Geographical mile
| Geographical mile | |
|---|---|
| 1 geographical mile in ... | ... is equal to ... |
| SI units | ~ 1855.3 m |
| imperial/US units | ~ 1.1528 mi or ~ 6,087 ft |
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 40075016.6856 m which makes the geographical mile 1855.3248 m, while the International Astronomical Union standard IAU-2000 takes the equator to be 40075035.5351 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.
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 (NIST) notes that: "The international nautical mile of 1 852 meters (6 076.115 49...feet) 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 (geographical or sea) mile." A separate reference also identifies the geographic mile as being identical to these international nautical miles (and slightly shorter than British nautical miles, which were identified as being equivalent to 1853.184 meters). The unit is not used much, but is cited in some United States laws (e.g., Section 1301(a) of the Submerged Lands Act, which defines state seaward boundaries in terms of geographic miles). 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".
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