Name
ST_Z — Return the Z coordinate of the point, or NULL if not
			available. Input must be a point.
Synopsis
float ST_Z(geometry  a_point);
Description
Return the Z coordinate of the point, or NULL if not
			available. Input must be a point.
 This method implements the SQL/MM specification.
 This method implements the SQL/MM specification.
 This function supports 3d and will not drop the z-index.
 This function supports 3d and will not drop the z-index.
Examples
SELECT ST_Z(ST_GeomFromEWKT('POINT(1 2 3 4)'));
 st_z
------
	3
(1 row)