ST_ExteriorRing
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Name

ST_ExteriorRing — Returns a LineString representing the exterior ring of a Polygon.

Synopsis

geometry ST_ExteriorRing(geometry a_polygon);

Description

Returns a LINESTRING representing the exterior ring (shell) of a POLYGON. Returns NULL if the geometry is not a polygon.

[Note]

This function does not support MULTIPOLYGONs. For MULTIPOLYGONs use in conjunction with ST_GeometryN or ST_Dump

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

Extract the exterior ring from a Polygon with a hole.

Code
WITH input(geom) AS (VALUES (
  'POLYGON((0 0,2 9,9 11,14 6,12 0,0 0),(4 3,9 3,10 7,5 8,4 3))'::geometry
))
SELECT geom AS input_polygon,
       ST_ExteriorRing(geom) AS exterior_ring
FROM input;
Output
POLYGON((0 0,2 9,9 11,14 6,12 0,0 0),(4 3,9 3,10 7,5 8,4 3)) | LINESTRING(0 0,2 9,9 11,14 6,12 0,0 0)
Figure
Geometry figure for visual-st-exteriorring-01

For a MultiPolygon, dump the component Polygons before extracting their exterior rings. Both component Polygons in this example have holes.

Code
WITH input(geom) AS (VALUES (
  'MULTIPOLYGON(
    ((0 0,2 9,9 11,14 6,12 0,0 0),(4 3,9 3,10 7,5 8,4 3)),
    ((18 1,18 10,29 10,31 4,25 0,18 1),(21 3,25 2,28 5,25 8,21 7,21 3))
  )'::geometry
))
SELECT part.geom AS input_polygon,
       ST_ExteriorRing(part.geom) AS exterior_ring
FROM input
CROSS JOIN LATERAL ST_Dump(geom) AS part
ORDER BY part.path;
Output
POLYGON((0 0,2 9,9 11,14 6,12 0,0 0),(4 3,9 3,10 7,5 8,4 3)) | LINESTRING(0 0,2 9,9 11,14 6,12 0,0 0)
POLYGON((18 1,18 10,29 10,31 4,25 0,18 1),(21 3,25 2,28 5,25 8,21 7,21 3)) | LINESTRING(18 1,18 10,29 10,31 4,25 0,18 1)
Figure
Geometry figure for visual-st-exteriorring-02

Extract exterior rings from a table of Polygons.

Code
SELECT gid, ST_ExteriorRing(geom) AS ering
FROM sometable;

For a table of MultiPolygons, dump the component Polygons and collect their exterior rings into a MultiLineString.

Code
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;

This example returns the 3D exterior ring of a Polygon.

Code
SELECT ST_ExteriorRing(
    'POLYGON Z ((0 0 5,4 0 5,4 3 5,0 3 5,0 0 5))'
    );
Output
LINESTRING(0 0 5,4 0 5,4 3 5,0 3 5,0 0 5)
Figure
Geometry figure for visual-st-exteriorring-03