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 a chain?
The chain is a surveying unit of 66 feet, which is exactly 20.1168 meters. It is divided into 100 links of 7.92 inches each, and 80 chains make a mile.
The chain was devised by the English mathematician Edmund Gunter around 1620 and used to survey land. Ten square chains make exactly an acre, which is why surveyors liked it. GPS has replaced it for most work, though US public land surveys are still recorded in chains and links.