Mile two points on the navigational chart, then place the open dividers against the minutes-of-latitude scale at the edge of the chart, and read off the distance in nautical miles. The Earth is not perfectly spherical but an oblate spheroid, so the length of a minute of latitude increases by 1% from the equator to the poles. Using the WGS84 ellipsoid, the commonly accepted Earth model for many purposes today, one minute of latitude at the WGS84 equator is 6,046 feet and at the poles is 6,107.5 feet. The average is about 6,076 feet (about 1,852 metres or 1.15 statute miles).
Nautical mile of latitude is not constant, but about at the poles and at the Equator, with a mean value of . Other countries measure the minute of arc at 45 degrees latitude, giving the nautical mile a length of . In 1929, the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as precisely 1,852 metres. Imperial units and United States customary units used a definition of the nautical mile based on the Clarke (1866) Spheroid. The United States nautical mile was defined as based in the Mendenhall Order foot of 1893. It was abandoned in