Geographical mile

Historical units

Historically, certain nations used slightly different divisions to create their geographical miles.

The Portuguese system derived their miles (milha geográfica) as one third of their league of three separate values. When each equatorial degree was divided into 18 leagues, the geographical mile was equal to degree or about 2.06 km; when divided into 20 leagues, the geographical mile was equal to degree, approximating the values provided above; and when divided into 25 leagues, the geographical mile was equal to degree or about 1.48 km.

The geographical miles of the traditional Dutch (geografische mijl), German (geographische Meile or Landmeile), and Danish systems (geografisk mil) all approximated their much longer milesequivalent to English leaguesby using a larger division of the equatorial degree. Instead of using one minute of arc, they all used four degreeto produce a distance now notionally equal to 7408 m but actually differing slightly depending on official measurements and computations. (For example, the Danish unit was computed as equivalent to about 7421.5 m by the astronomer Ole Rømer.)

Relationship with the nautical mile

Main article: Nautical mile

The geographical mile is 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 by treaty as exactly 1,852 m. This deprecated value of 6,080.2 feet is equivalent to 6,080.2 ft. A separate reference identifies the geographic mile as being identical to the international nautical mile of 1,852 m and slightly shorter than the British nautical mile of 6,080 ft.

Scandinavian nautical mile

Main article: Scandinavian nautical mile

Scandinavians used their own version of the geographical mile as their nautical mile up to the beginning of the 20th century (with continued regional use), indigenously known as a sea mile (, , ). It was defined as of an equatorial degree (° of longitude), equivalent to approximately four modern nautical miles or "medium meridian minutes" (4 × 1,852 m) – a nautical mile is approximately one sixtieth of a degree along a meridian (1/60 meridian degree).

During metrification in 1875, this brought it down to about 7420 m from its former equivalence of 3950 fathoms (, ) or about 7435 m.

Use

The unit is not used much in English-speaking countries but is cited in some United States laws. For example, Section 1301(a) of the Submerged Lands Act 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 6,086 and 4-10ths of a foot" and "sub-divided into lots of one mile square each, or 850 and 4-10ths of an acre".

References

References

1. Ministry of Defence Staff, Navy Dept, Great Britain Ministry of Defence. (1987). "Admiralty manual of navigation". H.M. Stationery Office.
2. "The American Practical Navigator". National Geospatial-Intelligence Agency.
3. This is used by the international (ITRS) and American WGS 84) coordinate reference system for the world.
4. (2008). "Biography of Ole Rømer". The Abraham Zelmanov Journal.
5. (1964). "Mapping". University of Chicago Press.
6. (November 2014). ""NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices, General Tables of Units of Measurement". [[NIST]].
7. (July 1, 1955). ""Units of Weight and Measure (United States Customary and Metric) Definition and Tables of Equivalents"". [[NIST.
8. "CRC Handbook of Chemistry and Physics, 62nd edition, 1981-1982". CRC Press.
9. [https://snl.no/sj%C3%B8mil Store norske leksikon] (in Norwegian)
10. (May 28, 1784). "A Century of Lawmaking for a New Nation: U.S. Congressional Documents and Debates, 1774 - 1875".