Why is a nautical mile different from a land mile?

The mile was originally a Roman linear measure of 1,000 paces — about 1,618 yards. (Its length has varied considerably at different periods and in different localities; the legal mile is now 1,760 yards, or 5,280 feet.) Such a linear measurement cannot be used at sea, so the nautical mile is based on the length of one minute of arc (or 1/60 of a degree) of a great circle of Earth. Owing to the fact that Earth is not a true sphere, the mile as thus defined varies considerably. Therefore, the British assigned it a standard value of 6,080 feet.