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Reference.org
Earth ellipsoid
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<p>An Earth ellipsoid or Earth spheroid is a mathematical figure approximating the <a href="/facts/Figure_of_the_Earth/z1LvpVgd">Earth's form</a>, used as a <a href="/facts/Frame_of_reference/15yYfB9M">reference frame</a> for computations in <a href="/facts/Geodesy/EJH0Te0m">geodesy</a>, <a href="/facts/Astronomy/0ArJVM1p">astronomy</a>, and the <a href="/facts/Geoscience/rQsXCSbM">geosciences</a>. Various different <a href="/facts/Ellipsoid/pe9vm0Sh">ellipsoids</a> have been used as approximations. </p><p>It is a <a href="/facts/Spheroid/GJyyLxQ7">spheroid</a> (an ellipsoid of <a href="/facts/Surface_of_revolution/yzpQMdzx">revolution</a>) whose <a href="/facts/Minor_axis/l7JES2wo">minor axis</a> (shorter diameter), which connects the geographical <a href="/facts/North_Pole/gjfItQJH">North Pole</a> and <a href="/facts/South_Pole/jBdpXTnh">South Pole</a>, is approximately aligned with the Earth's axis of rotation. The ellipsoid is defined by the <i>equatorial axis</i> (a) and the <i>polar axis</i> (b); their radial difference is slightly more than 21 km, or 0.335% of a (which is not quite 6,400 km). </p><p>Many methods exist for determination of the axes of an Earth ellipsoid, ranging from <a href="/facts/Meridian_arc/Ic30hDn1">meridian arcs</a> up to modern <a href="/facts/Satellite_geodesy/r9AAcApl">satellite geodesy</a> or the analysis and interconnection of continental <a href="/facts/Geodetic_network/2KNrnHmj">geodetic networks</a>. Amongst the different set of data used in <a href="/facts/National_survey/6o08XzS7">national surveys</a> are several of special importance: the <a href="/facts/Bessel_ellipsoid/cBq3cFq3">Bessel ellipsoid</a> of 1841, the international <a href="/facts/Hayford_ellipsoid/h7UrskxX">Hayford ellipsoid</a> of 1924, and (for <a href="/facts/GPS/DKVLnzF3">GPS</a> positioning) the <a href="/facts/WGS84/V9wSJnjX">WGS84</a> ellipsoid. </p>