WGS 84

WGS 84
Name	WGS 84
Type	Geodetic datum
Introduced	1984
Maintained by	National Geospatial-Intelligence Agency; United States Department of Defense
Epoch	1984.0 (initial)
Datum origin	Earth's center of mass
Ellipsoid	WGS 84 ellipsoid
Coordinate system	Geographic (latitude, longitude, ellipsoidal height)
Used for	Global Positioning System, international mapping, navigation

WGS 84 is a global geodetic datum and reference frame established to provide a consistent, Earth-centered coordinate system for mapping, navigation, and geospatial services. Developed for use with satellite technologies, WGS 84 underpins systems such as Global Positioning System and serves as a common platform for international surveying, cartography, and remote sensing. It interfaces with specialized frames used by agencies like National Aeronautics and Space Administration, European Space Agency, Japan Aerospace Exploration Agency, and Russian Federal Space Agency.

Overview

WGS 84 defines an Earth-centered, Earth-fixed coordinate system tied to the Earth's center of mass and an associated reference ellipsoid used for geographic coordinates. The frame was developed to satisfy requirements of United States Department of Defense programs including Global Positioning System and to align with international geodetic efforts such as International Earth Rotation and Reference Systems Service activities. It provides parameters for transforming between geocentric Cartesian coordinates and geodetic latitude, longitude, and height, interoperating with datasets from Landsat, Sentinel, and airborne sensors employed by National Oceanic and Atmospheric Administration and United States Geological Survey. WGS 84 is widely adopted by civil and commercial systems operated by companies like Garmin, Tesla, Apple Inc., Google LLC, and by aviation authorities including Federal Aviation Administration and International Civil Aviation Organization.

History and development

Work leading to the datum began in the 1970s with advances in satellite geodesy undertaken by Defense Mapping Agency, Navstar program teams, and researchers at institutions such as Massachusetts Institute of Technology and Scripps Institution of Oceanography. Significant milestones include coordination with the International Association of Geodesy and adoption milestones influenced by events like the deployment of GPS Block II satellites. Revisions and refinements involved cooperation among National Geospatial-Intelligence Agency, Defense Mapping Agency, and scientific bodies including United States Naval Observatory and groups at Harvard University and California Institute of Technology. Iterative updates paralleled developments in the International Terrestrial Reference Frame and advances in techniques like very long baseline interferometry used by Observatoire de Paris and Harvard-Smithsonian Center for Astrophysics teams.

Reference frame and ellipsoid

The WGS 84 reference ellipsoid approximates the size and shape of Earth with defined semi-major axis and flattening values established through satellite tracking and gravimetric measurements by organizations such as National Geospatial-Intelligence Agency and research centers like GFZ German Research Centre for Geosciences. The associated Earth-centered, Earth-fixed frame aligns with measurements from International GNSS Service networks and observatories including MIT Haystack Observatory and Jet Propulsion Laboratory. Parameters were chosen to be compatible with planetary ephemerides produced at Jet Propulsion Laboratory and astronomical references maintained at International Astronomical Union observatories. The ellipsoid supports coordinate conversions used by mapping agencies like Ordnance Survey, Institut Géographique National, and Geoscience Australia.

Geodetic components and transformations

WGS 84 specifies the mathematical relationships for converting between geocentric Cartesian coordinates and geodetic latitude, longitude, and height using algorithms implemented in software libraries such as those from GeographicLib, PROJ, and packages maintained by Open Source Geospatial Foundation. Transformations to regional datums like NAD83, ETRS89, GDA94, OSGB36, and CH1903 require Helmert seven-parameter or more complex time-dependent transformations influenced by tectonic motions measured by networks including International GNSS Service and research at Scripps Institution of Oceanography. Geoid models like EGM96, EGM2008, and national quasigeoid models from Bundesamt für Kartographie und Geodäsie and Instituto Geográfico Nacional are used to derive orthometric heights from ellipsoidal heights defined in WGS 84.

Realizations and maintenance (ITRF ties)

WGS 84 has undergone multiple realizations tied to the International Terrestrial Reference Frame (ITRF) series maintained by International Earth Rotation and Reference Systems Service. Specific realizations have been aligned with ITRF epochs such as ITRF2000, ITRF2005, ITRF2008, and later ITRF2014 through collaborations among National Geospatial-Intelligence Agency, European Space Agency, Geoscience Australia, and research institutes like CODATA-affiliated groups. Continuous tracking from stations in networks operated by US Geological Survey, Geological Survey of Canada, and national agencies provides velocity fields and station coordinates used to update WGS 84 realizations. Maintenance uses techniques from Space Geodesy including satellite laser ranging at Observatoire de la Côte d'Azur, Doppler tracking, and very long baseline interferometry at sites such as Wettzell.

Applications and accuracy considerations

WGS 84 underlies navigation for aviation managed by Federal Aviation Administration and Eurocontrol, maritime operations coordinated by International Maritime Organization, land surveying by national mapping agencies like Ordnance Survey and Instituto Geográfico Nacional, and location services by companies like Google LLC and Apple Inc.. Accuracy depends on receiver type, satellite geometry, atmospheric conditions, and realization epoch; high-precision geodesy using differential GNSS and post-processing centers such as Nevada Geodetic Laboratory and NRCan can achieve millimeter-to-centimeter accuracy, while consumer devices typically provide meter-level accuracy. Precise Point Positioning services offered by Jet Propulsion Laboratory and International GNSS Service improve absolute accuracy relative to WGS 84 realizations.

Criticisms and alternatives

Critiques of WGS 84 include issues of epoch-dependent coordinates, the need for tectonic plate motion modeling as emphasized by International Association of Geodesy and national agencies like Geoscience Australia, and regional offsets relative to local datums such as NAD83 and GDA2020. Alternatives and complements include regional reference frames like ETRS89 for Europe, NAD83 for North America, and national realizations such as GDA2020 implemented by Australian authorities. Scientific communities working at institutions like Scripps Institution of Oceanography, GFZ German Research Centre for Geosciences, and Jet Propulsion Laboratory continue to refine reference frames and promote interoperability between WGS 84 and ITRF-based systems.

Category:Geodetic datums