For more information

Office of Coast Survey

NOS Education: Currents

Nautical Charts, Diving Deeper audio podcast (03.23.09)

A nautical mile measures distance and a knot measures speed

NOAA nautical chart

NOAA depicts several maritime boundaries on its nautical charts. U.S. maritime limits and boundaries are measured by nautical miles.

A nautical mile is based on the circumference of the earth, and is equal to one minute of latitude. It is slightly more than a statute (land measured) mile (1 nautical mile = 1.1508 statute miles). Nautical miles are used for charting and navigating.

A knot is one nautical mile per hour (1 knot = 1.15 miles per hour). The term knot dates from the 17th century, when sailors measured the speed of their ship by using a device called a "common log." This device was a coil of rope with uniformly spaced knots, attached to a piece of wood shaped like a slice of pie. The piece of wood was lowered from the back of the ship and allowed to float behind it. The line was allowed to pay out freely from the coil as the piece of wood fell behind the ship for a specific amount of time. When the specified time had passed, the line was pulled in and the number of knots on the rope between the ship and the wood were counted. The speed of the ship was said to be the number of knots counted (Bowditch, 1984).

leaf

NOAA’s Office of Coast Survey is the nation’s nautical chartmaker. Originally formed by President Thomas Jefferson in 1807, Coast Survey updates charts, surveys the coastal sea floor, responds to maritime emergencies, and searches for underwater obstructions that pose a danger to navigation.