A file with the PRJ extension has become the industry standard for containing information related to a coordinate system.
Here is an example PRJ file, with an explanation of its contents. Note that normally a PRJ file contains all the text on a single line with no extra spaces. We have reformatted it here for clarity:
PROJCS["NAD_1983_StatePlane_Washington_South_FIPS_4602",
GEOGCS["GCS_North_American_1983",
DATUM["D_North_American_1983",
SPHEROID["GRS_1980",6378137.0,298.257222101]
],
PRIMEM["Greenwich",0.0],
UNIT["Degree",0.0174532925199433]
],
PROJECTION["Lambert_Conformal_Conic"],
PARAMETER["False_Easting",500000.0],
PARAMETER["False_Northing",0.0],
PARAMETER["Central_Meridian",-120.5],
PARAMETER["Standard_Parallel_1",45.83333333333334],
PARAMETER["Standard_Parallel_2",47.33333333333334],
PARAMETER["Latitude_Of_Origin",45.33333333333334],
UNIT["Meter",1.0],
AUTHORITY["EPSG",32149]
]
This PRJ file contains a projected coordinate system. It begins with a name for the projected coordinate system. Then it describes the geographic coordinate system. Then it defines the projection and all the parameters needed for the projection. It then defines the linear units used in the projection. The final entry (AUTHORITY) is optional and describes any standard designation for this projection.
Lines 2-8 define the geographic coordinate system. It begins with a name for the geographic coordinate system. Then it describes the datum. Then it defines the prime meridian used. Finally, it defines the angular units.
Lines 3-5 define the geodetic datum. It begins with a name for the datum. Then it defines the spheroid.
Line 4 defines the spheroid. It begins with a name for the spheroid. Then next parameter is the equatorial radius of the ellipsoid (in meters). The last parameter is the inverse flattening factor.