A mathematical formulation of the shape of the Earth which is defined by a semimajor axis and its eccentricity. There are 11 official ellipsoids in use throughout the world. The Clark Ellipsoid of 1866 is used in North America.
A three-dimensional mathematical model which approximates the shape of the geoid. Many different ellipsoids have been developed for continents or individual countries to minimize local deviations from the geoid. The standard global ellipsoid is the World Geodetic System of 1984 (WGS 84). Reference: Peter Dana, The Geographer's Craft
The Earth surface is approximately described by an ellipsoid, a closed surface all planar sections of which are ellipses. In general, an ellipsoid has three independent axes, and is usually specified by the length of the three semi-axes. If the lengths of two axes are the same, the ellipsoid is called “ellipsoid of revolution” or spheroid. Due to the rotation around its axis, the Earth has the shape of a spheroid. Several spheroids are used to model the Earth surface and project it onto a two-dimensional map; the choice of the reference spheroid depends on the region of the Earth to be represented and the required precision. The spheroids quoted in this work are Clarke 1866, WGS72 and WGS84. Clarke 1866 is used to map the North America and the Philippines. The World Geodetic System (WGS) spheroids have been developed to be used for global mapping; the number indicates the year of calculation. WGS84 is the most recent version, and is also used by the Global Positioning System.
spheroid The mathematical function used to describe the shape of the earth for geodetic computations. The figure is formed by rotating an ellipse about its minor (shorter) axis and is typically described by dimensions for the semimajor axis (a) together with the semiminor axis (b) or flattening (f) = (a-b)/a.
a squashed or stretched sphere in which each of the three axes can be of different lengths. (Contrast to a spheroid, in which two of the three axes have the same length.) An ellipsoid has the equation x²/a² + y²/b² + z²/c² = 1
An oblate spheroid used to roughly model the surface of the Earth. There are many possible ellipsoids available, parameterized by eccentricity and semi-major axis. When used as a datum, an ellipsoid also has an XYZ offset from the center of the Earth.
A geometric solid that simplifies the geoid's bulges and depressions to a smoother surface that is slightly wider at the equator than at the poles, and much more adaptable to mapping and survey measurements (see also Geodesy). Various reference ellipsoids have been developed and used in different parts of the world for large scale mapping purposes. the United states uses the Geodetic Reference System (GRS80) as the reference ellipsoid for surveying and topographic mapping.