In cartography, the Earth's shape has to be approximated: a *reference ellipsoid* specify the Earth's radius and a flattening constant different ones are in use also called *datum* or *geodetic system* for coordinates, a meridian (0° longitude) should also be specified Important ones are: *WGS84*: That's what GPS uses *OSGB36*: That's what Ordnance Survey uses *ED50*: That's what we use in most of Europe