Nautical mile

Last updated
Nautical mile
Unit system Non-SI unit
Unit of Length
SymbolM, NM, or nmi
1 M, NM, or nmi in ...... is equal to ...
    metre    1852 [1]
    foot    ≈6076
    statute mile    ≈1.15
    cable    10
Historical definition - 1 nautical mile Nautic mile definition v2 English.svg
Historical definition – 1 nautical mile
Visual comparison of a kilometre, statute mile, and nautical mile Nauticalmilecomparison.svg
Visual comparison of a kilometre, statute mile, and nautical mile

A nautical mile is a unit of measurement used in air, marine, and space navigation, [2] and for the definition of territorial waters. [3] Historically, it was defined as one minute (1/60 of a degree) of latitude along any line of longitude. Today the international nautical mile is defined as exactly 1852 metres (about 1.15 miles). The derived unit of speed is the knot, one nautical mile per hour.


Unit symbol

There is no single internationally agreed symbol, with several symbols in use. [1]


The word mile is from the Latin word for a thousand paces: mille passus. Navigation at sea was done by eye [10] until around 1500 when navigational instruments were developed and cartographers began using a coordinate system with parallels of latitude and meridians of longitude.

By the late 16th century, Englishmen knew that the ratio of distances at sea to degrees were constant along any great circle such as the equator or any meridian, assuming that Earth was a sphere. Robert Hues wrote in 1594 that the distance along a great circle was 60 miles per degree, that is, one nautical mile per arcminute. [11] Edmund Gunter wrote in 1623 that the distance along a great circle was 20 leagues per degree. [11] Thus, Hues explicitly used nautical miles while Gunter did not.

Since the Earth is not a perfect sphere but is an oblate spheroid with slightly flattened poles, a minute of latitude is not constant, but about 1861 metres at the poles and 1843 metres at the Equator. [12] France and other metric countries state that in principle a nautical mile is an arcminute of a meridian at a latitude of 45°, but that is a modern justification for a more mundane calculation that was developed a century earlier. By the mid 19th century France had defined a nautical mile via the original 1791 definition of the metre, one ten-millionth of a quarter meridian. [13] [14] Thus 10,000,000 m/90 × 60 = 1851.85 m ≈ 1852 m became the metric length for a nautical mile. France made it legal for the French Navy in 1906, and many metric countries voted to sanction it for international use at the 1929 International Hydrographic Conference.

Both the United States and the United Kingdom used an average arcminute, specifically, a minute of arc of a great circle of a sphere having the same surface area as the Clarke 1866 ellipsoid. [15] The authalic (equal area) radius of the Clarke 1866 ellipsoid is 6,370,997.2 metres (20,902,222 ft). [16] The resulting arcminute is 1853.2480 metres (6080.210 ft). The United States chose five significant digits for its nautical mile, 6080.2 feet, whereas the United Kingdom chose four significant digits for its Admiralty mile, 6080 feet.

In 1929, the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as exactly 1,852 metres. [1] The United States did not adopt the international nautical mile until 1954. [17] Britain adopted it in 1970, [18] but legal references to the obsolete unit are now converted to 1853 metres. [19]

Similar definitions

The metre was originally defined as 110,000,000 of the length of the meridian arc from the North pole to the equator, [lower-alpha 1] thus one kilometre of distance corresponds to one centigrad of latitude. The Earth's circumference is therefore approximately 40,000 km. The equatorial circumference is slightly longer than the polar circumference – the measurement based on this (40,075.017/60×360 = 1855.3 metres) is known as the geographical mile.

See also


  1. No meridian was specified in either 1791, 1793, 1795, or 1799. For example, the Law of 18 Germinal an III (April 7, 1795) states: "Meter, the measure of length equal to the ten-millionth part of a terrestrial meridian contained between the north pole and the equator." [20]

Related Research Articles

Minute and second of arc Units for measuring angles

A minute of arc, arcminute (arcmin), arc minute, or minute arc is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, one minute of arc is 1/21600 of a turn. The nautical mile was originally defined as a minute of latitude on a hypothetical spherical Earth so the actual Earth circumference is very near 21 600 nautical miles. A minute of arc is π/10800 of a radian.

The geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator. For the 1924 International Spheroid 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.

Latitude geographic coordinate specifying north–south position

In geography, latitude is a geographic coordinate that specifies the north–south position of a point on the Earth's surface. Latitude is an angle which ranges from 0° at the Equator to 90° at the poles. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Latitude is used together with longitude to specify the precise location of features on the surface of the Earth. On its own, the term latitude should be taken to be the geodetic latitude as defined below. Briefly, geodetic latitude at a point is the angle formed by the vector perpendicular to the ellipsoidal surface from that point, and the equatorial plane. Also defined are six auxiliary latitudes that are used in special applications.

Longitude geographic coordinate that specifies the east-west position of a point on the Earths surface

Longitude, is a geographic coordinate that specifies the east–west position of a point on the Earth's surface, or the surface of a celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ). Meridians connect points with the same longitude. The Prime Meridian, which passes near the Royal Observatory, Greenwich, England, is defined as 0° longitude by convention. Positive longitudes are east of the Prime Meridian, and negative ones are west.

Metre SI unit of length

The metre or meter is the base unit of length in the International System of Units (SI). The SI unit symbol is m. The metre is defined as the length of the path travelled by light in a vacuum in 1/299 792 458 of a second. The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's circumference is approximately 40000 km. In 1799, the metre was redefined in terms of a prototype metre bar. In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. The current definition was adopted in 1983 and slightly updated in 2019.

Mile Unit of length

The mile is an English unit of length of linear measure equal to 5,280 feet, or 1,760 yards, and standardised as exactly 1,609.344 metres by international agreement in 1959.

International System of Units Modern form of the metric system

The International System of Units is the modern form of the metric system. It is the only system of measurement with an official status in nearly every country in the world. It comprises a coherent system of units of measurement starting with seven base units, which are the second, metre, kilogram, ampere, kelvin, mole, and candela. The system allows for an unlimited number of additional units, called derived units, which can always be represented as products of powers of the base units. Twenty-two derived units have been provided with special names and symbols. The seven base units and the 22 derived units with special names and symbols may be used in combination to express other derived units, which are adopted to facilitate measurement of diverse quantities. The SI system also provides twenty prefixes to the unit names and unit symbols that may be used when specifying power-of-ten multiples and sub-multiples of SI units. The SI is intended to be an evolving system; units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves.

Geographic coordinate system Coordinate system to specify locations on Earth

A geographic coordinate system is a coordinate system that enables every location on Earth to be specified by a set of numbers, letters or symbols. The coordinates are often chosen such that one of the numbers represents a vertical position and two or three of the numbers represent a horizontal position; alternatively, a geographic position may be expressed in a combined three-dimensional Cartesian vector. A common choice of coordinates is latitude, longitude and elevation. To specify a location on a plane requires a map projection.

Celestial navigation Navigation using astronomical objects to determine position

Celestial navigation, also known as astronavigation, is the ancient and modern practice of position fixing that enables a navigator to transition through a space without having to rely on estimated calculations, or dead reckoning, to know their position. Celestial navigation uses "sights", or angular measurements taken between a celestial body and the visible horizon. The Sun is most commonly used, but navigators can also use the Moon, a planet, Polaris, or one of 57 other navigational stars whose coordinates are tabulated in the nautical almanac and air almanacs.

Gram Unit of mass 1/1000 of a kilogram

The gram is a metric system unit of mass.

History of geodesy

Geodesy (/dʒiːˈɒdɨsi/), also named geodetics, is the scientific discipline that deals with the measurement and representation of the Earth. The history of geodesy began in pre-scientific antiquity and blossomed during the Age of Enlightenment.

Nautical chart Topographic map of a maritime area and adjacent coastal regions

A nautical chart is a graphic representation of a sea area and adjacent coastal regions. Depending on the scale of the chart, it may show depths of water and heights of land, natural features of the seabed, details of the coastline, navigational hazards, locations of natural and human-made aids to navigation, information on tides and currents, local details of the Earth's magnetic field, and human-made structures such as harbours, buildings and bridges. Nautical charts are essential tools for marine navigation; many countries require vessels, especially commercial ships, to carry them. Nautical charting may take the form of charts printed on paper or computerized electronic navigational charts. Recent technologies have made available paper charts which are printed "on demand" with cartographic data that has been downloaded to the commercial printing company as recently as the night before printing. With each daily download, critical data such as Local Notices to Mariners are added to the on-demand chart files so that these charts are up to date at the time of printing.

Knot (unit) Unit of speed

The knot is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h. The ISO standard symbol for the knot is kn. The same symbol is preferred by the Institute of Electrical and Electronics Engineers (IEEE); kt is also common, especially in aviation, where it is the form recommended by the International Civil Aviation Organization (ICAO). The knot is a non-SI unit. The knot is used in meteorology, and in maritime and air navigation. A vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.

Gabriel Mouton was a French abbot and scientist. He was a doctor of theology from Lyon, but was also interested in mathematics and astronomy. His 1670 book, the Observationes diametrorum solis et lunae apparentium, proposed a natural standard of length based on the circumference of the Earth, divided decimally. It was influential in the adoption of the metric system in 1799.

Units of measurement in France

France has a unique history of units of measurement due to the radical decision to invent and adopt the metric system after the French Revolution.

Decimal degrees (DD) express latitude and longitude geographic coordinates as decimal fractions and are used in many geographic information systems (GIS), web mapping applications such as OpenStreetMap, and GPS devices. Decimal degrees are an alternative to using degrees, minutes, and seconds (DMS). As with latitude and longitude, the values are bounded by ±90° and ±180° respectively.

The French Geodesic Mission was an 18th-century expedition to what is now Ecuador carried out for the purpose of measuring the roundness of the Earth and measuring the length of a degree of latitude at the Equator. The mission was one of the first geodesic missions carried out under modern scientific principles, and the first major international scientific expedition.

History of the metre origins and previous definitions of the SI base unit for measurement

The history of the metre starts with the scientific revolution that began with Nicolaus Copernicus's work in 1543. Increasingly accurate measurements were required, and scientists looked for measures that were universal and could be based on natural phenomena rather than royal decree or physical prototypes. Rather than the various complex systems of subdivision in use, they also preferred a decimal system to ease their calculations.

Introduction to the metric system

The metric system was developed during the French Revolution to replace the various measures previously used in France. The metre is the unit of length in the metric system and was originally based on the dimensions of the earth, as far as it could be measured at the time. The litre, is the unit of volume and was defined as one thousandth of a cubic metre. The metric unit of mass is the kilogram and it was defined as the mass of one litre of water. The metric system was, in the words of French philosopher Marquis de Condorcet, "for all people for all time".

Earth's circumference is the distance around the Earth. Measured around the poles, the circumference is 39,940.653 km (24,817.971 mi). Measured around the equator, it is 40,075.017 km (24,901.461 mi).


  1. 1 2 3 Göbel, E.; Mills, I.M.; Wallard, Andrew, eds. (2006). The International System of Units (SI) (PDF) (8th ed.). Paris: Bureau International des Poids et Mesures. p. 127. ISBN   92-822-2213-6 . Retrieved 2017-06-20.
  2. "mile | unit of measurement". Encyclopædia Britannica. Retrieved 2016-06-10.
  4. Symboles, Abréviations et Termes utilisés sur les cartes marines [Symbols, Abbreviations and Terms used on Charts](PDF) (in French and English). 1D (INT1) (6th ed.). Service Hydrographique et Océanographique de la Marine (SHOM). 2016. Archived from the original (PDF) on 2016-08-21. Retrieved 2018-01-04. also available as Symbols and Abbreviations used on ADMIRALTY Paper Charts. NP5011 (6th ed.). United Kingdom Hydrographic Office. 2016. section B, line 45. ISBN   978-0-70-774-1741.
  5. "WS SIGMET Quick Reference Guide" (PDF). ICAO. ICAO. Retrieved 2016-06-09.
  6. International Standards and Recommended Practices, Annex 5 to the Convention on International Civil Aviation, “Units of measurement to be Used in Air and Ground Operations”, ICAO, 4th Edition, July 1979.
  7. "APPENDIX A: SYMBOLS AND PREFIXES". IEEE. Retrieved 2016-06-09.
  8. "U.S. Government Printing Office Style Manual". U.S. Government Printing Office. Retrieved 2016-06-10.
  9. Dutton's Navigation and Piloting (14th ed.). Naval Institute Press, Annapolis, MD. 1985. ISBN   0-87021-157-9.
  10. "Mile, Nautical and Statute – FREE Mile, Nautical and Statute information | Find Mile, Nautical and Statute research". Retrieved 2016-06-10.
  11. 1 2 Waters, David W. (1958), The Art of Navigation in England in Elizabethan and Early Stuart Times, p. 374
  12. McNish, Larry. "RASC Calgary Centre - Latitude and Longitude". The Royal Astronomical Society of Canada. Retrieved 30 August 2019.
  13. Bureau des Longitudes (1933), "Mesures employées sur les cartes marines", Annuaire pour l'an 1933: 392, The nautical mile [mille marin] is in principle the length of the sexagesimal minute of a meridian at a latitude of 45°. ... If we assume that the meter is exactly the ten-millionth part of the terrestrial quarter meridian, it would be equal to 1851.85 m. – Translation by Wikipedia.
  14. Bureau des Longitudes (1848), "Mesures itinéraires", Annuaire pour l'an 1848: 74
  15. Blazebrook, Richard (1922), A Dictionary of Physics, 1, p. 587
  16. Snyder, John P. (1987), Map Projections: A Working Manual, p. 16
  17. Astin, A.V.; Karo, H. Arnold (June 25, 1959). "Refinement of values for the yard and the pound" (PDF). National Bureau of Standards. Archived from the original (PDF) on March 9, 2013. Retrieved 2018-07-07.
  18. "Nautical mile definition and meaning | Collins English Dictionary". Collins Dictionary. Retrieved 1 September 2019.
  19. "The Units of Measurement Regulations 1995". Retrieved 2016-06-10.
  20. Hallock, William; Wade, Herbert T. (1906), Outlines of the Evolution of Weights and Measures and the Metric System, p. 54