# Airy spheroid

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The **Airy spheroid** or **Airy ellipsoid** is a mathematical model of the Earth, an [Earth ellipsoid](/source/Earth_ellipsoid), designed to fit the well for the [British Isles](/source/British_Isles). It is named after its inventor [George Biddell Airy](/source/George_Biddell_Airy), a nineteenth century English mathematician.

## Airy 1830 ellipsoid

The Airy 1830 ellipsoid has an equatorial radius of 6,377,563.396 m, a polar radius of 6,356,256.909 m and an inverse flattening of 299.3249646.

The original definition was in [feet](/source/Foot_(unit)) - using the 1796 definition of the foot, an equatorial radius of 20,923,713 ft and a polar radius of 20,853,810 ft.[1] When the [Ordnance Survey](/source/Ordnance_Survey) retriangulated in 1936 they defined a conversion to [metres](/source/Metres), namely a ratio of (10^0.48401603)/10 which is approximately 1 ft = 0.3048007491 m.[1]

## Airy Modified 1849

The 1849 ellipsoid (EPSG:7002), known as Airy Modified 1849, is the 1830 ellipsoid scaled by 0.999965 to better fit the primary triangulation of Ireland.[2]

## See also

- [Ordnance Survey National Grid](/source/Ordnance_Survey_National_Grid)

- [Irish grid reference system](/source/Irish_grid_reference_system)

## References

1. ^ [***a***](#cite_ref-epsg7001_1-0) [***b***](#cite_ref-epsg7001_1-1) ["Airy 1830 - EPSG:7001"](https://epsg.io/7001-ellipsoid). *epsg.io*.

1. **[^](#cite_ref-2)** ["Airy Modified 1849 - EPSG:7002"](https://epsg.io/7002-ellipsoid). *epsg.io*.

## Further reading

- Airy, George Biddell (1826). "XXXIII. On the figure of the earth". *Philosophical Transactions of the Royal Society of London*. **116**: 548–578.

- Lapaine, Miljenko (2016). "George Biddell Airy and his Contribution to Map Projections Theory". In Bandrova, Temenoujka; Konečný, Milan (eds.). [*6th International Conference on Cargography and GIS*](https://www.researchgate.net/publication/309772611_6th_International_Conference_on_Cartography_and_GIS). Bulgarian Cartographic Association. pp. 334–342.

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