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