A chain is a unit of length used in some older systems of measurement, particularly in surveying and land measurement. In the United States, one chain is equal to 66 feet or 22 yards. Internationally, it's defined as exactly 20.1168 meters.
Chains were historically used for measuring large plots of land and for surveying purposes, often in agriculture and real estate. However, the use of the chain as a standard unit of measurement has largely been replaced by the more modern and standardized metric and imperial systems, which use units like meters, feet, and yards.
The term "chain" comes from the practice of physically measuring land using a chain made of linked metal bars or wire. Each chain was typically 66 feet long, and surveyors would lay out chains end to end to measure distances across large areas. Today, modern surveying equipment such as electronic distance measuring devices (EDMs) and GPS technology have largely replaced the use of physical chains in land surveying.
The nautical mile in the United Kingdom, as in most countries, follows the international standard, which is approximately equal to 1.852 kilometers or 1.15078 statute miles (land miles). This international standard for the nautical mile is used for maritime and aviation navigation in the UK, as well as in many other countries around the world.
To provide some context, the nautical mile is based on the Earth's circumference and is defined as one minute of latitude along a meridian. It is commonly used in navigation because it provides a consistent and convenient way to measure distances at sea and in the air, where latitude is an essential factor. The use of the international nautical mile ensures compatibility and consistency in maritime and aviation operations across different regions and countries.