Geographic Coordinate SystemA geographic coordinate system (GCS) uses a three dimensional spherical surface to define locations onthe earth. A GCS is often incorrectly called datum, but datum is only one part of a GCS. A GCS includes anangular unit of measure, a prime meridian, and a datum (based on a spheroid).A point is referenced by its longitude and latitude values. Longitude and latitude are angles measuredfrom the earth's center to the point on the earth's surface. The angles often are measured in degree (orin grads). The following illustration shows the word as a globe with longitude and latitude values. In the spherical system, horizontal lines, or east-west lines, are lines of equal latitude, or parallels.Vertical lines, or north-south, are lines of equal longitude, or meridians. These lines encompass the globeand form a gridded network called graticule. The line of latitude midway between the poles is called the equator. It defines the line of zero latitude.