Nautical mile Nautical mile A nautical mile (also known as an air mile) is a unit of measurement defined as exactly . Historically, it was defined as one minute of latitude, which is one sixtieth of a degree of latitude – i.e. since a minute of a degree is x = , the Earth's polar circumference is approximately 21,600 nautical miles. It is a non-SI unit which today has a continued use in both air and marine navigation, and for the definition of territorial waters. There is no single internationally agreed symbol. While using M itself, the International Bureau of Weights and
Cylindrical equal-area projection many named specializations such as Gall–Peters or Gall orthographic, Behrmann, and Lambert cylindrical equal-area). This divides north-south distances by a factor equal to the secant of the latitude, preserving area but heavily distorting shapes. Any particular cylindrical equal-area map has a pair of identical latitudes of opposite sign (or else the equator) at which the east–west scale matches the north–south scale. All cylindrical equal-area projections use the formula: where "λ" is the longitude, "λ" is the central meridian, "φ" is the latitude, and "φ" is the standard latitude, all expressed in radians. Some cartographers prefer to work in degrees, rather