Nautical mile

1 nautical mile =
SI units
1.8520 km 1,852.0 m
US customary / Imperial units
1.1508 mi 6,076.1 ft

A nautical mile or sea mile is a unit of length. It corresponds approximately to one minute of latitude along any meridian. It is a non-SI unit used especially by navigators in the shipping and aviation industries.[1] It is commonly used in international law and treaties, especially regarding the limits of territorial waters. It developed from the geographical mile.

Contents

Definition

The international standard definition is: 1 nautical mile = 1,852 metres exactly.[1]

Unit symbol

There is no widely accepted international standard symbol for the unit nautical mile. The preferred abbreviation of the IEEE is nmi,[2] while M is used by the BIPM[1] and the maritime authorities of the USA[3] and Canada.[4] For aviation use, the preferred abbreviation of the ICAO is NM.[5] The abbreviation nm, though conflicting with the SI symbol for the nanometre, is also in widespread use.

Conversions to other units

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

One nautical mile converts to:

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.[6] 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 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. [7]

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 (1) 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.

Since the 1929 agreement, all nations have now adopted the international definition. In the United States, which formerly used a value of 1,853.248 m (6,080.2 ft), the new definition has been in use officially since July 1, 1954.[6]

British nautical mile (admiralty)

The British definition of the nautical mile originally related to the length on the surface of the Earth just south of Great Britain. It was not specified according to a calibrated measurement of the Earth, but chosen as exactly 800 feet longer than a statute mile, namely 6,080 feet. For disambiguation, this is sometimes called the "admiralty mile" after the British Admiralty. The precise definition of the foot varied slightly around the world until the international yard, always equal to exactly three feet, was standardized at exactly 0.9144 m in 1959, making the admiralty mile exactly 1,853.184 m. The Royal Hydrographic Office of the United Kingdom converted to the international definition in 1970.

US navy nautical mile (rocketry)

As a simpler approximation, designers of radar systems for ballistic and cruise missiles for use by the United States Navy in the 1950s would take 6,000 feet (1,828.8 m) as their equivalent of a nautical mile. In the past, some ship-borne computer systems developed for the Royal Navy also used the "data mile" of 6,000 feet, and the more unusual "foot*", equivalent to about nine inches, defined as 6,000/8,192 feet (223 mm).

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 term knot and log originally are derived from the practice of using a "log" tied to a knotted rope as a method of gauging speed of a ship. The log would be thrown into the water and the rope trailed 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, retractable hull-mounted unit, doppler, ultrasonics, or GPS.[8][9]

See also

Notes

  1. ↑ 1.0 1.1 1.2 Section 4.1 Table 8 in the International System of Units 8th ed. (2006) by the Bureau International des Poids et Mesures.
  2. ↑ IEEE guidelines for authors
  3. ↑ Positions, Distances, Directions, Compass; Office of Coastal Survey, NOAA, USA
  4. ↑ POSITIONS, DISTANCES, DIRECTIONS, COMPASS, Ministry of Fisheries and Oceans, Canada
  5. ↑ NOTIFICATION OF ANNEX DIFFERENCES (Presented by Australia)International Civil Aviation Organisation, Sixth Meeting of CNS/MET Sub Group of APANPIRG, Bangkok, Thailand, 15 to 19 July 2002,
  6. ↑ 6.0 6.1 Louis E. Barbrow and Lewis V. Judson (1976). "Appendix 4 The international nautical mile" (PDF). Weights and Measures Standards of the United States, A brief history. NIST Physics Laboratory. Retrieved on 2008-06-01.
  7. ↑ "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.
  8. ↑ "Origin of Naval Terminology". Naval Historical Center, US Naval Dept. Library (May 03, 2006).
  9. ↑ David, Fairhall (2005). "Pass your day skipper (2nd Edition)"

External links