The nautical mile is a unit of distance measurement, outside the International System of Units, used in maritime and air navigation.
The abbreviation for nautical mile is therefore not standardized. In France, we mainly use the M in upper case (we write for example 5M), but we also find Nm (for Nautical Mille or Nautique) and nmi is used for the nautical mile in the United States. As for the army, it abolishes the mile to use only Nautical, a way to avoid misunderstandings.
A little math lesson
The length of a nautical mile is easily explained:
It is the equivalent of one minute angle in latitude of the length of the arc formed by the Earth’s surface.
More simply if you draw a line that connects the equator to the pole you have an arc formed by the Earth’s surface.
If you divide this arc into degrees, you will get 0° at the equator and 90° at the pole.
And since one degree equals 60 minutes, you get 5400 minutes.
Knowing that the circumference of the earth 40 005 km.
A quarter of this distance gives 10,001 km from the equator to the pole.
Divided by the number of minutes, we get
One minute = 1,852 m
To make head conversions
To go from miles to miles, multiply the measurement by 2 and subtract 10% from the result (which is equivalent to multiplying by 1.8).
Find a scale on the map
To measure distances on a nautical chart, latitude scales (vertical on the chart) are used. Knowing that a minute of latitude measures 1 mile. Simply use the dry point compass to transfer distances.