Editing Geodesy (section)

== Units and measures on the ellipsoid ==
{{further|Geodetic coordinates}}
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[[File:Latitude and longitude graticule on an ellipsoid.svg|225px|thumb|right|The definition of latitude (φ) and longitude (λ) on an ellipsoid of revolution (or spheroid). The graticule spacing is 10 degrees. The latitude is defined as the angle between the normal to the ellipsoid and the equatorial plane.]]

Geographical [[latitude]] and [[longitude]] are stated in the units degree, minute of arc, and second of arc. They are ''angles'', not metric
measures, and describe the ''direction'' of the local normal to the [[reference ellipsoid]] of revolution. This direction is ''approximately'' the same as the direction of the plumbline, i.e., local gravity, which is also the normal to the geoid surface. For this reason, astronomical position determination – measuring the direction of the plumbline by astronomical means – works reasonably well when one also uses an ellipsoidal model of the figure of the Earth.

One geographical mile, defined as one minute of arc on the equator, equals 1,855.32571922 m. One [[nautical mile]] is one minute of astronomical latitude. The radius of curvature of the ellipsoid varies with latitude, being the longest at the pole and the shortest at the equator same as with the nautical mile.

A [[metre]] was originally defined as the 10-millionth part of the length from the equator to the North Pole along the meridian through Paris (the target was not quite reached in actual implementation, as it is off by 200 [[Parts-per notation#ppm|ppm]] in the current definitions). This situation means that one kilometre roughly equals (1/40,000) * 360 * 60 meridional minutes of arc, or 0.54 nautical miles. (This is not exactly so as the two units had been defined on different bases, so the international nautical mile is 1,852 m exactly, which corresponds to rounding the quotient from 1,000/0.54 m to four digits).