What is Reference ellipsoid?
[[Image:OblateSpheroid.PNG|250px|right|thumb|Flattened sphere (exaggerated)]] A '''reference ellipsoid''' is an [[ellipsoid]] which is used as a [[frame of reference]] for [[geodesy|geodetic]] calculations. It is a figure like the shape of the [[Earth]]. It is easier to work with than the [[geoid]], which is another shape like that of the Earth. It is relatively easy to describe the reference ellipsoid using [[mathematics|mathematical]] [[formula]]e. Describing the geoid is much more complex, as it results from precise measurements. The first such models were [[sphere]]s, used since [[Ancient Greece]]. In the 17th century, there were doubts that the earth was a [[sphere]]. In 1688, [[Isaac Newton]] solved a controversy with [[Giovanni Domenico Cassini]] by demonstrating mathematically that the rotation of the earth would lead to a flattening around the area of the poles, and not at the equator. In practice this was shown by [[Pierre Bouguer]] and [[Alexis-Claude Clairaut]] half a century later. The two had done an expedition to [[Peru]] and [[Lapland (region)|Lapland]] (1735-1741). Their measurement of the [[meridian arc]] lead to the definition of the [[metre]]. This was the ten-millionth part of the idealized distance between the pole and the equator, 1791. Because of different errors in measurement, it turned out to be 0.022% too short, and was redefined twice, in 1793, and 1799. The value of 1799 is still the official definition but it is about 0.197‰ too short. In 1983, the metre was redefined as the distance, [[light]] travels in a certain amount of time, in a [[vacuum]]. [[Category:Navigation]] [[Category:Geodesy]]