TheAiry spheroid orAiry ellipsoid is a mathematical model of the Earth, anEarth ellipsoid, designed to fit the well for theBritish Isles. It is named after its inventorGeorge Biddell Airy, a nineteenth century English mathematician.
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 infeet - 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 theOrdnance Survey retriangulated in 1936 they defined a conversion tometres, namely a ratio of (10^0.48401603)/10 which is approximately 1 ft = 0.3048007491 m.[1]
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]