nautical mile

Non-SI unit

Name	Symbol	Quantity	SI units
nautical mile	nmi	length	1852 m
The nautical mile, symbol nmi, NM or M, is a non-SI unit of length. The nautical mile is not accepted for use with the SI. One nautical mile is equal to 1852 m.
Definition			c Δν_Cs^-1
$1 \mspace{4mu} \text{nmi} \mspace{6mu} = \dfrac{(1 \mspace{4mu} 852)(9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770)}{299 \mspace{4mu} 792 \mspace{4mu} 458} \mspace{6mu} \dfrac{c}{\Delta \nu _{Cs}}\\ \\ \\ 1 \mspace{4mu} \text{nmi} \mspace{6mu} \approx 5.678 \mspace{4mu} 846 \mspace{4mu} 676 \mspace{4mu} 669 \mspace{4mu} 898 \mspace{4mu} 080 \times10^{4} \mspace{6mu} c \mspace{4mu} {\Delta \nu _{Cs}}^{-1}$

The nautical mile is a historic non-SI unit of length traditionally used in marine and aerial navigation.

One nautical mile was originally defined as the distance subtended along the Earth’s surface by an angle at the centre of the Earth equal to one arc minute. Thus, the distance along the Earth’s surface spanning one degree of latitude is equal to approximately 60 nautical miles.

A simple relationship between angle of latitude and distance along the Earth’s surface was also one of the objectives of the metric system when it was originally developed. Compared with the old nautical miles method, the metric system can simplify navigational calculations without the need for measurement units designed solely for navigation or mapping purposes.

By defining the metre as ¹⁄_{10 000 000} of the distance along a meridian from the North Pole to the Equator, and using a centesimal unit of angle measurement, which divides a right-angle into 100 gradians, or gons, the distance along the Earth’s surface spanning one gon of latitude is equal to approximately 100 kilometres, with one centigon corresponding to one kilometre.