| The ReferenceEllipsoid class defines a geodetic reference ellipsoid
used as a standard for geodetic measurements. The World Geodetic System
1984 (WGS84) ellipsoid is the current standard for most geographic and
geodetic coordinate systems, including GPS. The WGS84 ellipsoid is
provided as a static instance of this class.
The ellipsoid (actually an oblate spheroid) is uniquely specified by
two parameters, the semimajor (or equatorial) radius and the ellipticity
or flattening. In practice, the reciprocal of the flattening is
specified.
The ellipsoid is an approximation of the shape of the earth. Although
not exact, the ellipsoid is much more accurate than a spherical
approximation and is still mathematically simple. The geoid is
a still closer approximation of the shape of the earth (intended to
represent the mean sea level), and is generally specified by it's
deviation from the ellipsoid.
Different reference ellipsoids give more or less accurate results at
different locations, so it was previously common for different nations
to use ellipsoids that were more accurate for their areas. More recent
efforts have provided ellipsoids with better overall global accuracy,
such as the WGS84 ellipsiod, and these have now largely supplanted
the others.
author:
Paul D. Anderson
version:
3.0, February 18, 2006 | 