How do I convert UK Nautical Miles to International Nautical Miles?
Multiply by 1.00064. The UK (Admiralty) nautical mile is 6,080 ft, or 1,853.184 m, and the international one is exactly 1,852 m, so the UK mile is 1.184 m longer.
1,000 UK nautical miles is 1,000.64 international ones. Over most voyages that's smaller than the error in the plot, and the UK officially switched to the international mile in 1970, so the old one mostly turns up on older Admiralty charts.
What is a UK Nautical Mile?
The UK nautical mile was based on one minute of latitude, ie 1/60th of a degree. The Earth is not a perfect sphere, so a minute of latitude varies slightly with latitude, which is why a fixed length had to be chosen.
The UK nautical mile, also called the Admiralty mile, was fixed at 6,080 feet, which is 1,853.184 meters. That is 1.184 meters longer than the international nautical mile of 1,852 meters (6,076.11549 feet), which the UK adopted in 1970.
What is an international nautical mile?
An international nautical mile is exactly 1,852 meters, which is about 1.15078 statute miles. The word 'nautical' distinguishes it from the statute mile used on land.
Because the nautical mile began as one minute of latitude, the Earth's circumference works out at roughly 21,600 nautical miles (360 degrees of 60 minutes each). Navigators measure distance directly off the latitude scale on a chart.
A tenth of a nautical mile is a cable, which is 185.2 meters and is still used for short distances at sea. Minutes and seconds belong to the angles of latitude and longitude, not to the unit itself.