Geoscience Reference
In-Depth Information
Table 2.1. Defining parameters of the GRS 1980
Parameter and value
Description
a
= 6 378 137 m
semimajor axis of the ellipsoid
10
8
m
3
s
−
2
GM
= 3 986 005
·
geocentric gravitational constant of the
earth (including the atmosphere)
= 108 263
·
10
−
8
J
2
dynamical form factor of the earth (ex-
cluding the permanent tidal deforma-
tion)
10
−
11
rad s
−
1
ω
= 7 292 115
·
angular velocity of the earth
Table 2.1. Note that these parameters, as given in the table, are defined
as exact! Note also that
GM
, the “geocentric gravitational constant” of the
earth, may also more figuratively be denoted as “product of the (Newtonian)
gravitational constant and the earth's mass”.
On the basis of these defining parameters and by the computational
formulas given in Moritz (1980 b), the geometrical and physical constants of
Table 2.2 may be derived.
The GRS 1980 is still (2005) valid as the ocial reference system of the
IUGG and it forms the fundamental basis of the WGS 84.
World Geodetic System 1984 (WGS 84)
As just mentioned, the WGS 84 may be regarded as a descendant of the
GRS 1980. Due to its still increasing importance, we consider it appropriate
to describe the WGS 84 in some more detail.
Following the National Imagery and Mapping Agency (2000) of the USA,
the
definition
of the WGS 84 may be described in the following way. The
WGS 84 is a Conventional Terrestrial Reference System (CTRS). The def-
inition of this coordinate system follows the criteria as outlined by the In-
ternational Earth Rotation Service (IERS). The criteria for this system are
the following:
•
it is geocentric, the center of mass being defined for the whole earth
including oceans and atmosphere;
•
its scale is that of the local earth frame, in the meaning of a relativistic
theory of gravitation;
•
its orientation was initially given by the Bureau International de l'Heure
(BIH) orientation of 1984.0;
•
its time evolution in orientation will create no residual global rotation
with regards to the crust.
