13.4.5 Spatial Reference System Support

A spatial reference system (SRS) for spatial data is a coordinate-based system for geographic locations.

There are different types of spatial reference systems:

• A projected SRS is a projection of a globe onto a flat surface; that is, a flat map. For example, a light bulb inside a globe that shines on a paper cylinder surrounding the globe projects a map onto the paper. The result is georeferenced: Each point maps to a place on the globe. The coordinate system on that plane is Cartesian using a length unit (meters, feet, and so forth), rather than degrees of longitude and latitude.

  The globes in this case are ellipsoids; that is, flattened spheres. Earth is a bit shorter in its North-South axis than its East-West axis, so a slightly flattened sphere is more correct, but perfect spheres permit faster calculations.

• A geographic SRS is a nonprojected SRS representing longitude-latitude (or latitude-longitude) coordinates on an ellipsoid, in any angular unit.

• The SRS denoted in MySQL by SRID 0 represents an infinite flat Cartesian plane with no units assigned to its axes. Unlike projected SRSs, it is not georeferenced and it does not necessarily represent Earth. It is an abstract plane that can be used for anything. SRID 0 is the default SRID for spatial data in MySQL.

MySQL maintains information about available spatial reference systems for spatial data in the data dictionary mysql.st_spatial_reference_systems table, which can store entries for projected and geographic SRSs. This data dictionary table is invisible, but SRS entry contents are available through the INFORMATION_SCHEMA ST_SPATIAL_REFERENCE_SYSTEMS table, implemented as a view on mysql.st_spatial_reference_systems (see Section 28.3.36, “The INFORMATION_SCHEMA ST_SPATIAL_REFERENCE_SYSTEMS Table”).

The following example shows what an SRS entry looks like:

mysql> 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["Long",EAST],
                          AUTHORITY["EPSG","4326"]]
             DESCRIPTION:

This entry describes the SRS used for GPS systems. It has the name (SRS_NAME) WGS 84 and the ID (SRS_ID) 4326, which is the ID used by the European Petroleum Survey Group (EPSG).

SRS definitions in the DEFINITION column are WKT values, represented as specified in the Open Geospatial Consortium document OGC 12-063r5.

SRS_ID values represent the same kind of values as the SRID of geometry values or passed as the SRID argument to spatial functions. SRID 0 (the unitless Cartesian plane) is special. It is always a legal spatial reference system ID and can be used in any computations on spatial data that depend on SRID values.

For computations on multiple geometry values, all values must have the same SRID or an error occurs.

SRS definition parsing occurs on demand when definitions are needed by GIS functions. Parsed definitions are stored in the data dictionary cache to enable reuse and avoid incurring parsing overhead for every statement that needs SRS information.

To enable manipulation of SRS entries stored in the data dictionary, MySQL provides these SQL statements:

• CREATE SPATIAL REFERENCE SYSTEM: See Section 15.1.19, “CREATE SPATIAL REFERENCE SYSTEM Statement”. The description for this statement includes additional information about SRS components.

• DROP SPATIAL REFERENCE SYSTEM: See Section 15.1.31, “DROP SPATIAL REFERENCE SYSTEM Statement”.