PostGIS
Description
Returns a LINESTRING representing the exterior ring (shell) of a POLYGON. Returns NULL if the geometry is not a polygon.
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This method
implements the OGC Simple
Features Implementation Specification for SQL 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(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(geom)) AS erings
FROM (SELECT gid, (ST_Dump(geom)).geom As 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)