Definition The coordinate origin of WGS 84 is meant to be located at the Earth's center of mass; the uncertainty is believed to be less than 2 cm . Currently, WGS 84 uses the Earth Gravitational Model 2008. This geoid defines the nominal sea level surface by means of a spherical harmonics series of degree 2160. It is an improvement over the 1996 EGM96 model, which was in turn an improvement over the original WGS 84 geoid, referred to as EGM84. The deviations of the EGM96 geoid from the WGS 84 reference ellipsoid range from about −105 m to about +85 m . 1966 version Improvements to the global system included 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 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 value for the semimajor axis ( a ) of the WGS 72 Ellipsoid is 6 378 135 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 6 378 135 meters and a flattening of 1/298.26 were adopted. The new world geodetic system was called WGS 84. It is the reference system used by the Global Positioning System. It is geocentric and globally consistent within ±1 m . Current geodetic realizations of the geocentric reference system family International Terrestrial Reference System (ITRS) maintained by the IERS are geocentric, and internally consistent, at the few-cm level, while still being metre-level consistent with WGS 84. 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 298.257 222 100 882 711... WGS 84 6 378 137.0 m ≈ 6 356 752.314 245 m 298.257 223 563 upright=1.15|Equatorial ( a ), polar ( b ) and mean Earth radii as defined in the 1984 World Geodetic System revision (not to scale) The very small difference in the flattening thus results in a tiny difference of 0.105 mm in the semi polar axis. The geoid component of WGS84 is being continuously updated as a separate Earth Gravitational Model (EGM). The Earth Gravitational Model 1996 (EGM96) was first published in 1996, with revisions as recent as 2004. The geoid has roughly 100 km resolution versus 200 km for the original WGS 84. The use of EGM96 was confirmed in the 1997 update. Many of the original authors of WGS 84 contributed to a new higher-fidelity model, called EGM2008. This new model has a geoid with accuracy approaching 10 cm , requiring over 4.6 million terms in the spherical expansion (versus 130,317 in EGM96 and 32,757 in WGS 84). The use of EGM2008 was confirmed in the 2014 update. An update was planned for EGM2020.
Definition The coordinate origin of WGS 84 is meant to be located at the Earth's center of mass; the uncertainty is believed to be less than . Currently, WGS 84 uses the Earth Gravitational Model 2008. This geoid defines the nominal sea level surface by means of a spherical harmonics series of degree 2160. It is an improvement over the 1996 EGM96 model, which was in turn an improvement over the original WGS 84 geoid, referred to as EGM84. The deviations of the EGM96 geoid from the WGS 84 reference ellipsoid range from about to about . 1966 version Improvements to the global system included 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 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 ( 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 value for the semimajor axis ( ) of the WGS 72 Ellipsoid is . The adoption of an -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 -value of and a flattening of 1/298.26 were adopted. The new world geodetic system was called WGS 84. It is the reference system used by the Global Positioning System. It is geocentric and globally consistent within . Current geodetic realizations of the geocentric reference system family International Terrestrial Reference System (ITRS) maintained by the IERS are geocentric, and internally consistent, at the few-cm level, while still being metre-level consistent with WGS 84. Ellipsoid reference Semi-major axis Semi-minor axis Inverse flattening 1/ GRS 80 ≈ ... WGS 84 ≈ upright=1.15|Equatorial ( ), polar ( ) and mean Earth radii as defined in the 1984 World Geodetic System revision (not to scale) The very small difference in the flattening thus results in a tiny difference of in the semi polar axis. The geoid component of WGS84 is being continuously updated as a separate Earth Gravitational Model (EGM). The Earth Gravitational Model 1996 (EGM96) was first published in 1996, with revisions as recent as 2004. The geoid has approximately resolution versus for the original WGS 84. The use of EGM96 was confirmed in the 1997 update. Many of the original authors of WGS 84 contributed to a new higher-fidelity model, called EGM2008. This new model has a geoid with accuracy approaching , requiring over 4.6 million terms in the spherical expansion (versus 130,317 in EGM96 and 32,757 in WGS 84). The use of EGM2008 was confirmed in the 2014 update. An update was planned for EGM2020.
D
2 cm
R
−105 m to about +85 m
to about
D
6,378,145 meters
D
a
D
6 378 135 meters
R
a-value
-value
R
a-value of 6 378 135 meters
-value of
D
±1 m
D
a
R
b Inverse flattening (
Inverse flattening
D
f)
D
6 378 137.0 m
R
6 356 752.314 140 m 298.257 222 100 882 711...
...
D
6 378 137.0 m
D
6 356 752.314 245 m 298.257 223 563
D
a
D
b
D
0.105 mm
R
roughly 100 km resolution versus 200 km
approximately resolution versus
D
10 cm