Description

Returns a line string representing the exterior ring of the POLYGON geometry. Return NULL if the geometry is not a polygon.

	Only works with POLYGON geometry types

This method implements the OpenGIS Simple Features Implementation Specification for SQL. OGC SFSQL 1.1 - 2.1.5.1

This method implements the SQL/MM specification: SQL-MM 3: 8.2.3, 8.3.3

This function supports 3d and will not drop the z-index.

Examples

--If you have a table of polygons
SELECT gid, ST_ExteriorRing(the_geom) AS ering
FROM sometable;

--If you have a table of MULTIPOLYGONs
--and want to return a MULTILINESTRING composed of the exterior rings of each polygon
SELECT gid, ST_Collect(ST_ExteriorRing(the_geom)) AS erings
	FROM (SELECT gid, (ST_Dump(the_geom)).geom As the_geom
			FROM sometable) As foo
GROUP BY gid;

--3d Example
SELECT ST_AsEWKT(
	ST_ExteriorRing(
	ST_GeomFromEWKT('POLYGON((0 0 1, 1 1 1, 1 2 1, 1 1 1, 0 0 1))')
	)
);

st_asewkt
---------
LINESTRING(0 0 1,1 1 1,1 2 1,1 1 1,0 0 1)

See Also

ST_Boundary, ST_NumInteriorRings