A Coordinate Reference System, also referred to as Spatial Reference System (SRS) or a map projection, identifies what method was used to convert the three dimensional sphere-like Earth into a flat X,Y coordinate surface. In other words, it explains what method was used to project three dimensions onto a two dimensional surface.

There are literally thousands of defined ways to do this, each with its own pluses and minuses. Each defined method is a separate CRS, the most common being World Geodetic System 1984 or WGS 84, which is used by the Global Positioning System (GPS).

In order to correctly identify a position and accurately calculate things such as surface distance between two points, the CRS for a set of spatial data must be known. It is typically identified and stored as part of the file or geospatial database. One CRS can be converted into another CRS as long as both are known.