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It is commonly used in international law and treaties, especially regarding the limits of territorial waters. It developed from the geographical mile. DefinitionThe international standard definition is: 1 nautical mile = 1852 metres exactly.
Unit symbolThere is no official international standard symbol for the unit nautical mile. The symbols M, NM, nm and nmi are commonly used in some areas (not to be confused with nm, the official symbol for nanometre). The International Bureau of Weights and Measures (BIPM) in its International System of Units brochure lists the nautical mile in the table of units "currently accepted" for use with SI using M as a symbol, mentioning in a footnote other symbols, and stating that as yet there is no internationally agreed symbol. Although nm is the official symbol for nanometre, there is little confusion because it is used in very different contexts, and differs by twelve orders of magnitude (one nautical mile = 1852 billion nanometres). Listings of aircraft flight ranges typically include both the "nm" (nautical mile) and "km" (kilometre) equivalents next to each other. There are several national unit symbols in use (for example, mpk (meripeninkulma, "sea league") in Finnish, sm (Seemeile, "sea mile") in German, and M (sjómíla, "sea mile") in Icelandic). The People's Republic of China uses n mile as the national standard symbol with no s added for plural.
Conversions to other unitsOne nautical mile converts to: 1.852 km (exact); 1.150779 mile (statute); 2025.372 yard; 6076.1155 feet; 1 meridian arc minute at sea-level surface distance;
HistoryThe nautical mile was historically defined as a minute of arc along a meridian of the Earth. It can therefore be used for approximate measures on a meridian as change of latitude on a nautical chart. According to WGS84, radius of curvature in a meridian plane is 6 399 593.6258 metres at the poles and 6 335 439.3273 metres at the Equator. By the definition of geodetic latitude, length of the minute of arc depends on the radius of curvature; distance to the angle (1 minute, in this case), not to the Earth's centre. This length equals to approx. 1861.57 metres at the poles and 1842.90 metres at the Equator. Length of a minute of arc defined by the geocentric latitude depends on the distance from the Earth's center (and curvature). This length is greater at the Equator.
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