Nautical mile 1 sea mile", followed by "For most practical purposes distance is measured from the latitude scale, assuming that one minute of latitude equals one nautical mile". One tenth of a nautical mile is a cable length. The derived unit of speed is the knot, defined as one nautical mile per hour. The metre was originally defined as of the meridian arc from the North pole to the equator passing through Dunkirk. The Earth’s circumference is therefore approximately 40,000 km. The equatorial circumference is slightly longer than the polar circumference – the measurement based on this (40,075.017 km x x =
Grade measurement Grade measurement Grade measurement is the geodetic determination of the local radius of curvature of the figure of the Earth by determining the difference in astronomical latitude between two locations on the same meridian, the metric distance between which is known. The first known grade measurement was performed by Eratosthenes (240 BC) between Alexandria and Syene in what is now Egypt, determining the radius of the Earth with remarkable correctness. The Dutch geodesist Snellius (~1620) repeated the experiment between Alkmaar and Bergen op Zoom using more modern geodetic instrumentation. Later grade measurements aimed at determining the flattening of the Earth