A reference system used to correlate locations on a map with real locations on the Earth’s surface.  Coordinate systems are generally based on grids where locations are described based on distance from the origin of the grid.  Geographic coordinates, for example, are a spherical coordinate system expressed in latitude, longitude, and sometimes elevation, meaning degrees north from the equator, west from the prime meridian and distance above sea level, where the degrees are radial from the earth’s centre.  Projected coordinate systems are based on a cartesian grid expressed in distance north and east from the origin of the grid cell, where the curved surface of the Earth has been conformally projected to flat plane.  Geographic coordinate systems work well for worldwide applications, projected coordinate systems, of which there are many, work best for local level applications.