2024年4月4日发(作者：)

World Geodetic System

----世界大地测量系统（主要参量、历史演变和基本原理）

The World Geodetic System is a standard for use in cartography, geodesy, and navigation. It

comprises a standard coordinate frame for the Earth, a standard spheroidal reference surface (the

datum or reference ellipsoid) for raw altitude data, and a gravitational equipotential surface (the

geoid) that defines the nominal sea level. The latest revision is WGS 84 (dating from 1984 and last

revised in 2004), which was valid up to about 2010[1]. 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 [2].

The WGS 84 meridian of zero longitude is the IERS Reference Meridian[3], 5.31 arc

seconds or 102.5 metres (336.3 ft) east of the Greenwich meridian at the latitude of the Royal

Observatory[4]&[5].

The WGS 84 datum surface is an oblate spheroid (ellipsoid) with major (transverse) radius a

= 6378137 m at the equator and flattening f = 1/298.257223563[6]. The polar semi-minor

(conjugate) radius b then equals a times (1−f), or 6356752.3142 m.[6]

Presently WGS 84 uses the EGM96 (Earth Gravitational Model 1996) geoid, revised in 2004.

This geoid defines the nominal sea level surface by means of a spherical harmonics series of

degree 360 (which provides about 100 km horizontal resolution)[7]. The deviations of the EGM96

geoid from the WGS 84 reference ellipsoid range from about −105 m to about +85 m[8]. 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

(Mathematical and Physical Theory of Physical Geodesy). Austria and Germany founded the

Zentralbüro für die Internationale Erdmessung (Central Bureau of International Geodesy), and a

series of global ellipsoids of the Earth were derived (e.g., Helmert 1906, Hayford 1910/ 1924).

A unified geodetic system for the whole world became essential in the 1950s for several reasons:

(1) International space science and the beginning of astronautics.

(2) The lack of inter-continental geodetic information.

(3) The inability of the large geodetic systems, such as European Datum (ED50), North American

Datum (NAD), and Tokyo Datum (TD), to provide a worldwide geo-data basis

(4) Need for global maps for navigation, aviation, and geography.

(5) 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

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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 (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 from 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 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 [9] and Canadian SHORAN surveys were used to define a best-fitting

ellipsoid and an earth-centered orientation for each of initially selected datum. (Every datum is

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.

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.

Gravimetric datum orientation

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