Navigation maps are printed with a coordinate grid that is a two-dimensional projection of the three-dimensional Earth surface, based on a given Coordinate System. Historically the countries over the world use many different coordinate systems. Earth is shaped not like a sphere; it is a geoid that has no exact mathematical definition and whose form can be at best approximated by an ellipsoid. To make an exact projection of a region into a coordinate system, the region is first projected onto a so-called reference ellipsoid that would fit the region best. The reference ellipsoid is mathematically defined and can be used for a coordinate system projection. Historically countries all over the world developed and used different and often more than one local and global Reference Systems.
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