International Nautical Miles
Abbreviation/Symbol:
NM
nmi
Unit of:
Worldwide use:
The nautical mile (NM) is based on the size of the Earth: it began as one minute of latitude, so a degree of latitude is 60NM and the Earth's circumference is roughly 21,600NM.
On a chart you can measure distance straight off the latitude scale, since a minute of latitude is one nautical mile. Speed is in knots, ie nautical miles per hour, and aircraft navigate in the same units.
The international nautical mile is exactly 1,852 meters, or about 1.151 statute miles. It was agreed at the International Extraordinary Hydrographic Conference in Monaco in 1929, and the United States adopted it in 1954.
Definition:
The international nautical mile (NM) is exactly 1,852 meters, or 1.852km, and is also called the sea mile. It began as the length of one minute of latitude, so the Earth's circumference is roughly 21,600NM.
Before 1929 countries used slightly different nautical miles, as the length of a minute of latitude varies a little with latitude. The international figure of 1,852m was agreed that year at a conference in Monaco.
Origin:
The nautical mile began as one minute of latitude – a sixtieth of a degree – so that distances could be read directly from the latitude scale on a chart. (Counting knots in a rope was a way of measuring speed, not distance.)
A minute of latitude varies slightly, from about 1,843m at the equator to 1,862m at the poles, so countries used different values until the International Hydrographic Conference in Monaco in 1929 adopted 1,852 meters. The US adopted it in 1954 and the UK in 1970. The Earth works out at about 21,600 nautical miles round.
The 1,852m figure was chosen because it is close to the average length of a minute of latitude. Even with GPS, speed at sea and in the air is still given in knots.
Common references:
One international nautical mile is 1,852 meters
Usage context:
The international nautical mile is exactly 1,852 meters, which is about 6,076 feet or 1.15 statute miles. It was fixed at that figure in 1929, close to the average length of one minute of latitude.
Ships and aircraft both give distance in nautical miles and speed in knots, which already means nautical miles per hour – so 'knots per hour' is a mistake, and 20 knots is 20 nautical miles in an hour.
Furthermore, International Nautical Miles are also used in meteorology and oceanography. Weather forecasts often provide information on the distance between weather systems or the location of storms using nautical miles. Oceanographers use nautical miles to measure the length of coastlines, the distance between research stations, and the extent of ocean currents.