Spatial reference systems (SRS)

A spatial reference system (SRS) is a coordinate-based system for geographic locations. seekdb currently supports only the SRS that is supported by default.

Spatial reference systems generally include the following types:

• Projected SRS. A projected SRS is a projection of the Earth onto a plane, which is a flat map. The coordinate system on this plane is a Cartesian coordinate system, using units of length (such as meters and feet) instead of latitude and longitude.

• Geographic SRS. A geographic SRS is a non-projected SRS that represents latitude and longitude (or latitude-longitude) coordinates on an ellipsoid using any angular unit.

In addition, there is a flat Cartesian plane represented by SRID 0, which has no assigned units for its axes. Unlike projected SRS, it is an abstract plane without geographic reference and does not necessarily represent the Earth. SRID 0 is the default SRID for spatial data.

You can obtain SRS information from the INFORMATION_SCHEMA ST_SPATIAL_REFERENCE_SYSTEMS table, as shown in the following example:

SELECT *  FROM INFORMATION_SCHEMA.ST_SPATIAL_REFERENCE_SYSTEMS
     WHERE SRS_ID = 4326\G

The result is as follows:

*************************** 1. row ***************************
                SRS_NAME: WGS 84
                  SRS_ID: 4326
            ORGANIZATION: EPSG
ORGANIZATION_COORDSYS_ID: 4326
              DEFINITION: GEOGCS["WGS 84",DATUM["World Geodetic System 1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.017453292519943278,AUTHORITY["EPSG","9122"]],AXIS["Lat",NORTH],AXIS["Lon",EAST],AUTHORITY["EPSG","4326"]]
             DESCRIPTION: NULL
1 row in set (0.002 sec)

In the preceding example, the SRS used for the GPS system is described. Its name (SRS_NAME) is WGS 84, and its ID (SRS_ID) is 4326.

The SRS definitions in the DEFINITION column are WKT values. WKT is defined based on EBNF (Extended Backus Naur Form). WKT can be used as a data format (referred to as WKT-Data in this text) or for defining SRS in GIS.

The SRS_ID value represents a value of the same type as the SRID of a geometry value or a value passed as the SRID parameter to a spatial function. SRID 0 (a unitless Cartesian plane) is a special and valid spatial reference system ID that can be used for any spatial data calculations that depend on the SRID value.

For calculations involving multiple geometry values, all values must have the same SRID; otherwise, an error will occur.