SRS

The spatial reference system (SRS) of spatial data is a coordinate-based geographic location system. OceanBase Database supports only the system default SRS.

Generally, SRSs can be classified into the following types:

• Projected SRSs. A projected SRS is a projection of the earth onto a flat surface, that is, a flat map. The coordinate system on this plane is a Cartesian coordinate system and uses length units such as meters and feet, rather than degrees of longitude and latitude.

• Geographic SRSs. A geographic SRS is a non-projected SRS that represents the longitude-latitude or latitude-longitude coordinates on an ellipsoid, in any angular unit.

SRID 0 represents an infinite flat Cartesian plane with no units assigned to its axes. Unlike a projected SRS, it is not georeferenced and does not necessarily represent Earth. It is an abstract plane. SRID 0 is the default SRID for spatial data.

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

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

The preceding example describes an SRS used in a GPS. Its name (SRS_NAME) is WGS 84 and its ID (SRS_ID) is 4326.

The SRS definition in the DEFINITION column is a well-known text (WKT) value. WKT is defined in the extended Backus–Naur form (EBNF). WKT can be used either as a data format (WKT-Data) or as an SRS definition in a GIS.

The SRS_ID value represents a geometry value of the same type as SRID or is passed as an SRID parameter to a spatial function. SRID 0 (the Cartesian plane without units) is a special, valid SRS ID that can be used for any spatial data calculation that depends on SRID values.

For the calculation of multiple geometry values, all values must have the same SRID; otherwise, an error occurs.