The World Geodetic System is a standard for use in cartography, geodesy, and navigation.
It comprises a standard coordinate system for the Earth, a standard spheroidal reference
surface for raw altitude data, and a gravitational equipotential surface that defines the nominal
sea level. The latest revision is WGS 84, established
in 1984 and last revised in 2004. Earlier schemes included WGS 72, WGS 66, and WGS 60.
WGS 84 is the reference coordinate system used by the Global Positioning System.
Main parameters The coordinate origin of WGS 84 is meant to
be located at the Earth's center of mass; the error is believed to be less than 2 cm.
The WGS 84 meridian of zero longitude is the IERS Reference Meridian, 5.31 arc seconds
or 102.5 metres east of the Greenwich meridian at the latitude of the Royal Observatory.
The WGS 84 datum surface is an oblate spheroid with major radius a = 6378137 m at the equator
and flattening f = 1/298.257223563. The polar semi-minor axis b then equals a times, or
6356752.3142 m. Currently, WGS 84 uses the EGM96 geoid, revised
in 2004. This geoid defines the nominal sea level surface by means of a spherical harmonics
series of degree 360. The deviations of the EGM96 geoid from the WGS 84 reference ellipsoid
range from about −105 m to about +85 m. EGM96 differs from the original WGS 84 geoid,
referred to as EGM84. History
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 founded the Zentralbüro für die Internationale Erdmessung,
and a series of global ellipsoids of the Earth were derived.
A unified geodetic system for the whole world became essential in the 1950s for several
reasons: International space science and the beginning
of astronautics. The lack of inter-continental geodetic information.
The inability of the large geodetic systems, such as European Datum, North American Datum,
and Tokyo Datum, to provide a worldwide geo-data basis
Need for global maps for navigation, aviation, and geography.
Western Cold War preparedness necessitated a standardised, NATO-wide geospatial reference
system, in accordance with the NATO Standardisation Agreement
In the late 1950s, the United States Department of Defense, together with scientists of other
institutions and countries, began to develop the needed world system to which geodetic
data 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. 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 from a local horizontal determined by the spirit level, plumb line, or an equivalent
device that depends on the local gravity field. As a result, the elevations in the data 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 data that were not
only made separately, for different regions, but to re-reference the elevations to an ellipsoid
model rather than to the geoid. 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 initially selected datum. 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.
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 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. The United States 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, 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 United States 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. The results of the adjustment consisted of corrections to
initial station coordinates and coefficients of the gravitational field.
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. 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. Additional electronic satellite data was provided by the SECOR 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. Data
from the Smithsonian Astrophysical Observatory was also used which included camera and some
laser ranging.
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.
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 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.
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: WGS 84
In the early 1980s the need for a new world geodetic system was generally recognized by
the geodetic community, also within the US 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 Very Long Baseline Interferometry
observations constituted significant new information. 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. 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 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 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. The very small difference in the flattening
thus results in a tiny difference of 0.105 mm in the semi polar axis.
Longitudes on WGS 84 WGS 84 uses the IERS Reference Meridian as
defined by the Bureau International de l'Heure, 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. The longitude positions on WGS 84 agree with
those on the older North American Datum 1927 at roughly 85° longitude west, in the east-central
United States. Updates and new standards
The latest major revision of WGS 84 is also referred to as "Earth Gravitational Model
1996", 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.
Many of the original authors of WGS 84 contributed to a new higher fidelity model, called EGM2008.
This new model will have a geoid with a resolution approaching 10 km, requiring over 4.6 million
terms in the spherical expansion. See also
GPS TRANSIT system
NAD83 ETRS89
EGM96 Geo - for marking up WGS84 coordinates inHTML
Degree Confluence Project Geotagging
Point of Interest Geodetic system
References
External links Geodesy for the Layman, Chapter VIII, "The
World Geodetic System" 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.
The original WGS 84 is documented in versions prior to 1996.
Main NGA page on Earth gravity models Technical Manual DMA TM 8358.1 - Datums, Ellipsoids,
Grids, and Grid Reference Systems at the National Geospatial-Intelligence Agency
Description of the difference between the geoid and the ellipsoid from the US NOAA National
Geodetic Survey GEOID page NASA GSFC Earth gravity page
GeographicLib provides a utility GeoidEval to evaluate the geoid height for the EGM84,
EGM96, and EGM2008 earth gravity models. Here is an online version of GeoidEval.
Spatial reference EPSG Projection 4326 - WGS 84
This article incorporates public domain material from websites or documents of the
National Geodetic Survey.