6.6 Distance on the Earth 

Distance on the Earth's surface may be expressed using one of three common units:

• The Nautical Mile (NM)

• The Statute Mile (SM)

• The Kilometre (km)

Because navigation is closely related to the angular measurement of the Earth, and traditionally uses a system based on the number 60, the Nautical Mile is preferred in aviation and marine navigation rather than the Statute Mile or Kilometre.

It is therefore appropriate to define all three units.

6.6.2 Define: 

(a) Statute Mile (Sm)

The Statute Mile, often referred to as the land mile, originated from the Roman mile, which was defined as 1000 Roman paces.

The word mile comes from the Latin word “mille”, meaning one thousand.

Unlike the nautical mile, the Statute Mile is an arbitrary unit of distance and is not related to the size or geometry of the Earth.

Conversions:

• 1 SM = 5280 ft

• 1 NM = 1.15 SM

• 1 NM = 1.85 km

(b) Nautical Mile (Nm)

The Nautical Mile is directly related to the circumference of the Earth.

In navigation, distance is measured using angular measurement along a great circle, such as a meridian or the Equator. These measurements are expressed in degrees (°), minutes ('), and seconds (") of arc.