# nautical mile

Physical unit
Unit name Nautical mile, nautical mile
Unit symbol ${\ displaystyle \ mathrm {M, sm, NM}}$
Physical quantity (s) length
Formula symbol ${\ displaystyle l}$
dimension ${\ displaystyle {\ mathsf {L}}}$
In SI units ${\ displaystyle \ mathrm {1 \, sm = 1 \, 852 \; m}}$
Derived from Arc minute

The nautical mile or nautical mile ( M , German: sm , English: NM ) is a unit of length commonly used in shipping and aviation . It should correspond to 1/60 degree of longitude at the equator or 1/60 degree of latitude, i.e. one angular minute . However, it was later defined to be exactly 1852.0 m. The unit of speed derived from this, nautical miles per hour , is called the knot .

## definition

In the international standard EN ISO 80000–3 , 1 nautical mile is set to exactly 1852  m , the same can also be found in the German DIN 1301 , Part 2, from February 1978.

## history

Originally, the measure of the nautical mile was based on the arc length of an arc minute on a great circle - e.g. B. the equator or a meridian - the globe-like body of the earth. This corresponds to the 60th part of the distance between two neighboring whole-numbered longitudes on the equator - or between two neighboring longitudes. The length of a meridian from the equator to the pole of approx. 10,001.966 km (ellipsoid parameter of the WGS84 ) results in an average value of

${\ displaystyle 1 \, {\ text {sm}} = {\ frac {10001 {,} 966 \, \, {\ text {km}}} {90 \ cdot 60}} = 1852 {,} 216 \, \, {\ text {m}}}$.

Due to the flattening of the earth , a meridian minute of the geographical latitude measures 1,842.90 m at the equator, but 1,861.57 m at the poles. In contrast , for a minute of arc measured from the center of the earth at the equator, an arc length of 1855.31 m results.

### Admiralty Mile, US nautical mile, International Nautical Mile

So until 1929 Great Britain added a length of exactly 800  English feet to the length of the English mile (a total of 6080 feet) for the measure of the nautical mile , resulting in the measure of 1853.18 meters for the English Admiralty Mile .

In the USA until 1954 the US nautical mile was used as the measure of the nautical mile , which was set at 6080.20 feet, which corresponds to a length of 1853.24 meters.

Internationally, the dimension of the International Nautical Mile was set at 1852.01 m at the International Hydrographic Conference in Monaco in 1929  . Deviating from this, however, the German DIN 1301-1 and the BIPM brochure mentioned below state that at that time the value 1852 m was assumed for the international nautical mile .

### Nautical mile, nautical mile

The Admiralty Manual of Navigation (1964) differentiates between nautical mile and sea ​​mile . The nautical mile corresponds to the official but non-legal international SI unit nautical mile (French name definition: mille marin; literally German: nautical mile, literally English: sea mile) with the length of 1852 m defined by international treaties, i.e. without decimal places . The introduction of the new name was necessary because the term sea ​​mile, with its historical definition and its current and future relevance, enjoyed absolute priority . This English sea mile is the length of one minute of arc on any great circle of the earth, that fluctuates due to the Earth flattening between 1843 m (Meridian at the equator) and 1,862 m (Meridian at the poles) and is still only in the immediate vicinity of the ship's location recommended. On this occasion, the further use of the admiralty mile (1853.18 m) was declared undesirable. If necessary, the term admirality mile can be replaced with nautical mile . Because of the accuracy that can actually be achieved in location determination and measurement technology at sea ( astronavigation , radio direction finding , DECCA , LORAN , visual direction finding, log ), the different definitions ( GPS ) do not play a practical role to this day . Errors only arise when the computational accuracy of modern computers is used to work over long distances with an unsuitable scale .

After reliable data about the flattening of the earth had been determined in the middle of the 19th century, especially by Friedrich Wilhelm Bessel , the geographical land mile and the geographical nautical mile were introduced in Germany in addition to the great-circle-based definition of the nautical mile : The geographical land mile corresponds to the historical nautical mile on the equator (1855.46 m), the geographical nautical mile corresponds to the nautical mile on a meridian (1843 m to 1862 m). Both should be useful when surveying on land and at sea, but could not prevail in practice and are hardly known today. With the adoption of the nautical mile as an “official but non-statutory SI unit”, all great-circle-based definitions in Germany were deleted without replacement due to their practical irrelevance. Only in astronomical navigation is a nautical mile still equated with a minute of arc on a great circle. However, it is unnecessary to know a conversion factor in meters, so that the modern definition is irrelevant. Ultimately, for over a thousand years astronavigation has used the spherical shape of the earth and ignored the flattening.

The historical definition of the nautical mile is still used daily in seafaring for map work insofar as Mercator maps are used almost exclusively for navigation. All lines in north-south direction are always parts of meridians, i.e. great circles. As a result, the scaling of the left and right edge of the map with the degrees of latitude and minutes of latitude is an approximately accurate measure of nautical miles in the area of ​​the relevant latitude. The map edges are deliberately designed that length measurements with the Divider are particularly simple and clear.

When it comes to navigation, it is of no interest how many meters a nautical mile is, because all measuring instruments and maps always refer to the nautical mile (or arc minute) anyway. In astronavigation, the angular minutes read on the sextant also correspond to arc minutes on the globe. These equivalences are never exact, but more than sufficiently accurate for constant use.

## Aviation: Nautical and Statute Miles

Until the end of World War II , German aviation used meters for altitude and kilometers for distance. The United States took over the war a leading role in the creation of national regulators for aviation ( FAA ) and later also in establishing international aviation organizations ( ICAO , IATA ). As a result, nautical miles established themselves as distance information in aviation almost all over the world.

Exceptions are China , Russia and other countries of the CIS and formerly the Soviet Union . These countries apply the international system of units consistently in aviation. Pilots must switch between SI units and feet / miles / ... when crossing the airspace borders. These countries are also increasingly using the usual non-SI units for global standardization.

Conversions between nautical miles and kilometers are seldom necessary as nautical miles are used throughout. In the USA and Great Britain in particular, there is greater confusion with the distinction between nautical mile and statute mile .

In radiotelephony , the expression “miles” always refers to nautical miles. Distance information on US road signs and maps (e.g. Daytona 6 miles ), however, always relate to statute miles (= 1609.344 m). When defining the US airspace structure, statute miles are decisive for the cloud distances and visibility to be observed.

## conversion

From the original definition of nautical mile (1 meridian minute) and the oldest definition of 1 meter (the 10 millionth part of the distance between the North Pole and the equator), the fact that 54 nautical miles corresponds to about 100 kilometers. Approximately, x nautical miles can be converted into kilometers using the rule of thumb “double and subtract 10% from the result” , which corresponds to a multiplication by a factor of 1.8. With this formula, the nautical mile value in kilometers can be calculated by almost 3% too tight, but -  calculated in the head , for a rough estimate  - in the vast majority of cases this means a practicable and sufficient accuracy.

## Abbreviations and derived units

There is no general agreement on the unit symbol of the nautical mile. The abbreviation sm is common in German , internationally it is often abbreviated as nautical mile with nm , NM or nm . The IHO standard for charts requires the international abbreviation M . NM is used in air traffic control .

The 1999 Encyclopaedia of Scientific Units, Weights and Measures names the internationally used abbreviation for nautical mile mi . According to a BIPM brochure , however, the following symbols are used: M , NM , Nm and nmi . Confusion with the SI units nanometer ( nm ) and newton meter ( N • m ) can usually be excluded from the context. In Austria and Germany , the nautical mile is permitted as a “legal unit in metrology” due to international agreements, although it is outside the SI .

• a nautical mile is divided into 10  cable lengths ( kbl )
• one nautical mile is approximately 2000 yards
• approximately equal to 1/1000 nautical mile (1.852 m) - exactly: 6  feet , i.e. 1.8288 m - is the thread ( fm ) that is almost only used for depth information in some nautical charts for shipping.
• 3600ste the part of the nautical mile is the Meridiantertie ( mtr ), which for the second speed in the log is used

The speed of vessels in maritime and aircraft is computationally usually expressed in nautical miles per hour, the unit symbol kn it is based, however, on the notion node (English: knot ).

## Earlier "nautical miles"

The Turkish nautical mile corresponded

• 1 nautical mile = 1296 meters

The Dutch as well as the French "mile" at sea corresponded

## literature

• Sobel / Andrewes, Längengrad, Berlin 1999
• The International System of Units. Bureau International des Poids et Mesures (BIPM): 8th edition 2006, ISBN 92-822-2213-6 . There especially p. 124, 127 and 128 in the English part. (PDF file; 3.88 MB)
• Bureau International des Poids et Mesures: Le Système International (SI). 6th edition. 1991, ISBN 92-822-2112-1 .
• Standard DIN 1313 April 1978, physical quantities and equations, terms, notations.
• Directive of the Council for the approximation of the laws of the member states on units in metrology (as amended) of the proposal of June 21, 1991 (91 / C185 / 06) AbL. No. C 185 v. July 17, 1991, pp. 13-21)
• Units and Time Act , Units Ordinance
• A. Sacklowski: Unit dictionary , origin, application, explanation of law and norms. (= Beuth comments ). Beuth-Verlag, Berlin 1986, ISBN 3-410-11988-4 .
• P. Anderton, PH Bigg: Changing to the metric system, conversion factors, symbols and definitions. Her Majesty's Stationery Office , London 1965.
• Standard ANSI / IEEE Std 268-1982. American National Standard, Metric Practice
• Admiralty Manual of Navigation. Volume 1 (BR 45 (1)) Her Majesty's Stationery Office , London 1964. (consolidated edition 1970)

Wiktionary: nautical mile  - explanations of meanings, word origins, synonyms, translations

## Individual evidence

1. DIN EN ISO 80000-3: 2013 Sizes and units - Part 3: Space and time , Section 3-1.a
2. The Earth according to WGS 84 (every degree of latitude), calculated by Sigurd Humerfelt ( Memento from April 14, 2011 in the Internet Archive )
3. ^ Declan Selleck: Special Report: Russia transition to ICAO RVSM. In: International Ops 2019 - OPSGROUP. November 18, 2011. Retrieved June 27, 2019 (American English).
4. Declan Selleck: Big change: Russia finally moving to QNH. In: International Ops 2019 - OPSGROUP. February 22, 2017. Retrieved June 27, 2019 (American English).
5. China Reduced Vertical Separation Minima (RVSM) - SKYbrary Aviation Safety. Retrieved June 27, 2019 .
6. Activities, Courses, Seminars & Webinars - ALC_Content - FAA - FAASTeam - FAASafety.gov. Retrieved June 27, 2019 .
7. bipm.org
8. ptb.de ( Memento from September 24, 2015 in the Internet Archive )
9. ICAO (2010) Official definition, ANNEX 5 Units of Measurement to be Used in Air and Ground Operations
10. EUROCONTROL (1997) doc. "Radar Sensor Performance Analysis" SUR.ET1.ST03.1000-STD-01-01, p. 24 [1]
11. ^ Johann Ph. Neumann: Textbook of Physics. Volume 1, Verlag Carl Gerold, Vienna 1930, p. 506.
12. ^ Johann Ph. Neumann: Textbook of Physics. Volume 1, Verlag Carl Gerold, Vienna 1930, p. 505.
13. For example, is the novel " 20,000 leagues sous les Mers " by Jules Verne in German "20,000 Miles Under the Sea" instead of "20,000 Leugen under the sea".