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 Deletions are marked like this. Additions are marked like this. Line 7: Line 7: [http:/images/bmwiki/ellipsoid.png] [[http:/images/bmwiki/ellipsoid.png]] Line 9: Line 9: An ellipsoid is independent of a ["datum"], but a datum will always specify which ellipsoid is to be used. Various ellipsoids have been used to approximate the shape of the earth. A few are listed below. An ellipsoid is independent of a [[datum]], but a datum will always specify which ellipsoid is to be used. Various ellipsoids have been used to approximate the shape of the earth. A few are listed below.

An ellipsoid is a 3-dimensional shape formed by rotating an ellipse either around its major axis or around its minor axis. The Earth's shape can be modeled by an ellipsoid formed by rotating an ellipse around its minor axis. This is because the centrifugal force resulting from its rotation has distorted it slightly. The figure below is enormously exaggerated; the Earth would appear to be spherical at the scale of the image.

An ellipsoid is described by two numbers. In the figure below, a is the semi-major axis or equatorial radius, and b is the semi-minor axis or polar radius. Flattening and eccentricity, which are derivable from a and b, are convenient for use in the calculations involved in datums and surveying.

• f is the flattening, and is defined to be (a - b) / a.

• e is the eccentricity, and is defined as e2 = 2f - f2.

An ellipsoid is independent of a datum, but a datum will always specify which ellipsoid is to be used. Various ellipsoids have been used to approximate the shape of the earth. A few are listed below.

 Ellipsoid Date a (meters) b (meters) flattening Description WGS 84 1984 6,378,137.0 6,356,752.3 1/298.257223563 Used by the WGS84 datum and the NAVSTAR GPS system. The ellipsoid is very nearly the same as the GRS80. The minor difference in the flattening has no effect on practical calculations. GRS 80 1980 6,378,137.0 6,356,752.3 1/298.257222101 Used by the NAD83 datum. Clarke 1866 6,378,206.4 6,356,583.8 1/294.978 Used by the NAD27 datum, and for older maps within the U.S. Airy 1830 6,377,563.4 6,356,256.9 1/299.325 Used by older maps within Great Britain Bessel 1841 6,377,397.2 6,356,079.0 1/299.153 Used by older maps within Central Europe. Also used by the CGS prior to 1875, when the Clarke 1866 spheriod was adopted for use.

Ellipsoid (last edited 2009-03-22 11:18:03 by JimIrwin)