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| Ellipsoid and geoid |
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In order to make the GPS system able to measure an accurate position on the Earth, there should be an underlying framework describing coordinates and references. Geodesy is the science dealing with the form, gravitation and changing of the Earth. A geodetical system is a framework for deciding coordinates on the surface of the Earth in relation to a reference, ellipsoid and geoids.
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Click on the cutting in order to see the whole figure.
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The geoid is an imagined surface with similar gravitational force all over, a so-called equi-gravitational surface. The flat is formed by the surface of the oceans and its imagined continuation under the landmasses. If the whole surface of the Earth was covered by the sea and there was no motion in the ocean or the atmosphere, the surface of the ocean would have been an equi-gravitational surface.
If calculations of positions, distances or directions on the surface of the Earth are to be made, it is necessary to have a mathematical surface of reference. The most common surface of reference is the ellipsoid. An ellipsoid is defined as a closed, curved surface which on any plane will be cut into an ellipse. This ellipsoid is used as a reference for the calculation of positions. It is a mathematical surface representing the Earth. The ellipsoid is used because it is a mathematical figure with an even surface that is easy to handle, as opposed to the geoids that are being affected by the different density at the centre of the Earth. The ellipsoid differs often somewhat from the geoids. For datum level of heights the average sea level (the geoid) of the Earth is used.
The figure above shows polssible differences between geoids, ellipsoid and topology. |
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