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The length of one minute of arc along a meridian is equal to one NM (1 852 m) at approximately latitude:
  • A
  • B
  • C
  • D

Because the Earth is flattened at the Poles, the radius of curvature is increased and a greater arc is required to subtend an angle of one minute at the centre of curvature, hence a nautical mile is longer at the Poles, at about 1 862 m.

The amount of curvature is greatest at the Equator (the radius of curvature is decreased) and therefore the length of arc required to generate an angle of one minute at the centre of curvature is less. A nautical mile is shortest at the Equator and measures about 1 844 m.

The average value is 1 852 m, which would correspond to 1 minute of arc of a Meridian measured at a latitude of approximately 45°.

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