SRID
SRID(g Geometry)
Alias: ST_SRID
Result type: integer
Returns the Spatial Reference Identifier (SRID) of g; returns ‑1 if the SRID is not specified.
There are many different ways to take points on the Earth and represent them on a flat surface. Each spatial reference system describes a different method of accomplishing this. Many spatial reference systems are listed at: http://spatialreference.org/ (http://spatialreference.org/).
Examples:
Get all the available spatial reference systems (some results omitted):
SELECT (SRID) FROM spatial_ref_sys;
-------------
srid
-------------
2000
2001
2002
...
32760
32761
32766
-------------
(3748 rows)
Point with SRID specified:
SELECT SRID(POINTFROMTEXT('POINT(49 3)', 4326));
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col1
-------------
4326
-------------