From Wikipedia, the free encyclopedia Nautical mile
Nautical mile
1 nautical mile = nautical mile was defined as 1853.248 metres[4]
SI units (6080.20 U.S. feet, based on the definition of the foot in
the Mendenhall Order of 1893): it was abandoned in
1.85200 km 1,852.00 m
favour of the international nautical mile in 1954.[5] The
US customary / Imperial units Imperial (UK) nautical mile, also known as the Admiralty
mile,
mile was defined in terms of the knot such that one
1.15078 mi 6,076.12 ft
nautical mile was exactly 6080 feet (1853.184 m):[6] it was
The nautical mile (symbol M, NM or nmi is a unit of
nmi) abandoned in 1970[6] and, for legal purposes, old refer-
length that is about one minute of arc of latitude along ences to the obsolete unit are now converted to 1853 me-
any meridian, but is approximately one minute of arc tres exactly.[7]
of longitude only at the equator. By international agree-
ment it is exactly 1,852 metres (approximately 6,076 Sea mile
feet). In English usage, a sea mile is, for any latitude, the length
It is a non-SI unit (although accepted for use in the In- of one minute of latitude at that latitude. It varies from
ternational System of Units by the BIPM) used especially about 1,842.9 metres (6,046 ft) at the equator to about
by navigators in the shipping and aviation industries,[1] 1,861.7 metres (6,108 ft) at the poles, with a mean value
and also in polar exploration. It is commonly used in in- of 1,852.3 metres (6,077 ft).[6] The international nautical
ternational law and treaties, especially regarding the lim- mile was chosen as the integer number of metres closest
its of territorial waters. It developed from the sea mile to the mean sea mile.
and the related geographical mile. American use has changed recently. The glossary in
The nautical mile remains in use by sea and air navi- the 1966 edition of Bowditch defines a "sea mile" as a
gators worldwide because of its convenience when work- "nautical mile".[8] In the 2002 edition, the glossary says:
ing with charts. Most nautical charts are constructed on "An approximate mean value of the nautical mile equal
the Mercator projection whose scale varies by approxi- to 6,080 feet; the length of a minute of arc along the
mately a factor of six from the equator to 80° north or meridian at latitude 48°."[9]
south latitude. It is, therefore, impossible to show a single The sea mile has also been defined as 6000 feet or
linear scale for use on charts on scales smaller than about 1000 fathoms, for example in Dresner’s Units of Measure-
1/80,000.[2] Since a nautical mile is, for practical naviga- ment. Dresner includes a remark to the effect that this
tion, the same as a minute of latitude, it is easy to mea- must not be confused with the nautical mile.
sure a distance on a chart with dividers, using the lati-
tude scale on the side of the chart directly to the east or Geographical mile
west of the distance being measured. The geographical mile is the length of one minute of lon-
gitude along the Equator, about 1855.4 m on the Interna-
Definition tional (1924) Spheroid[6] or about 1855.325 m on the WGS
84 ellipsoid. Bowditch defines it as 6,087.08 feet, which is
The international nautical mile was defined by the First 1,855.34 metres.[9] The term "geographical mile" has also
International Extraordinary Hydrographic Conference, been used to refer to the mean sea mile, which would lat-
Monaco (1929) as exactly 1852 metres.[1] This is the only er become the international nautical mile.[3]
definition in widespread current use, and is the one ac- Do not confuse this with the similar-sounding unit
cepted by the International Hydrographic Organization the geografische Meile, seen in historical German measure-
and by the International Bureau of Weights and Measures ments. This unit was intended to be the length of four
(BIPM). Before 1929, different countries had different de- minutes of arc along the equator and is standardized as
finitions, and the Soviet Union, the United Kingdom and 7421.6 metres. In Germany, the Mile, Uhr or Stunde typi-
the United States did not immediately accept the inter- cally refers to 24,000 of the local foot. This is the distance
national value. one might walk in an hour (Stunde).
Both the Imperial and U.S. definitions of the nautical
mile were based on the Clarke (1866) Spheroid: they were Telegraphic mile
different approximations to the length of one minute of
A telegraphic mile is the rounded length of a minute of
arc along a great circle of a sphere having the same sur-
arc along the Equator.
face area as the Clarke Spheroid.[3] The United States
1
From Wikipedia, the free encyclopedia Nautical mile
Tactical mile or data mile chart. The originally intended definition of the metre as
10−7 of a half-meridian arc makes the mean historical
As an approximation, designers of radar systems for bal-
nautical mile exactly (2×107)/10,800 = 1,851.851851… his-
listic, cruise and anti-ship missiles used by NATO navies
torical metres. Based on the current IUGG meridian of
use 6,000 feet (1,828.8 m) as their equivalent of a nautical
20,003,931.4585 (standard) metres the mean historical
mile.
mile. In the Royal Navy, this is also known as a data mile
nautical mile is 1,852.216 m.
The historical definition differs from the length-
Radar mile based standard in that a minute of arc, and hence a nau-
In radar theory, the data mile (6,000 feet) is the length tical mile, is not a constant length at the surface of the
unit. A radar mile is the time it takes a radar pulse to Earth but gradually lengthens in the north-south direc-
travel one datamile forth and a data mile back again, tion with increasing distance from the equator, as a
which equals 12.277 μs. This value corresponds with the corollary of the Earth’s oblateness, hence the need for
speed of light (c. 3×108 m/s). A radar’s range can be de- "mean" in the last sentence of the previous paragraph.
termined by dividing the listening time (pulse repetition This length equals about 1,861 metres at the poles and
time minus pulsewidth) by a radar mile. 1,843 metres at the Equator.[13]
Other nations had different definitions of the nautical
Unit symbol mile. This variety in combination with the complexity
of angular measure described above along with the in-
The International Hydrographic Organization, whose trinsic uncertainty of geodetically derived units mitigat-
membership includes essentially all seafaring nations, ed against the extant definitions in favor of a simple unit
and the International Bureau of Weights and Measures of pure length. International agreement was achieved in
use M as the abbreviation for the nautical mile.[1][10] The 1929 when the International Extraordinary Hydrograph-
preferred abbreviation of the International Civil Aviation ic Conference held in Monaco adopted a definition of one
Organization is nm [11] The abbreviation nm though con-
nm. nm, international nautical mile as being equal to 1,852 metres
flicting with the SI symbol for the nanometre, is also exactly, in excellent agreement (for an integer) with both
widely used. The SI symbol for the newton metre is n m the above-mentioned values of 1,851.851 historical me-
n·m, nm,
(with a space) or n·m not nm because only prefixes may tres and 1,852.216 standard metres.
abut a unit symbol.[12] Use of angle-based length was first suggested by E.
Gunter (of Gunter’s chain fame).[14] During the 18th cen-
History tury, 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 kilo-
metre was selected to represent a centisimal minute of
arc, on the same basis, with the circle divided into 400 de-
grees of 100 minutes.
Conversions to other units
Historical definition - 1 nautical mile
The nautical mile was historically defined as a minute
of arc along a meridian of the Earth (north-south), mak- Visual comparison of a kilometre, statute mile, and nautical
ing a meridian exactly 180×60 = 10,800 historical nautical mile
miles.[5] It can therefore be used for approximate mea-
sures on a meridian as change of latitude on a nautical One international nautical mile converts to:
2
From Wikipedia, the free encyclopedia Nautical mile
• 1.852 kilometres (exact) Imagery and Mapping Agency, pp. 34–35,
• 1.150779 miles (statute) (exact: 57,875/50,292 miles) http://msi.nga.mil/MSISiteContent/StaticFiles/
• 2,025.372 yards (exact: 2,315,000/1,143 yards) NAV_PUBS/APN/Chapt-03.pdf
• 6,076.1155 feet (exact: 2,315,000/381 feet or [3] ^ Glazebrook, Richard (1922), "Measurement, Units
1,822,831/300 survey feet) of", Dictionary of Applied Physics, 1, pp. 580–88,
• 1,012.6859 fathoms (exact: 1,157,500/1,143 fathoms) http://www.archive.org/details/
• 10 international cables (exact) dictionaryofappl025484mbp .
• 10.126859 imperial (100-fathom) cables (exact: [4] Aside from rounding this is the exact length of a
11,575/1,143 imperial cables) great-circle minute on a sphere of radius
• 8.439049 U.S. customary (120-fathom) cables (exact: 6370997.2406 meters, which is the sphere that has
57,875/6,858 U.S. customary cables) the same area as the Clarke 1866 spheroid as
• 0.998383 equatorial arc minutes (traditional usually defined.
geographical miles) [5] ^ National Bureau of Standards (August 1954),
• 0.9998834 mean meridian arc minutes (mean "Adoption of International Nautical Mile", Technical
historical nautical miles) News Bulletin, http://physics.nist.gov/Pubs/SP447/
app4.pdf .
Associated units [6] ^ Ministry Of Defence, Gran Bretaña (1987),
Admiralty Manual of Navigation, London: HMSO,
The derived unit of speed is the knot, defined as one nau- pp. 6–7, ISBN 0117728802,
tical mile per hour. The term "log" is used to measure the http://books.google.com/?id=GCgXCxG4VLcC .
distance a vessel has moved through the water. This term [7] "The Units of Measurement Regulations 1995". The
can also be used to measure the speed through the water National Archives. http://www.legislation.gov.uk/
(see chip log), as the speed and distance are directly re- uksi/1995/1804/schedule/made. Retrieved
lated. February 2011.
The terms "knot" and "log" are derived from the [8] Bowditch, Nathaniel, LLD; et al (1966 - Corrected
practice of using a "log" tied to a knotted rope as a Print), The American Practical Navigator, Washington:
method of gauging the speed of a ship. A log attached to U.S. Navy Hydrographic Office, p. 945
a knotted rope was thrown into the water, trailing be- [9] ^ Bowditch, Nathaniel, LLD; et al, The American
hind the ship. The number of knots that passed off the Practical Navigator (2002 ed.), Washington: National
ship and into the water in a given time would determine Imagery and Mapping Agency, pp. 716–854,
the speed in "knots". The present day measurement of http://164.214.12.45/MSISiteContent/StaticFiles/
knots and log are determined using a mechanical tow, NAV_PUBS/APN/Gloss-1.pdf
electronic tow, hull-mounted units (which may or may [10] Chart No. 1, Positions, Distances, Directions, Compass,
not be retractable), Doppler (either ultrasonic or radar), Jointly by NOAA and Department of Commerce,
or GPS.[15][16] Speeds measured with a GPS differ from USA, http://www.nauticalcharts.noaa.gov/mcd/
those measured by other means in that they are Speed chartno1.htm The cited book incorporates IHO
Over Ground (accounting for the effect of current) while Chart INT 1 and therefore represents the practice
the others are Speed Through the Water, which does not of the members of the IHO, most of the seafaring
account for current. nations.
[11] NOTIFICATION OF ANNEX DIFFERENCES (Presented
See also by Australia), International Civil Aviation
Organisation, Sixth Meeting of CNS/MET Sub
• Conversion of units Group of APANPIRG, Bangkok, Thailand, 15–19 July
• Knot (unit) for the unit of speed 2002.
• Mile for other types of mile [12] SI unit symbols
• Orders of magnitude (length) [13] "For a point on the spheroid of the IAU System at
• Units of measurement geodetic latitude (Φ): 1 degree of latitude [=]
(110.575 + 1.110 sin2Φ) km." Seidelmann, P. K. (Ed.),
Notes (1992), Explanatory supplement to the Astronomical
almanac, Sausalito, CA: University Science Books,
[1] ^ International Bureau of Weights and Measures 700.
(2006), The International System of Units (SI) (8th ed.), [14] W. Waters, The Art of Navigation in England in
p. 127, ISBN 92-822-2213-6, http://www.bipm.org/ Elizabethan and Stuart Times, (London, 1958)
utils/common/pdf/si_brochure_8_en.pdf [15] Origin of Naval Terminology, Naval Historical Center,
[2] Bowditch, Nathaniel, LLD; et al, The American US Naval Dept. Library,
Practical Navigator (2002 ed.), Washington: National
3
From Wikipedia, the free encyclopedia Nautical mile
http://www.history.navy.mil/library/online/
origin.htm#kno, retrieved May 3, 2006
External links
[16] Fairhall, David (2005), Pass your day skipper (2nd • National Bureau of Standards: Refinement of values
ed.), A&C Black, ISBN 0713674008 . for the yard and the pound (1959)
• Moritz, H. (1980), "Geodetic Reference System", • Measure Distances on a map in Nautical miles
Bulletin Geodesique 54 (3). (IUGG/WGS-84 data) • Nautical Unit Converter Nautical miles converted to
• Taff, Laurence G. (1981), Computational Spherical and from miles and kilometres
Astronomy, John Wiley and Sons (IAU data)
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