6.6 Distance on the Earth 

Distance on the Earth's surface may be expressed in one of the following three units:

• The Nautical Mile,
• The Statute Mile, and
• The Kilometre.

Since navigation is greatly concerned with angular measurement of the Earth, and best used by a system based on the number 60, the Nautical Mile is selected over the Metre or Statute Mile.

It is appropriate therefore to define all 3 units.

6.6.2 Define: 

(a) Statute Mile (Sm)

The Statute Mile, or land mile, was derived from the Roman mile, a unit of 1000 Roman paces.  

The word Mile coming from the Latin 'mille', meaning thousand.

This mile was an arbitrary measure, and not related to the size of the Earth.

• 1 sm = 5280 ft
• 1 nm = 1.15 sm = 1.85 km

(b) Nautical Mile (Nm)

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

For angular measurement of distance in navigation we use;

• degrees, minutes and seconds of arc along a great circle, which is a meridian or the Equator.

1 Nautical Mile is derived by taking the length of arc of a great circle at a distance of 1 minute.

• 1 minute = 1 nm
• 1 degree = 60 nm
• 1 nm = 6080 ft
• 1 nm = 1.15 sm = 1.85 km

(c) Kilometre (Km)

A Kilometre is 1/10,000 part of the distance between the equator and either pole.