The Geoid and Different Height Systems - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 145

Fig. 4.8 Earth ellipsoid

Table 4.1 Earth ellipsoid parameters (semimajor axis a, flattening f, gravitational constant
total mass GM, and angular velocity ω)
Name of the
ellipsoid

GM (10 14
m 3 /s 2 )

ω (10 5
rad/s) Year a (m) f

Krassowski
Ellipsoid

1940 6,378,245 1/298.3 - -

GRS75 1975 6,378,140 1/298.257 3.986005 7.292115

GRS80 1980 6,378,137 1/298.257222101 3.986005 7.292115

WGS84 1996 6,378,137 1/298.257223563 3.986004418 7.292115

CGCS2000 2008 6,378,137 1/298.257222101 3.986004418 7.292115

called the normal ellipsoid. Due to the irregularity of the actual Earth's shape, a
regular surface should be chosen as the reference surface on which geodetic
observations and computations are performed. The Earth ellipsoid introduced
hereby is called the reference ellipsoid (see Sect. 5.2).

4.2.2 Normal Ellipsoid and Normal Gravity

The normal ellipsoid is an imaginary rotational ellipsoid with regular shape and
homogeneous mass distribution that satisfies certain conditions. It is the regular
shape of the geoid and is used to represent the ideal body of the Earth. The gravity
field generated by the normal ellipsoid is termed the normal gravity field.
Corresponding gravity, gravity potential, and level surface are called normal
gravity, normal gravity potential, and the spheropotential surface (spherop), respec-
tively. The normal ellipsoid is artificially chosen. The normal gravity potential on
the normal ellipsoid is specified as being constant, and its value is equal to the
gravity potential W 0 on the geoid (Fig. 4.7 ).