Frequently asked question

A datum is defined on an oblate spheroid, which is a flattened oval shape, with a larger radius in the horizontal (equatorial) plane than the vertical (polar) plane. The radius of the spheroid changes continuously from the greatest length at the equator as the radius is measured from the equator to the poles.

Oblate spheroid

The radius of a sphere has the same measurement in every direction.

Because the curve defining the spheroid changes continuously, while the curvature of the sphere is always the same, no geographic (datum) transformation can be calculated that will accurately perform a transformation between the two surfaces.

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