Geoscience Reference
In-Depth Information
The change of normal gravity with elevation is given by the formula
(2-216), which for the International Ellipsoid becomes
0 . 000 45 sin 2 ϕ ) h +0 . 000 072 h 2 ,
γ h = γ
(0 . 308 77
(2-222)
where γ h and γ are measured in gal, and h is the elevation in kilometer.
Although the International Ellipsoid can no longer be considered the
closest approximation of the earth by an ellipsoid, it may still be used as
a reference ellipsoid for geodetic purposes. An ocial change of a reference
system must be very carefully considered because a large amount of data
maybereferredtosuchasystem.
The eastern countries have used the ellipsoid of Krassowsky:
a = 6 378 245 m ,
f =1 / 298 . 3 .
(2-223)
Contemporary data
After the start of Sputnik, the first artificial satellite, in 1957, the Interna-
tional Astronomical Union, in 1964, adopted a new set of constants, among
them a = 6 378 160 m and f =1 / 298 . 25. The value of a , which is consid-
erably smaller than that for the International Ellipsoid, incorporates astro-
geodetic determinations; the change in the value of J 2 , and consequently of
f , is due to the results from artificial satellites.
In 1967, these values were taken by the International Union of Geodesy
and Geophysics (IUGG) as the Geodetic Reference System 1967 .
This decision was soon seen to be wrong; especially the value of a was
recognized to be too large: now we believe to be on the order of 6 378 137 m,
the value of the Geodetic Reference System 1980 (GRS 1980) and, based on
it, the World Geodetic System 1984 (WGS 84). More details of these two
systems are given below.
Geodetic Reference System 1980 (GRS 1980)
The GRS 1980 has been adopted at the XVII General Assembly of the IUGG
in Canberra, December 1979, by Resolution No. 7. Inherently, this resolu-
tion recognizing that the Geodetic Reference System 1967 adopted at the
XIV General Assembly of IUGG, Lucerne, 1967, no longer represents the
size, shape, and gravity field of the earth to an accuracy adequate for many
geodetic, geophysical, astronomical, and hydrographic applications and con-
sidering that more appropriate values are now available, recommends that
the Geodetic Reference System 1967 be replaced by the new Geodetic Refer-
ence System 1980 which is also based on the theory of the geocentric equipo-
tential ellipsoid. The four defining parameters of the GRS 1980 are given in
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