An ellipsoid is a mathematical approximation of the shape of the Earth formed by rotating an ellipse about an axis. This page outlines the parameters that define ellipsoids commonly used in New Zealand geodetic datums.

An oblate ellipsoid (where the ellipsoid is formed by rotation along the minor ellipsoid axis) is the best fit to the Earth, although it can still depart by up to 100 metres from its actual shape.

EllipsoidSemi-major axis ( a )Inverse flattening ( 1 / f )
GRS806 378 137 m298.257 222 101
International 19246 378 388 m297
WGS846 378 137 m298.257 223 563