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A reference ellipsoid, also known as a reference spheroid, is a mathematically defined ellipsoid that approximates the shape of the Earth. It serves as a fundamental geodetic datum for establishing horizontal coordinates, such as latitude and longitude. The Earth's shape is not a perfect sphere but is slightly flattened at the poles and bulges at the equator due to its rotation. A reference ellipsoid aims to model this slightly oblate spheroid.

Various reference ellipsoids have been developed over time by different organizations and for different purposes. These ellipsoids are defined by two main parameters: the semi-major axis (equatorial radius) and the semi-minor axis (polar radius), or equivalently, the semi-major axis and the flattening. The flattening represents the difference between the equatorial and polar radii relative to the equatorial radius.

Commonly used reference ellipsoids include the GRS 80 (Geodetic Reference System 1980) and the WGS 84 (World Geodetic System 1984). These systems are widely adopted for global positioning and navigation systems, including GPS. Different ellipsoids can lead to slight variations in calculated coordinates, which is why it is important to specify the reference ellipsoid when defining geodetic coordinates. While ellipsoids provide a smooth mathematical surface, the actual Earth's surface is more complex and is represented by a geoid, which is a more irregular equipotential surface of gravity.