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The OGC well-known text representation of spatial reference systems |
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Release 9.2
Last modified April 3, 2008 |
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The well-known text representation of spatial reference systems provides a standard textual representation for spatial reference system information.
The definitions of the well-known text representation are modeled after the POSC/EPSG coordinate system data model.
A spatial reference system, also referred to as a coordinate system, is a geographic (latitude-longitude), a projected (x,y), or a geocentric (x,y,z) coordinate system.
The coordinate system is composed of several objects. Each object has a keyword in uppercase (for example, DATUM or UNIT) followed by the defining, comma-delimited parameters of the object in brackets.
Some objects are nested; the objects themselves are composed of objects. Implementations are free to substitute standard brackets ( ) for square brackets [ ] and should be prepared to read both forms of brackets.
The Extended Backus Naur Form (EBNF) definition for the string representation of a coordinate system is as follows, using square brackets (see note above):
<coordinate system> = <projected cs> | <geographic cs> | <geocentric cs>
<projected cs> = PROJCS["<name>", <geographic cs>, <projection>, {<parameter>,}* <linear unit>]
<projection> = PROJECTION["<name>"]
<parameter> = PARAMETER["<name>", <value>]
<value> = <number>
A dataset's coordinate system is identified by the PROJCS keyword if the data is in projected coordinates, by GEOGCS if in geographic coordinates, or by GEOCCS if in geocentric coordinates. The PROJCS keyword is followed by all of the pieces that define the projected coordinate system. The first piece of any object is always the name. Several objects follow the projected coordinate system name: the geographic coordinate system, the map projection, one or more parameters, and the linear unit of measure. All projected coordinate systems are based on a geographic coordinate system, so the pieces specific to a projected coordinate system are described first. For example, UTM zone 10N on the NAD83 datum is defined as:
PROJCS["NAD_1983_UTM_Zone_10N",
<geographic cs>,
PROJECTION["Transverse_Mercator"],
PARAMETER["False_Easting",500000.0],
PARAMETER["False_Northing",0.0],
PARAMETER["Central_Meridian",-123.0],
PARAMETER["Scale_Factor",0.9996],
PARAMETER["Latitude_of_Origin",0.0],
UNIT["Meter",1.0]]
The name and several objects define the geographic coordinate system object in turn: the datum, the prime meridian, and the angular unit of measure.
<geographic cs> = GEOGCS["<name>", <datum>, <prime meridian>, <angular unit>] <datum> = DATUM["<name>", <spheroid>] <spheroid> = SPHEROID["<name>", <semi-major axis>, <inverse flattening>] <semi-major axis> = <number> <inverse flattening> = <number> <prime meridian> = PRIMEM["<name>", <longitude>] <longitude> = <number>
The geographic coordinate system string for UTM zone 10N on NAD83 is:
GEOGCS["GCS_North_American_1983", DATUM["D_North_American_1983", SPHEROID["GRS_1980",6378137,298.257222101]], PRIMEM["Greenwich",0], UNIT["Degree",0.0174532925199433]]
The UNIT object can represent angular or linear units of measure.
<angular unit> = <unit> <linear unit> = <unit> <unit> = UNIT["<name>", <conversion factor>] <conversion factor> = <number>
Conversion factor specifies number of meters (for a linear unit) or number of radians (for an angular unit) per unit and must be greater than zero.
Therefore, the full string representation of UTM zone 10N is:
PROJCS["NAD_1983_UTM_Zone_10N", GEOGCS["GCS_North_American_1983", DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137,298.257222101]], PRIMEM["Greenwich",0],UNIT["Degree",0.0174532925199433]], PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",500000.0], PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",-123.0], PARAMETER["Scale_Factor",0.9996],PARAMETER["Latitude_of_Origin",0.0], UNIT["Meter",1.0]]
A geocentric coordinate system is quite similar to a geographic coordinate system. It is represented by:
<geocentric cs> = GEOCCS["<name>", <datum>, <prime meridian>, <linear unit>]