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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.

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