The text representation of a spatial system

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.

A coordinate system is composed of several objects. Each object is defined by an uppercase keyword (for example, DATUM or UNIT) followed by the defining, comma-delimited parameters of the object in brackets. Some objects are composed of other 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:

<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 coordinate system of a data set is identified by one of the following three keywords:

PROJCS: if the data is in projected coordinates
GEOGCS: if in geographic coordinates
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 name: the geographic coordinate system, the map projection, one or more parameters, and the linear unit of measure.

As an 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: 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> 
NOTE: semi-major axis is measured in meters and must be > 0.

<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>

The conversion factor specifies the number of meters (for a linear unit) or the 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>]

You can use the SE_CreateSrtext() function to assist you in constructing these spatial reference system text strings.

The remainder of this appendix shows the OGC well-known text “building blocks” of spatial reference systems that are supported by the HCL Informix® spatial data types.

These text strings can be generated by the SE_CreateSrtext() function; you use the factory ID number in the first column of the table as the input argument to SE_CreateSrtext().