Why do the nautical mile and statute mile both exist?
The nautical mile is the distance used in navigation and based on the length of one minute of arc taken along a great circle. This is also sometimes referred to as the International Nautical Mile or an International Air Mile. The nautical mile is frequently confused with the geographical mile, which is equal to 1 minute of arc on the Earth’s equator (6087.15 feet). On the ocean, distances are calculated slightly differently than on land, hence nautical miles are used, as opposed to land or statute miles.