The international nautical mile is defined as exactly 1,852 metres. This definition was adopted in 1929 by the International Extraordinary Hydrographic Conference, Monaco. The United States adopted it in 1954. Prior to the adoption of the international nautical mile, the nautical mile used by the US and the UK was 6080 feet, or 1853.184 metres.
A nautical mile is approximately a minute of arc along a great circle of the Earth and was formerly defined so. The earth is not a perfect sphere, so a minute of arc can be less than, or more than, a nautical mile by a few metres.
For purposes of practical navigation, the former definition is still widely used: the length of a minute of arc of latitude is found on any nautical chart and can be taken to represent approximately one nautical mile on that particular chart.
The abbreviation nm is often used, although this means nanometre in the SI.
See also: conversion of units\n