Geodetic datum
Geodesy |
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A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame) is a global
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
A standard datum specification (whether horizontal or vertical) consists of several parts: a model for Earth's shape and dimensions, such as a
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]
History
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
However, the scientific advances of the
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
Dimensions
Horizontal datum
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
Vertical 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
Geodetic coordinates
In geodetic coordinates, Earth's surface is approximated by an
Earth reference ellipsoid
Defining and derived parameters
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
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)
Parameter | Notation | Value |
---|---|---|
Semi-major axis | 6378137 m | |
Reciprocal of flattening | 298.257222101 |
World Geodetic System 1984 (WGS 84)
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.
Parameter | Notation | Value |
---|---|---|
Semi-major axis | 6378137.0 m | |
Reciprocal of flattening | 298.257223563 |
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
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
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
Discussion and examples
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
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 (
WGS 84 is the
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
Examples of map datums are:
- WGS 84, 72, 66 and 60 of the World Geodetic System
- NAD83, the North American Datumwhich 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]
- Hong Kong Principal Datum, a vertical datum used in Hong Kong.[20][21]
- SAD69- South American Datum 1969
Plate movement
The Earth's
If a global reference frame (such as
See also
- Axes 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 System
Footnotes
- ^ The plural is not "data" in this case
- ^ About the right/left-handed order of the coordinates, i.e., or , see Spherical coordinate system#Conventions.
References
- ^ 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.
- 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.
{{cite web}}
: 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
- List of geodetic parameters for many systems from University of Colorado
- Gaposchkin, E. M. and Kołaczek, Barbara (1981) Reference Coordinate Systems for Earth Dynamics Taylor & 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 Survey
External links
- 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