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. The American Practical Navigator 2017 defines the geographical mile as. Greater precision depends more on choice of ellipsoid than on more careful measurement: the length of the equator in the World Geodetic System WGS-84 is which makes the geographical mile 1855.3248 m, while the IERS Conventions takes the equator to be making the geographical mile 1855.3250 m, 0.2 millimetres longer. In any ellipsoid, the length of a degree of longitude at the equator is thus exactly 60 geographical miles. The shape of the Earth is a slightly flattened sphere, which results in the Earth's circumference being 0.168% larger when measured around the equator as compared to through the poles. The geographical mile is slightly larger than the nautical mile ; one geographic mile is equivalent to approximately.
Related units
It was closely related to the nautical mile, which was originally determined as 1 minute of arc along a great circle of the Earth, but is nowadays defined as exactly 1852 metres. The US National Institute of Standards and Technology notes that: "The international nautical mile of 1 852 meters was adopted effective July 1, 1954, for use in the United States. The value formerly used in the United States was 6 080.20 feet = 1 nautical mile." A separate reference identifies the geographic mile as being identical tothe international nautical mile of 1852 metres. The unit is not used much, but is cited in some United States laws. While debating what became the Land Ordinance of 1785, Thomas Jefferson's committee wanted to divide the public lands in the west into "hundreds of ten geographical miles square, each mile containing 6086 and 4-10ths of a foot" and "sub-divided into lots of one mile square each, or 850 and 4-10ths of an acre". The Danish and German geographical mile is 4 minutes of arc, and was defined as approximately 7421.5 metres by the astronomer Ole Rømer of Denmark. In Norway and Sweden, this 4-minute geographical mile was mainly used at sea, up to the beginning of the 20th century.
