Nautical mile

1 nautical mile =
SI units
1.85200 km 1,852.00 m
US customary / Imperial units
1.15078 mi 6,076.12 ft

The nautical mile (symbol M, NM, Nm or nmi) is a unit of length corresponding approximately to one minute of arc of latitude along any meridian. By international agreement it is exactly 1,852 metres (approximately 6,076 feet).

It is a non-SI unit (although accepted for use in the International System of Units by the BIPM) used especially by navigators in the shipping and aviation industries,[1] and also in polar exploration. It is commonly used in international law and treaties, especially regarding the limits of territorial waters. It developed from the sea mile and the related geographical mile.

The nautical mile remains in use by sea and air navigators worldwide because of its convenience when working with charts. Most nautical charts are constructed on the Mercator projection whose scale varies by approximately a factor of six from the equator to 80° north or south latitude. It is, therefore, impossible to show a single linear scale for use on charts on scales smaller than about 1/80,000.[2] Since a nautical mile is, for practical navigation, the same as a minute of latitude, it is easy to measure a distance on a chart with dividers, using the latitude scale on the side of the chart directly to the east or west of the distance being measured.

Contents

Definition

The international nautical mile was defined by the First International Extraordinary Hydrographic Conference, Monaco (1929) as exactly 1852 metres.[1] This is the only definition in widespread current use, and is the one accepted by the International Hydrographic Organization and by the International Bureau of Weights and Measures (BIPM). Before 1929, different countries had different definitions, and the Soviet Union, the United Kingdom and the United States did not immediately accept the international value.

Both the Imperial and U.S. definitions of the nautical mile were based on the Clarke (1866) Spheroid: specifically, they were different approximations to the length of one minute of arc along a great circle of a hypothetical sphere having the same surface area as the Clarke Spheroid.[3] The United States nautical mile was defined as 1853.248 metres (6080.20 U.S. feet, based on the definition of the foot in the Mendenhall Order of 1893): it was abandoned in favour of the international nautical mile in 1954.[4] The Imperial (UK) nautical mile, also known as the Admiralty mile, was defined in terms of the knot such that one nautical mile was exactly 6080 feet (1853.184 m):[5] it was abandoned in 1970[5] and, for legal purposes, old references to the obsolete unit are now converted to 1853 metres exactly.[6]

Sea mile

In English usage, a sea mile is, for any latitude, the length of one minute of latitude at that latitude. It varies from approximately 1842.9 m at the Equator to approximately 1861.7 m at the Poles, with a mean value of 1852.3 m.[5] The international nautical mile was chosen as the integer number of metres closest to the mean sea mile.

American use has changed recently. The glossary in the 1966 edition of Bowditch defines a "sea mile" as a "nautical mile".[7] In the 2002 edition, the glossary says: "An approximate mean value of the nautical mile equal to 6,080 feet; the length of a minute of arc along the meridian of latitude 48°."[8]

One also encounters a sea mile of 6000 feet or 1000 fathoms, such as in Dresner 'Units of Measurement'. Dresner includes a remark to the effect that this must not be confused with the nautical mile.

Geographical mile

The geographical mile is equal to one minute of arc of longitude along the Equator: it is equal to approximately 1855.4 m for the International (1924) Spheroid,[5] or approximately 1855.325 m for the WGS 84 ellipsoid. On the other hand, Bowditch defines it as 6,087.08 feet, which is 1,855.34 metres.[8] The term "geographical mile" has also been used to refer to the mean sea mile, which would later become the international nautical mile.[3]

Care must be taken not to confuse this with the similar-sounding German unit called the geografische Meile, if one is dealing with historical German measurements (or one is German). This unit is intended to signify four minutes of arc along the equator and is standardized as 7421.6 metres. In Germany, the mile, uhr or stunde typically refers to 24,000 of the local foot. This is the distance one might walk in an hour (Stunde).

Telegraphic mile

A telegraphic mile, is the rounded length of a minute of arc along the Equator.

Tactical mile or data mile

As an approximation, designers of radar systems for ballistic, cruise and anti-ship missiles used by NATO navies use 6,000 feet (1,828.8 m) as their equivalent of a nautical mile. In the Royal Navy, this is also known as a data mile.

Unit symbol

The International Hydrographic Organization, whose membership includes essentially all seafaring nations, and the International Bureau of Weights and Measures use M as the abbreviation for the nautical mile.[1][9] The preferred abbreviation of the International Civil Aviation Organization is NM.[10] The abbreviation nm, though conflicting with the SI symbol for the nanometre, is also widely used. The SI symbol for the newton metre is N m.

History

Historical definition - 1 nautical mile

The nautical mile was historically defined as a minute of arc along a meridian of the Earth, making a meridian exactly 180×60 = 10,800 historical nautical miles.[4] It can therefore be used for approximate measures on a meridian as change of latitude on a nautical chart. The originally intended definition of the metre as 10−7 of a half-meridian arc makes the mean historical nautical mile exactly (2 × 107)/10,800 = 1,851.851851… historical metres. Based on the current IUGG meridian of 20,003,931.4585 (standard) metres the mean historical nautical mile is 1,852.216 m.

The historical definition differs from the length-based standard in that a minute of arc, and hence a nautical mile, is not a constant length at the surface of the Earth but gradually lengthens with increasing distance from the equator, as a corollary of the Earth's oblateness, hence the need for "mean" in the last sentence of the previous paragraph. This length equals about 1,861 metres at the poles and 1,843 metres at the Equator.[11]

Other nations had different definitions of the nautical mile. This variety in combination with the complexity of angular measure described above along with the intrinsic uncertainty of geodetically derived units mitigated against the extant definitions in favor of a simple unit of pure length. International agreement was achieved in 1929 when the International Extraordinary Hydrographic Conference held in Monaco adopted a definition of one international nautical mile as being equal to 1,852 metres exactly, in excellent agreement (for an integer) with both the above-mentioned values of 1,851.851 historical metres and 1,852.216 standard metres.

Use of angle-based length was first suggested by E. Gunter (of Gunter's chain fame), reference: W. Waters, The Art of Navigation in England in Elizabethan and Stuart Times, ( London, 1958). During the 18th century, the relation of a mile of 6000 (geometric) feet, or a minute of arc on the earth surface had been advanced as a universal measure for land and sea. The metric Kilometre was selected to represent a centisimal minute of arc, on the same basis, with the circle divided into 400 degrees of 100 minutes.

Conversions to other units

Visual comparison of a kilometre, statute mile, and nautical mile

One nautical mile converts to:

Associated units

The derived unit of speed is the knot, defined as one nautical mile per hour. The term "log" is used to measure the distance a vessel has moved through the water. This term can also be used to measure the speed through the water (see chip log), as the speed and distance are directly related.

The terms "knot" and "log" are derived from the practice of using a "log" tied to a knotted rope as a method of gauging the speed of a ship. A log attached to a knotted rope was thrown into the water, trailing behind the ship. The number of knots that passed off the ship and into the water in a given time would determine the speed in "knots". The present day measurement of knots and log are determined using a mechanical tow, electronic tow, hull-mounted units (which may or may not be retractable), Doppler (either ultrasonic or radar), or GPS.[12][13] Speeds measured with a GPS differ from those measured by other means in that they are Speed Over Ground (accounting for the effect of current) while the others are Speed Through the Water, which does not account for current.

See also

Notes

  1. ↑ 1.0 1.1 1.2 International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 127, ISBN 92-822-2213-6, http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf 
  2. ↑ Bowditch, Nathaniel, LLD; et al, The American Practical Navigator (2002 ed.), Washington: National Imagery and Mapping Agency, pp. 34–35, http://www.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Chapt-03.pdf 
  3. ↑ 3.0 3.1 Glazebrook, Richard (1922), "Measurement, Units of", Dictionary of Applied Physics, 1, pp. 580–88, http://www.archive.org/details/dictionaryofappl025484mbp .
  4. ↑ 4.0 4.1 National Bureau of Standards (August 1954), "Adoption of International Nautical Mile", Technical News Bulletin, http://physics.nist.gov/Pubs/SP447/app4.pdf .
  5. ↑ 5.0 5.1 5.2 5.3 Ministry Of Defence, Gran Bretaña (1987), Admiralty Manual of Navigation, London: HMSO, pp. 6–7, ISBN 0117728802, http://books.google.com/?id=GCgXCxG4VLcC .
  6. ↑ Schedule to the Units of Measurement Regulations 1995 No. 1804.
  7. ↑ Bowditch, Nathaniel, LLD; et al (1966 - Corrected Print), The American Practical Navigator, Washington: U.S. Navy Hydrographic Office, p. 945 
  8. ↑ 8.0 8.1 Bowditch, Nathaniel, LLD; et al, The American Practical Navigator (2002 ed.), Washington: National Imagery and Mapping Agency, pp. 716–854, http://164.214.12.45/MSISiteContent/StaticFiles/NAV_PUBS/APN/Gloss-1.pdf 
  9. ↑ Chart No. 1, Positions, Distances, Directions, Compass, Jointly by NOAA and Department of Commerce, USA, http://www.nauticalcharts.noaa.gov/mcd/chartno1.htm  The cited book incorporates IHO Chart INT 1 and therefore represents the practice of the members of the IHO, most of the seafaring nations.
  10. ↑ NOTIFICATION OF ANNEX DIFFERENCES (Presented by Australia), International Civil Aviation Organisation, Sixth Meeting of CNS/MET Sub Group of APANPIRG, Bangkok, Thailand, 15–19 July 2002.
  11. ↑ "For a point on the spheroid of the IAU System at geodetic latitude (Φ): 1 degree of latitude [=] (110.575 + 1.110 sin2Φ km." Seidelmann, P. K. (Ed.), (1992), Explanatory supplement to the Astronomical almanac, Sausalito, CA: University Science Books, 700.
  12. ↑ Origin of Naval Terminology, Naval Historical Center, US Naval Dept. Library, http://www.history.navy.mil/library/online/origin.htm#kno, retrieved May 3, 2006 
  13. ↑ Fairhall, David (2005), Pass your day skipper (2nd ed.), A&C Black, ISBN 0713674008 .

External links