The geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator. For the international ellipsoid 1924 this equalled 1855.4 metres. The American Practical Navigator 2017 defines the geographical mile as 6087.08 feet (1855.342 m). 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 40075016.6856 m which makes the geographical mile 1855.3248 m, while the IERS Conventions (2010) takes the equator to be 40075020.4555 m 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.
|1 geographical mile in ...||... is equal to ...|
|SI units||~ 1855.3 m|
|imperial/US units||~ 1.1528 mi or ~ 6,087 ft|
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 (which was historically linked to the circumference measured through both poles); one geographic mile is equivalent to approximately 1.00178 nautical 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." (The deprecated value, 6080.2 feet, is 1,853.24496 mn) A separate reference identifies the geographic mile as being identical to the international nautical mile of 1852 metres (and slightly shorter than the British nautical mile of 6080 feet (1853.184 m)). 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".
The Danish and German geographical mile (geografisk mil and geographische Meile or geographische Landmeile, respectively) 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 (sjømil), up to the beginning of the 20th century.
- Ministry of Defence Staff, Navy Dept, Great Britain Ministry of Defence (1987). Admiralty manual of navigation. H.M. Stationery Office. p. 7. ISBN 9780117728806.CS1 maint: multiple names: authors list (link)
- "Glossary of Marine Navigation", The American Practical Navigator, II (2017 ed.), National Geospatial-Intelligence Agency, p. 346
- Department of Defense World Geodetic System 1984 (third ed.), National Imagery and Mapping Agency, 2004, p. 3-2. Equatorial radius = 6,378,137 m.
- Petit, Gérard; Luzum, Brian, eds. (2010), "General definitions and numerical standards", IERS Conventions (2010), p. 18. Equatorial radius = 6,378,137.6 m.
- David Greenhood; Gerard L. Alexander (1964). Mapping. University of Chicago Press. pp. 51–52. ISBN 9780226306971.
- "NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices, General Tables of Units of Measurement (PDF). NIST (Report). November 2014. p. C-15 (Appendix C, footnote 14). Retrieved April 9, 2019.
- "Units of Weight and Measure (United States Customary and Metric) Definition and Tables of Equivalents" (PDF). National Bureau of Standards (Report). July 1, 1955. p. 4. Retrieved May 5, 2021.
- Weast, Robert C. (ed.). CRC Handbook of Chemistry and Physics, 62nd edition, 1981-1982. Boca Raton, Florida: CRC Press. p. F-297. ISBN 978-0849304620.
- "Journal of Continental Congress, Vol. 27". A Century of Lawmaking for a New Nation: U.S. Congressional Documents and Debates, 1774 - 1875. Library of congress. May 28, 1784. p. 446. Retrieved April 9, 2019.
- Rabounski, Dmitri (2008). "Biography of Ole Rømer" (PDF). The Abraham Zelmanov Journal. 1: 2. Retrieved 1 February 2018.