The positions of objects on the earth's spherical surface are measured in degrees of latitude and longitude, also known as geographic coordinates. Latitude and longitude are not uniform units of measure; only along the equator does the distance represented by one degree of longitude approximate the same distance of latitude.
To overcome measurement discrepancies, data is often transformed from the three-dimensional geographic coordinate system to a two-dimensional planar surface in a projected coordinate system.
Each coordinate system is defined by both a well-known ID (WKID) or a definition string (well-known text [WKT]). For a complete list of supported IDs and their corresponding definition strings, see the following: