A Spatial Reference System defines the projection of a point on the Earth. Commonly, a point is represented as longitude and latitude, where the Prime Meridian in Greenwich, England is 0° longitude and the equator is 0° latitude.

The Earth is an ellipsoid and cannot be accurately represented on a flat two-dimensional map, so different projection systems have been developed by local groups based on the specific requirements of the area of the world that is being mapped. These projection systems are registered and maintained by the OGP Surveying and Positioning committee as the EPSG Geodetic Parameter Dataset at http://www.epsg.org/.