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In geodesy, a **reference ellipsoid** is a mathematically defined surface that approximates the geoid, which is the truer, imperfect figure of the Earth, or other planetary body, as opposed to a perfect, smooth, and unaltered sphere, which factors in the undulations of the bodies' gravity due to variations in the composition and density of the interior, as well as the subsequent flattening caused by the centrifugal force from the rotation of these massive objects (for planetary bodies that do rotate).
Because of their relative simplicity, reference ellipsoids are used as a preferred surface on which geodetic network computations are performed and point coordinates such as latitude, longitude, and elevation are defined.

In the context of standardization and geographic applications, a *geodesic reference ellipsoid* is the mathematical model used as foundation by spatial reference system or geodetic datum definitions.

In 1687 Isaac Newton published the Principia in which he included a proof that a rotating self-gravitating fluid body in equilibrium takes the form of a flattened ("oblate") ellipsoid of revolution, generated by an ellipse rotated around its minor diameter; a shape which he termed an oblate spheroid.^{[1]}^{[2]}

In geophysics, geodesy, and related areas, the word 'ellipsoid' is understood to mean 'oblate ellipsoid of revolution', and the older term 'oblate spheroid' is hardly used.^{[3]}^{[4]} For bodies that cannot be well approximated by an ellipsoid of revolution a triaxial (or scalene) ellipsoid is used.

The shape of an ellipsoid of revolution is determined by the shape parameters of that ellipse. The semi-major axis of the ellipse, a, becomes the equatorial radius of the ellipsoid: the semi-minor axis of the ellipse, b, becomes the distance from the centre to either pole. These two lengths completely specify the shape of the ellipsoid.

In geodesy publications, however, it is common to specify the semi-major axis (equatorial radius) a and the flattening f, defined as:

That is, f is the amount of flattening at each pole, relative to the radius at the equator. This is often expressed as a fraction 1/m; *m* = 1/*f* then being the "inverse flattening". A great many other ellipse parameters are used in geodesy but they can all be related to one or two of the set a, b and f.

A great many ellipsoids have been used to model the Earth in the past, with different assumed values of a and b as well as different assumed positions of the center and different axis orientations relative to the solid Earth. Starting in the late twentieth century, improved measurements of satellite orbits and star positions have provided extremely accurate determinations of the earth's center of mass and of its axis of revolution; and those parameters have been adopted also for all modern reference ellipsoids.

The ellipsoid WGS-84, widely used for mapping and satellite navigation has f close to 1/300 (more precisely, 1/298.257223563, by definition), corresponding to a difference of the major and minor semi-axes of approximately 21 km (13 miles) (more precisely, 21.3846857548205 km). For comparison, Earth's Moon is even less elliptical, with a flattening of less than 1/825, while Jupiter is visibly oblate at about 1/15 and one of Saturn's triaxial moons, Telesto, is highly flattened, with f between 1/3 to 1/2 (meaning that the polar diameter is between 50% and 67% of the equatorial.

Geodetic coordinates are a type of curvilinear orthogonal coordinate system used in geodesy based on a *reference ellipsoid*.
They include geodetic latitude (north/south) ϕ, *longitude* (east/west) λ, and ellipsoidal height h (also known as geodetic height^{[5]}).

Currently the most common reference ellipsoid used, and that used in the context of the Global Positioning System, is the one defined by WGS 84.

Traditional reference ellipsoids or *geodetic datums* are defined regionally and therefore non-geocentric, e.g., ED50. Modern geodetic datums are established with
the aid of GPS and will therefore be geocentric, e.g., WGS 84.

**^**Heine, George (September 2013). "Euler and the Flattening of the Earth".*Math Horizons*.**21**(1): 25–29. doi:10.4169/mathhorizons.21.1.25.**^**Choi, Charles Q. (12 April 2007). "Strange but True: Earth Is Not Round".*Scientific American*. Retrieved 4 May 2021.**^**Torge, W (2001) Geodesy (3rd edition), published by de Gruyter, ISBN 3-11-017072-8**^**Snyder, John P. (1993).*Flattening the Earth: Two Thousand Years of Map Projections*. University of Chicago Press. p. 82. ISBN 0-226-76747-7.**^**National Geodetic Survey (U.S.).; National Geodetic Survey (U.S.) (1986).*Geodetic Glossary*. NOAA technical publications. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Ocean Service, Charting and Geodetic Services. p. 107. Retrieved 2021-10-24.**^**Awange, J.L.; Grafarend, E.W.; Paláncz, B.; Zaletnyik, P. (2010).*Algebraic Geodesy and Geoinformatics*. Springer Berlin Heidelberg. p. 156. ISBN 978-3-642-12124-1. Retrieved 2021-10-24.

- P. K. Seidelmann (Chair), et al. (2005), “Report Of The IAU/IAG Working Group On Cartographic Coordinates And Rotational Elements: 2003,”
*Celestial Mechanics and Dynamical Astronomy*, 91, pp. 203–215.- Web address: https://astrogeology.usgs.gov/Projects/WGCCRE

*OpenGIS Implementation Specification for Geographic information - Simple feature access - Part 1: Common architecture*, Annex B.4. 2005-11-30- Web address: http://www.opengeospatial.org

- Geographic coordinate system
- Coordinate systems and transformations (SPENVIS help page)
- Coordinate Systems, Frames and Datums