Note on WGS84 coordinate reference systems and datums
WGS84 is an Earth-centered, Earth-fixed terrestrial reference system and geodetic datum. WGS84 is based on a consistent set of constants and model parameters that describe the Earth's size, shape, and gravity and geomagnetic fields. WGS84 is the standard U.S. Department of Defense definition of a global reference system for geospatial information and is the reference system for the Global Positioning System (GPS). It is compatible with the International Terrestrial Reference System (ITRS). The current realization WGS84 (G1762) follows the criteria outlined in the International Earth Rotation Service (IERS) Technical Note 21 (TN 21). The responsible organization is the National Geospatial-Intelligence Agency (NGA). NGA plans to conduct a WGS84 reference frame network adjustment in 2013 to incorporate IERS Conventions 2010 Technical Note 36 (TN 36).
- Origin: Earth’s center of mass being defined for the whole Earth including oceans and atmosphere.
- Z-Axis: The direction of the IERS Reference Pole (IRP). This direction corresponds to the direction of the BIH Conventional Terrestrial Pole (CTP) (epoch 1984.0) with an uncertainty of 0.005".
- X-Axis: Intersection of the IERS Reference Meridian (IRM) and the plane passing through the origin and normal to the Z-axis. The IRM is coincident with the BIH Zero Meridian (epoch 1984.0) with an uncertainty of 0.005".
- Y-Axis: Completes a right-handed, Earth-Centered Earth-Fixed (ECEF) orthogonal coordinate system.
- Scale: Its scale is that of the local Earth frame, in the meaning of a relativistic theory of gravitation. Aligns with ITRS.
- Orientation: Given by the Bureau International de l’Heure (BIH) orientation of 1984.0.
- Time Evolution: Its time evolution in orientation will create no residual global rotation with regards to the crust.
WGS84 identifies four defining parameters. These are the semi-major axis of the WGS84 ellipsoid, the flattening factor of the Earth, the nominal mean angular velocity of the Earth, and the geocentric gravitational constant as specified below.
Flattening Factor of the Earth
Nominal Mean Angular Velocity
7292115 10-11 rad/s
Geocentric Gravitational Constant
|GM||3986004.418 108 m3/s2|
The value of GM includes the mass of the Earth's atmosphere. GPS users should retain the original WGS84 GM value of 3986005.0 108 m3/s2 as specified in the GPS interface control document (ICD-GPS-200) and the NIMA Technical Report 8350.2.
Both the EPSG database and the NGS website use 'WGS 84' with spaces between 'WGS' and '84'. The EPSG database contains no specific WGS84 datum realizations.
|Geog 2D Code||Datum Code||Short Name||Datum Epoch||Area Code||Area Name||Remarks||Shift|
First realization established by DoD in 1987 using Doppler observations.
|WGS84 (G730)||1994.0||Realization introduced by DoD on 1994-06-29 based on GPS observations. |
G stands for 'GPS' and 730 is GPS week number. Based on ITRF91.
Realization introduced by DoD on 1997-01-29 based on GPS observations.
|WGS84 (G1150)||2001.0||Realization introduced by DoD on 2002-01-20 based on GPS observations.|
G stands for 'GPS' and 1150 is GPS week number. Based on ITRF2000.
|WGS84 (G1674)||2005.0||Realization introduced by DoD on 2012-02-08 based on GPS observations.|
G stands for 'GPS' and 1674 is GPS week number. Based on ITRF2008.
|WGS84 (G1762)||2005.0||Realization introduced by DoD on 2013-10-16 based on GPS observations.|
G stands for 'GPS' and 1762 is GPS week number. Based on ITRF2008.
Transformation parameters between WGS84 (G1762) and past WGS84 realizations as well as some ITRF realizations.
The transformation parameters between various ITRF realizations can be found in ITRF Transformation Parameters.xlsx.
Rotations are for the position vector rotation convention. Units are meters, mas (milliarcsecons) and ppb (parts-per-billion).
1 mas = 0.001 " = 2.77778 e-7 degrees = 4.84814 e-9 radians. 0.001 " corresponds to about 0.030 m at the earth's surface.
WGS84 and ITRF
In general the ITRS (and its realizations ITRFyy) are identical to WGS84 at one meter level. Meanwhile there are two types of WGS84 realization.
- Old realization based on U.S. Navy Navigation Satellite System, commonly known as DOPPLER Transit, and provided station coordinates with accuracies of about one meter. With respect to this realization the International Earth Rotation Service published transformation parameters between ITRF90 and this Doppler realized system: WGS84.TXT.
- New realizations of WGS84 based on GPS data, such as G730, G873, G1150 and G1674. These new WGS84 realizations are coincident with ITRF at about 10-centimeter level. For these realizations there are no official transformation parameters. This means that one can consider that ITRF coordinates are also expressed in WGS84 at 10 cm level. However, the most recent G1762 realization adopted ITRF2008 coordinates for more than half of the reference stations and velocities of nearby sites for the others. Thus, ITRF2014, ITRF2008 and WGS84 (G1762) are likely to agree at the centimeter level, yielding conventional 0-transformation parameters.
The OGP Surveying & Positioning Committee recommends in their Guidance note 4, 'Use of the International Terrestrial Reference Frame (ITRF) as the reference geodetic system for surveying and real-time positioning', to keep the old wording "from local coordinate reference system to WGS84" only when the published transformation parameter values allow a coordinate transformation to an accuracy worse than one meter, and to use the new wording "from local coordinate reference system to ITRFyy at epoch yyyy.y" for the publication of transformation parameter values at a sub-meter precision.
WGS84, ITRF and NAD83
The original WGS84 realization essentially agrees with NAD83 (1986). Subsequent WGS84 realizations, however, approximate certain ITRS realizations.
The North American Datum of 1983 (NAD83) is used everywhere in North America except Mexico. This datum is realized in the conterminous United States and Alaska (North American Plate) through the National CORS (Continuously Operating Reference Stations) which provides the basis for obtaining rigorous transformations between the ITRF series and NAD83 as well as a myriad of scientific applications.
As of November 2011, the CORS network contains over 1,800 stations, contributed by over 200 different organizations, and the network continues to expand. The latest realization of NAD83 is technically called NAD83 (2011/PA11/MA11) epoch 2010.00, constituting the framework for the definition of the National Spatial Reference System (NSRS). In Canada NAD83 is also monitored through the Canadian Active Control System. Thus, the two organizations responsible for monitoring and making changes to the NAD83 are the National Geodetic Survey (NGS), http://www.ngs.noaa.gov, and the Natural Resources of Canada (NRCan), http://www.nrcan.gc.ca.
Mexican Datum of 1993
Mexico’s Instituto Nacional de Estadística, Geografía, e Informática (INEGI), http://www.inegi.org.mx, the Federal agency responsible for geodesy and mapping in the country, adopted the geocentric frame ITRF92, epoch 1988.0, as the basis for their datum definition. The realization of the datum is achieved through the Red Geodésica Nacional Activa (RGNA) a 14 station network of permanent GPS receivers. Recently, they adopted ITRF2008,.epoch 2010.0, as the new basis for the Mexican Datum definition.
WGS84, ITRF and SIRGAS
The Sistema de Referencia Geocentrico para America del Sur 1995 (SIRGAS 1995) was established to support a unified geodetic and mapping frame for the South American continent. Most South American and Caribbean countries participated in this enterprise with 58 reference stations which was later extended to Central and North America. The adopted reference frame was ITRF94, epoch 1995.42. The Sistema de Referencia Geocentrico para las AmericaS 2000 (SIRGAS 2000) was realised by a frame of 184 stations observed in 2000 and adjusted in the ITRF2000, epoch 2000.40. SIRGAS 2000 includes ties to tide gauges and replaces SIRGAS 1995 for South America while expanding SIRGAS to Central America. The name was changed in 2001 for use in all of Latin America. There are several web pages with information about SIRGAS, such as: http://www.ibge.gov.br/home/geociencias/geodesia/sirgas.
WGS84, ITRF and ETRS89
The ETRS89 (European Terrestrial Reference System of 1989) is based on ITRF89, epoch 1989.0 and monitored by a network of about 250 permanent GNSS tracking stations known as the EUREF Permanent Network (EPN). The IAG Sub-commission EUREF is responsible for the maintenance of the European Terrestrial Reference System (ETRS89). Check the EUREF web site: http://www.euref.eu. The EPN Central Bureau is located at the Royal Observatory of Belgium, http://www.epncb.oma.be.
WGS84, ITRF and GDA94
The Geocentric Datum of Australia of 1994 (GDA94) was originally referred to the frame ITRF92, at epoch 1994.0. GDA94 is controlled by the Australian Regional GNSS Network (ARGN) which is presently comprised of a network of 15 GPS stations permanently tracking in Australia and its Territories, with the 8 stations in Australia known as the Australian Fiducial Network (AFN). The organization responsible for monitoring GDA94 is Geoscience Australia, http://www.auslig.gov.au.
- WGS84 - http://earth-info.nga.mil/GandG/publications/tr8350.2/tr8350_2.html
- WGS84 (G730), (G873) and (G1150) - http://www.ngs.noaa.gov/CORS/Articles
- ITRFyy, ITRF2000, ITRF2005, ITRF2008 and ITRF2014 - http://itrf.ensg.ign.fr