Geographical mile

The geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator. For the 1924 International Spheroid this equalled 1855.4 metres. Any greater precision depends more on choice of standard than on more careful measurement: the length of the equator in the World Geodetic System WGS-84 is 40,075,016.6856 m which makes the geographical mile 1855.3248 m, while the International Astronomical Union standard IAU-2000 takes the equator to be 40,075,035.5351 m making the geographical mile 1855.3257 m, almost a millimetre longer. In any standard, the length of a degree of longitude at the equator is thus exactly 60 geographical miles.