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Encyclopedia > World Geodetic System

The World Geodetic System defines a reference frame for the earth, for use in geodesy and navigation. The latest revision is WGS 84 dating from 1984 (last revised in 2004), which will be valid up to about 2010. It has been suggested that geodetic system be merged into this article or section. ... Table of geography, hydrography, and navigation, from the 1728 Cyclopaedia. ... This article does not cite any references or sources. ...


Earlier schemes included WGS 72, WGS 66 and WGS 60.

Contents

History of the World Geodetic System

Efforts to supplement the various national surveying systems began in the 19th century with F.R. Helmert's famous books Mathematische und Physikalische Theorien der Physikalischen Geodäsie. Austria and Germany initiated the foundation of a Central Bureau of "Internationale Erdmessung", and a series of global ellipsoids of the Earth were derived (e.g. Helmert 1906, Hayford 1910/ 1924). Surveyor at work with a leveling instrument. ... Friedrich Robert Helmert (* July 31, 1843 in Freiberg, Saxonia; † June 15, 1917 in Potsdam) was a celebrated German geodesist and an important writer on the theory of errors. ... It has been suggested that geodetic system be merged into this article or section. ... 3D rendering of an ellipsoid In mathematics, an ellipsoid is a type of quadric that is a higher dimensional analogue of an ellipse. ... John Fillmore Hayford (1868-1925), eminent United States geodesist. ... 1924 (MCMXXIV) was a leap year starting on Tuesday (link will display the full calendar). ...


A unified World Geodetic System became essential in the 1950s for several reasons: Year 1950 (MCML) was a common year starting on Sunday (link will display the full calendar) of the Gregorian calendar. ...

In the late 1950s the United States DOD, together with scientists of other institutions and countries, began to develop the needed world system to which geodetic datums could be referred and compatibility established between the coordinates of widely separated sites of interest. Efforts of the U.S. Army, Navy and Air Force were combined leading to the DoD World Geodetic System 1960 (WGS 60). The term datum as used here refers to a smooth surface somewhat arbitrarily defined as "zero elevation," consistent with a set of surveyor's measures of distances between various stations, and differences in elevation, all reduced to a grid of latitudes, longitudes, and elevations. Heritage surveying methods found elevation differences off a local horizontal determined by the spirit level, plumb line, or an equivalent device that depends on the local gravity field (see physical geodesy). As a result, the elevations in the datums are referenced to the geoid, a surface that is not readily found using satellite geodesy. The latter observational method is more suitable for global mapping. Therefore, a motivation, and a substantial problem in the WGS and similar work is to patch together datums that were not only made separately, for different regions, but to re-reference the elevations to an ellipsoid model rather than to the geoid. Space science, or the space sciences, are fields of science that are concerned with the study or utilization of outer space. ... Astronautics is the branch of engineering that deals with machines designed to work outside of Earths atmosphere, whether manned or unmanned. ... It has been suggested that this article or section be merged with Datum. ... ED 50 (European Datum 1950) is a geodetic datum which was defined after World War II for the international connection of geodetic networks. ... The North American Datum is the official reference ellipsoid used for the primary geodetic network in North America. ... The adjective global and adverb globally imply that the verb or noun to which they are applied applies to the entire Earth and all of its species and regions. ... Table of geography, hydrography, and navigation, from the 1728 Cyclopaedia. ... The United States Department of Defense (DOD or DoD) is the federal department charged with coordinating and supervising all agencies and functions of the government relating directly to national security and the military. ... This article does not cite any references or sources. ... Latitude,usually denoted symbolically by the Greek letter phi, , gives the location of a place on Earth north or south of the equator. ... Longitude, sometimes denoted by the Greek letter λ (lambda),[1][2] describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. ... Basic Definition In geography, the elevation of a geographic location is its height above mean sea level (or some other fixed point). ... A spirit level A spirit level or bubble level is an instrument designed to indicate whether a surface is level or plumb. ... It has been suggested that this article or section be merged with plumb line. ... Definition Physical geodesy is the study of the physical properties of the gravity field of the Earth, the geopotential, with a view to their application in geodesy. ... The GOCE project will measure high-accuracy gravity gradients and provide an accurate geoid model based on the Earths gravity field. ... 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 Earths gravity field by means of satellite techniques. ... The GOCE project will measure high-accuracy gravity gradients and provide an accurate geoid model based on the Earths gravity field. ...


In accomplishing WGS 60, a combination of available surface gravity data, astro-geodetic data and results from HIRAN and Canadian SHORAN surveys were used to define a best-fitting ellipsoid and an earth-centered orientation for each of the initially selected datums (Chapter IV). (The datums are relatively oriented with respect to different portions of the geoid by the astro-geodetic methods already described.) The sole contribution of satellite data to the development of WGS 60 was a value for the ellipsoid flattening which was obtained from the nodal motion of a satellite. Gravity is a force of attraction that acts between bodies that have mass. ... Astro-geodetic methods are a group of important methods in geodesy, satellite techniques and astrometry. ... 3D rendering of an ellipsoid In mathematics, an ellipsoid is a type of quadric that is a higher dimensional analogue of an ellipse. ... An Earth observation satellite, ERS 2 For other uses, see Satellite (disambiguation). ... 3D rendering of an ellipsoid In mathematics, an ellipsoid is a type of quadric that is a higher dimensional analogue of an ellipse. ...

Gravimetric datum orientation

Prior to WGS 60, the U.S. Army and U.S. Air Force had each developed a world system by using different approaches to the gravimetric datum orientation method. To determine their gravimetric orientation parameters, the Air Force used the mean of the differences between the gravimetric and astro-geodetic deflections and geoid heights (undulations) at specifically selected stations in the areas of the major datums. The Army performed an adjustment to minimize the difference between astro-geodetic and gravimetric geoids. By matching the relative astro-geodetic geoids of the selected datums with an earth-centered gravimetric geoid, the selected datums were reduced to an earth-centered orientation. Since the Army and Air Force systems agreed remarkably well for the NAD, ED and TD areas, they were consolidated and became WGS 60. Image File history File links Size of this preview: 564 × 600 pixelsFull resolution (650 × 691 pixel, file size: 39 KB, MIME type: image/gif) improved image quality File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Size of this preview: 564 × 600 pixelsFull resolution (650 × 691 pixel, file size: 39 KB, MIME type: image/gif) improved image quality File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Seal of the Air Force. ... The vertical deflections (deflections of the plumb line, astro-geodetic deflections) are important parameters of the local gravity field. ... Gravimetry is the measurement of gravitational force, weight, or density. ... The GOCE project will measure high-accuracy gravity gradients and provide an accurate geoid model based on the Earths gravity field. ...


The Department of Defense World Geodetic System 1966

Steps to the improvement of a global system were the Astrogeoid of Irene Fischer and the astronautic Mercury datum. In January 1966, a World Geodetic System Committee composed of representatives from the United States Army, Navy and Air Force, was charged with the responsibility of developing an improved WGS needed to satisfy mapping, charting and geodetic requirements. Additional surface gravity observations, results from the extension of triangulation and trilateration networks, and large amounts of Doppler and optical satellite data had become available since the development of WGS 60. Using the additional data and improved techniques, WGS 66 was produced which served DoD needs for about five years after its implementation in 1967. The defining parameters of the WGS 66 Ellipsoid were the flattening (1/298.25), determined from satellite data and the semimajor axis (6,378,145 meters), determined from a combination of Doppler satellite and astro-geodetic data. A worldwide 5° × 5° mean free air gravity anomaly field provided the basic data for producing the WGS 66 gravimetric geoid. Also, a geoid referenced to the WGS 66 Ellipsoid was derived from available astrogeodetic data to provide a detailed representation of limited land areas. The word mapping has several senses: In mathematics and related technical fields, it is some kind of function: see map (mathematics). ... Triangulation can be used to find the distance from the shore to the ship. ... Standing at B, you want to know your location relative to the reference points P1, P2, and P3. ... This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... See also list of optical topics. ... Gravity anomalies are widely used in geodesy and geophysics. ...


The Department of Defense World Geodetic System 1972

After an extensive effort extending over a period of approximately three years, the Department of Defense World Geodetic System 1972 was completed. Selected satellite, surface gravity and astrogeodetic data available through 1972 from both DoD and non-DoD sources were used in a Unified WGS Solution (a large scale least squares adjustment). The results of the adjustment consisted of corrections to initial station coordinates and coefficients of the gravitational field. In regression analysis, least squares, also known as ordinary least squares analysis is a method for linear regression that determines the values of unknown quantities in a statistical model by minimizing the sum of the residuals (difference between the predicted and observed values) squared. ...


The largest collection of data ever used for WGS purposes was assembled, processed and applied in the development of WGS 72. Both optical and electronic satellite data were used. The electronic satellite data consisted, in part, of Doppler data provided by the U.S. Navy and cooperating non-DoD satellite tracking stations established in support of the Navy's Navigational Satellite System (NNSS). Doppler data was also available from the numerous sites established by GEOCEIVERS during 1971 and 1972. Doppler data was the primary data source for WGS 72 (Figure 38). Additional electronic satellite data was provided by the SECOR (Sequential Collation of Range) Equatorial Network completed by the U.S. Army in 1970. Optical satellite data from the Worldwide Geometric Satellite Triangulation Program was provided by the BC-4 camera system (Figure 39). Data from the Smithsonian Astrophysical Observatory was also used which included camera (Baker Nunn) and some laser ranging.

DOPPLER SATELLITE GROUND STATIONS PROVIDING DATA FOR WGS 72 DEVELOPMENT
WORLDWIDE GEOMETRIC SATELLITE TRIANGULATION NETWORK, BC-4 CAMERAS

The surface gravity field used in the Unified WGS Solution consisted of a set of 410 10° × 10° equal area mean free air gravity anomalies determined solely from terrestrial data. This gravity field includes mean anomaly values compiled directly from observed gravity data wherever the latter was available in sufficient quantity. The value for areas of sparse or no observational data were developed from geophysically compatible gravity approximations using gravity-geophysical correlation techniques. Approximately 45 percent of the 410 mean free air gravity anomaly values were determined directly from observed gravity data. This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ...


The astrogeodetic data in its basic form consists of deflection of the vertical components referred to the various national geodetic datums. These deflection values were integrated into astrogeodetic geoid charts referred to these national datums. The geoid heights contributed to the Unified WGS Solution by providing additional and more detailed data for land areas. Conventional ground survey data was included in the solution to enforce a consistent adjustment of the coordinates of neighboring observation sites of the BC-4, SECOR, Doppler and Baker-Nunn systems. Also, eight geodimeter long line precise traverses were included for the purpose of controlling the scale of the solution.


The Unified WGS Solution, as stated above, was a solution for geodetic positions and associated parameters of the gravitational field based on an optimum combination of available data. The WGS 72 ellipsoid parameters, datum shifts and other associated constants were derived separately. For the unified solution, a normal equation matrix was formed based on each of the mentioned data sets. Then, the individual normal equation matrices were combined and the resultant matrix solved to obtain the positions and the parameters.


The value for the semimajor axis (a) of the WGS 72 Ellipsoid is 6378135 meters. The adoption of an a-value 10 meters smaller than that for the WGS 66 Ellipsoid was based on several calculations and indicators including a combination of satellite and surface gravity data for position and gravitational field determinations. Sets of satellite derived station coordinates and gravimetric deflection of the vertical and geoid height data were used to determine local-to-geocentric datum shifts, datum rotation parameters, a datum scale parameter and a value for the semimajor axis of the WGS Ellipsoid. Eight solutions were made with the various sets of input data, both from an investigative point of view and also because of the limited number of unknowns which could be solved for in any individual solution due to computer limitations. Selected Doppler satellite tracking and astro-geodetic datum orientation stations were included in the various solutions. Based on these results and other related studies accomplished by the Committee, an a-value of 6378135 meters and a flattening of 1/298.26 were adopted.


In the development of local-to WGS 72 datum shifts, results from different geodetic disciplines were investigated, analyzed and compared. Those shifts adopted were based primarily on a large number of Doppler TRANET and GEOCEIVER station coordinates which were available worldwide. These coordinates had been determined using the Doppler point positioning method.


A new World Geodetic System: WGS84

In the early 1980s the need for a new world geodetic system was generally recognized by the geodetic community, also within the Department of Defense. WGS 72 no longer provided sufficient data, information, geographic coverage, or product accuracy for all then current and anticipated applications. The means for producing a new WGS were available in the form of improved data, increased data coverage, new data types and improved techniques. GRS 80 parameters together with available Doppler, satellite laser ranging and VLBI (Very Long Baseline Interferometry) observations constituted significant new information. Also, an outstanding new source of data had become available from satellite radar altimetry. Also available was an advanced least squares method called collocation which allowed for a consistent combination solution from different types of measurements all relative to the Earth's gravity field, i.e. geoid, gravity anomalies, deflections, dynamic Doppler, etc. Definition GRS 80, or Geodetic Reference System 1980, is a geodetic reference system consisting of a global reference ellipsoid and a gravity field model. ... This article needs cleanup. ...


The new World Geodetic System was called WGS 84. It is currently the reference system being used by the Global Positioning System. It is geocentric and globally consistent within ±1 m. Current geodetic realizations of the geocentric reference system family ITRS (International Terrestrial Reference System) maintained by the IERS are geocentric, and internally consistent, at the few-cm level, while still being metre-level consistent with WGS 84. The Global Positioning System (GPS) is currently the only fully functional Global Navigation Satellite System (GNSS). ... The International Terrestrial Reference System (ITRS) describes procedures for creating reference frames suitable for use with measurements on or near the Earths surface. ... The International Earth Rotation Service (IERS) is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Paramater (EOP) and International Celestial Reference System (ICRS) groups. ...


The WGS 84 originally used the GRS 80 reference ellipsoid, but has undergone some minor refinements in later editions since its initial publication. Most of these refinements are important for high-precision orbital calculations for satellites, but have little practical effect on typical topographical uses. The following table lists the primary ellipsoid parameters. In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body. ... Earth orbit is an orbit around the planet Earth. ...

Ellipsoid
reference
Semi-major
axis a
Semi-minor
axis b
Inverse flattening
(1/f)
 GRS 80 6,378,137.0 m 6,356,752.314 140 m   298.257 222 101
 WGS 84 6,378,137.0 m 6,356,752.314 245 m   298.257 223 563
 "WGRS 80/84"  6,378,137.0 m   6,356,752.3 m 298.257

The very small difference in the flattening thus results in a—very theoretical—difference of 105 µm in the semi polar axis. For most purposes, the differing polar axes can be merged to 6,356,752.3 m, with the inverse flattening rounded to 298.257.


Longitudes on WGS84

WGS84 uses the zero meridian as defined by the Bureau International de l'Heure,[1] which was defined by compilation of star observations in different countries. The mean of this data caused a shift of about 100 metres east away from the Prime Meridian at Greenwich, UK.[2] The Prime Meridian, Greenwich The Prime Meridian is the meridian (line of longitude) passing through the Royal Greenwich Observatory, Greenwich, England; it is the meridian at which longitude is 0 degrees. ... The Bureau International de lHeure (BIH) or the International Time Bureau, seated at the Paris Observatory, was the international bureau responsible for combining different measurements of Universal Time. ... Location of the Prime Meridian Prime Meridian in Greenwich The Prime Meridian, also known as the International Meridian or Greenwich Meridian, is the meridian (line of longitude) passing through the Royal Greenwich Observatory, Greenwich, England — it is the meridian at which longitude is 0 degrees. ... Greenwich is a town, now part of the south eastern urban sprawl of London, England, on the south bank of the River Thames in the London Borough of Greenwich. ...


The longitude positions on WGS84 agree with those on the older North American Datum 1927 at roughly 85° longitude west, in the east-central USA. The North American Datum is the official reference ellipsoid used for the primary geodetic network in North America. ...


Updates and New Standards

The latest major revision of WGS 84 is also referred to as "Earth Gravity Model 1996" (EGM96), first published in 1996, with revisions as recent as 2004. This model has the same reference ellipsoid as WGS 84, but has a higher-fidelity geoid (roughly 100 km resolution versus 200 km for the original WGS 84). Year 1996 (MCMXCVI) was a leap year starting on Monday (link will display full 1996 Gregorian calendar). ... EGM96 (or Earth Geopotential Model 1996) is a geopotential model of the Earth consisting of spherical harmonic coefficients complete to degree and order 360. ...


Many of the original authors of WGS 84 are currently working on a new higher fidelity model, tentatively called EGM06. This new model will have a geoid with a resolution approaching 10 km, requiring over 4.6 million terms in the spherical expansion (versus 130,317 in EGM96 and 32,757 in WGS 84).


Notes

  1. ^ European Organisation for the Safety of Air Navigation and IfEN: "WGS 84 Implementation Manual", page 13. 1998
  2. ^ National Marine Museum: "The Longitude of Greenwich"

EUROCONTROL is the European Organisation for the Safety of Air Navigation, an international organisation whose primary objective is the development of a seamless, pan-European Air Traffic Management (ATM) system. ...

See also

Over fifty GPS satellites such as this NAVSTAR have been launched since 1978. ... The North American Datum is the official reference ellipsoid used for the primary geodetic network in North America. ... The European Terrestrial Reference System 1989 is a three-dimensional gedoesic frame of reference. ... EGM96 (or Earth Geopotential Model 1996) is a geopotential model of the Earth consisting of spherical harmonic coefficients complete to degree and order 360. ... Geo is a microformat used for marking up WGS84 geographical coordinates (latitude;longitude) in (X)HTML. This allows parsing tools (for example other websites, or Firefoxs Operator extension) to extract the locations, and display them using some other website or mapping tool, or to load them into a GPS... A point of interest, or POI, is a specific point location that someone may find useful or interesting. ...

External links

  • NIMA Technical Report TR8350.2 Department of Defense World Geodetic System 1984, Its Definition and Relationships With Local Geodetic Systems, Third Edition, National Geospatial-Intelligence Agency. This is the official publication of the standard, including addenda. Note this report actually documents the EGM 96 model (a revision of WGS 84). The original WGS 84 is documented in versions prior to 1996.

The first version of this text was taken from the public domain booklet Geodesy for the Layman at http://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/TR80003E.HTM#ZZ11 -- please Wikify as necessary. This document was written in 1984 and may need to be updated. The public domain comprises the body of all creative works and other knowledge—writing, artwork, music, science, inventions, and others—in which no person or organization has any proprietary interest. ...


  Results from FactBites:
 
World Geodetic System - Wikipedia, the free encyclopedia (1839 words)
To determine their gravimetric orientation parameters, the Air Force used the mean of the differences between the gravimetric and astro-geodetic deflections and geoid heights (undulations) at specifically selected stations in the areas of the major datums.
In January 1966, a World Geodetic System Committee composed of representatives from the Army, Navy and Air Force, was charged with the responsibility of developing an improved WGS needed to satisfy mapping, charting and geodetic requirements.
Current geodetic realizations of the geocentric reference system family ITRS (International Terrestrial Reference System) maintained by the IERS are geocentric, and internally consistent, at the few-cm level, while still being metre-level consistent with WGS 84.
Geodetic system - Wikipedia, the free encyclopedia (309 words)
Geodetic systems or geodetic datums are used in geodesy, navigation, surveying by cartographers and satellite navigation systems to translate positions indicated on their products to their real position on earth.
The systems are needed because the earth is not a perfect sphere.
The main reason that there are a number of datums is that before the advent of GPS positioning, national map making organisations did not have a common surveying reference point and only produced maps for their locality.
  More results at FactBites »

 
 

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