What is a US Nautical Mile?
A US nautical mile is a unit of measurement used primarily in navigation and marine activities. It is defined as exactly 1,852 meters or approximately 6,076.12 feet. The term "nautical mile" originated from the need for sailors to measure distances on the Earth's surface while taking into account the curvature of the Earth.
The US nautical mile is based on the international nautical mile, which was agreed at the First International Extraordinary Hydrographic Conference in Monaco in 1929. The length was set at 1,852 meters, close to one minute of latitude. The Earth is not a perfect sphere, so a minute of latitude varies slightly with latitude, but for practical purposes the US nautical mile is a fixed value.
Speed over water is measured in knots, one knot being one nautical mile per hour, so 10 knots is 18.52km/h. The US only adopted the 1,852 meter nautical mile in 1954 – it previously used a slightly longer one.
What is a decimeter?
A decimeter (dm) is one tenth of a meter, or 10 centimeters. The 'deci' is the SI prefix for 10^-1.
The decimeter is rarely used by itself, since people go from centimeters straight to meters. You mostly meet it as the cubic decimeter, which is exactly one liter.
A decimeter is about 3.937 inches, so just under four. Divide inches by 3.937 – or multiply by 0.254 – to go the other way.