 TWD67 and TWD97 different datum currently used in Taiwan 12 BJS54 and XAS80 old geodetic datum used in China 13 GCJ 02 and BD 09 Chinese encrypted geodetic datum PZ 90 11 the current geodetic reference used by GLONASS 14 GTRF the geodetic reference used by Galileo currently defined as ITRF2005 15 CGCS2000 or CGS 2000 the geodetic reference used by BeiDou Navigation Satellite System based on ITRF97 15 16 17 International Terrestrial Reference Frames ITRF88 89 90 91 92 93 94 96 97 2000 2005 2008 2014 different realizations of the ITRS 18 19 Hong Kong Principal Datum a vertical datum used in Hong Kong 20 21 SAD69 South American Datum 1969Plate movement editThe Earth s tectonic plates move relative to one another in different directions at speeds on the order of 50 to 100 mm 2 0 to 3 9 in per year 22 Therefore locations on different plates are in motion relative to one another For example the longitudinal difference between a point on the equator in Uganda on the African Plate and a point on the equator in Ecuador on the South American Plate increases by about 0 0014 arcseconds per year citation needed These tectonic movements likewise affect latitude If a global reference frame such as WGS84 is used the coordinates of a place on the surface generally will change from year to year Most mapping such as within a single country does not span plates To minimize coordinate changes for that case a different reference frame can be used one whose coordinates are fixed to that particular plate Examples of these reference frames are NAD83 for North America and ETRS89 for Europe See also editAxes conventions ECEF ECI coordinates Engineering datum Figure of the Earth Geographic coordinate conversion Grid reference International Terrestrial Reference System Kilometre zero Local tangent plane coordinates Ordnance Datum Milestone Planetary coordinate system Reference frame World Geodetic SystemFootnotes edit The plural is not data in this case About the right left handed order of the coordinates i e l ϕ displaystyle lambda phi nbsp or ϕ l displaystyle phi lambda nbsp see Spherical coordinate system Conventions References edit Jensen John R Jensen Ryan R 2013 Introductory Geographic Information Systems Pearson p 25 Geoid Help ArcGIS for Desktop Archived from the original on 2017 02 02 Retrieved 2017 01 23 Datums Help ArcGIS for Desktop Archived from the original on 2017 02 02 Retrieved 2017 01 23 Geocentric Datum of Australia Technical Manual PDF Intergovernmental Committee on Surveying and Mapping 2 December 2014 Archived from the original PDF on 2018 03 20 Retrieved 2017 02 20 NGA DoD World Geodetic System 1984 Archived from the original on 2017 07 04 Retrieved 2007 03 01 McFadyen GPS An Explanation of How it Works Michael McFadyen s Scuba Diving Web Site Archived from the original on 2006 08 19 Frequently Asked Questions National Geodetic Survey Archived from the original on 2011 10 19 Craven Alex GDA94 Frequently Asked Questions Geoproject Solutions Archived from the original on 2016 08 15 日本測地系2011 JGD2011 とは 空間情報クラブ club informatix co jp 2015 08 20 Archived from the original on 2016 08 20 座標変換ソフトウェア TKY2JGD 国土地理院 www gsi go jp Archived from the original on 2017 11 05 Yang H Lee Y Choi Y Kwon J Lee H Jeong K 2007 The Korean Datum Change to a World Geodetic System AGU Spring Meeting Abstracts 2007 G33B 03 Bibcode 2007AGUSM G33B 03Y 台灣地圖夢想家 SunRiver 大地座標系統與二度分帶座標解讀 上河文化 www sunriver com tw Archived from the original on 2016 08 20 Analysis of Conversion Method and Map Merging from BJS54 XA80 Surveying and Mapping Results to CGCS2000 Archived 2016 09 18 at the Wayback Machine The transition to using the terrestrial geocentric coordinate system Parametry Zemli 1990 PZ 90 11 in operating the GLObal NAvigation Satellite System GLONASS has been implemented www glonass iac ru Archived from the original on 2015 09 07 a b Use of international references for GNSS operations and applications PDF unoosa org Archived PDF from the original on 2017 12 22 Handbook of Satellite Orbits From Kepler to GPS Table 14 2 BeiDou Navigation Satellite System Signal In Space Interface Control Document Open Service Signal Version 2 0 Archived 2016 07 08 at the Wayback Machine section 3 2 Archived copy PDF Archived PDF from the original on 2017 01 26 Retrieved 2016 08 19 a href Template Cite web html title Template Cite web cite web a CS1 maint archived copy as title link General concepts itrf ensg ign fr Archived from the original on 2008 12 04 Vertical Datum used in China Hong Kong onshore Archived from the original on 2012 11 13 Explanatory Notes on Geodetic Datums in Hong Kong PDF geodetic gov hk Archived from the original PDF on 2016 11 09 Retrieved 2016 08 19 Read HH Watson Janet 1975 Introduction to Geology New York Halsted pp 13 15 Further reading editList of geodetic parameters for many systems from University of Colorado Gaposchkin E M and Kolaczek Barbara 1981 Reference Coordinate Systems for Earth Dynamics Taylor amp Francis ISBN 9789027712608 Kaplan Understanding GPS principles and applications 1 ed Norwood MA 02062 USA Artech House Inc 1996 GPS Notes P Misra and P Enge Global Positioning System Signals Measurements and Performance Lincoln Massachusetts Ganga Jamuna Press 2001 Peter H Dana Geodetic Datum Overview Large amount of technical information and discussion US National Geodetic SurveyExternal links edit nbsp Look up datum in Wiktionary the free dictionary GeographicLib includes a utility CartConvert which converts between geodetic and geocentric ECEF or local Cartesian ENU coordinates This provides accurate results for all inputs including points close to the center of Earth A collection of geodetic functions that solve a variety of problems in geodesy in Matlab NGS FAQ What is a geodetic datum About the surface of the Earth on kartoweb itc nl Retrieved from https en wikipedia org w index php title Geodetic datum amp oldid 1192834348, wikipedia, wiki, book, books, library,

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