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
which can store entries for projected and geographic SRSs. This
data dictionary table is invisible, but SRS entry contents are
available through the
implemented as a view on
Section 25.27, “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 a name
SRS_NAME) of WGS 84 and an ID
SRS_ID) of 4326, which is the ID used by the
European Petroleum Survey
SRS_ID values represent the same kind of
values 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 cached in the data dictionary cache so that parsing overhead is not incurred 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 13.1.19, “CREATE SPATIAL REFERENCE SYSTEM Syntax”. The description for this statement includes additional information about SRS components.