ST_ExteriorRing

Syntax

geometry ST_ExteriorRing(geometry aPolygon);

Parameters

Usage notes

• Returns NULL if aPolygon is not a polygon geometry.

• Supports 3D geometries and preserves Z coordinates.

Examples

Extract the exterior ring of a polygon

SELECT ST_AsText(ST_ExteriorRing('POLYGON((1 0,3 0,0 3,1 0),(1 0,2 0, 0 2,1 0))'::geometry));

Output:

          st_astext
-----------------------------
 LINESTRING(1 0,3 0,0 3,1 0)
(1 row)

Extract exterior rings from a table of polygons

SELECT gid, ST_ExteriorRing(geom) AS exterior_ring
FROM parcels;

Related topics

• ST_InteriorRingN: Returns the Nth interior ring of a polygon.

• ST_NumInteriorRings: Returns the number of interior rings in a polygon.

• ST_Boundary: Returns the closure of the combinatorial boundary of a geometry.