ST_Boundary

Signatures

GEOMETRY ST_Boundary(GEOMETRY geom);
GEOMETRY ST_Boundary(GEOMETRY geom, INT srid);

Description

Returns the boundary of geom, optionally setting its SRID to srid. The boundary of a Geometry is a set of Geometries of the next lower dimension.

Warning

This function does not work on GEOMETRYCOLLECTIONs

Note

Implements the OpenGIS Simple Features Implementation Specification for SQL version 1.2.1

Examples

SELECT ST_Boundary('LINESTRING(4 6, 1 1, 5 5)');
-- Answer: MULTIPOINT((4 6), (5 5))

SELECT ST_Boundary('POLYGON((0 0, 10 0, 10 10, 0 10, 0 0))', 2154);
-- Answer: LINEARRING(0 0, 10 0, 10 10, 0 10, 0 0)

This polygon has a hole

SELECT ST_Boundary('POLYGON((5 4, 5 1, 2 1, 2 4, 4 5, 5 4),
                    (3 2, 3 3, 4 3, 4 2, 3 2))');
-- Answer: MULTILINESTRING((5 4, 5 1, 2 1, 2 4, 4 5, 5 4),
--                          (3 2, 3 3, 4 3, 4 2, 3 2))

A point has no boundary

SELECT ST_Boundary('POINT(2 2)');
-- Answer: GEOMETRYCOLLECTION EMPTY

Comparison with `ST_ExteriorRing`

SELECT ST_ExteriorRing('POLYGON((0 0, 10 0, 10 6, 0 6, 0 0),
                                (1 1, 2 1, 2 5, 1 5, 1 1))') ER,
       ST_Boundary('POLYGON((0 0, 10 0, 10 6, 0 6, 0 0),
                             (1 1, 2 1, 2 5, 1 5, 1 1))') BDRY;

Answer:

ER	BDRY
LINEARRING(0 0, 10 0, 10 6, 0 6, 0 0)	MULTILINESTRING( (0 0, 10 0, 10 6, 0 6, 0 0), (1 1, 2 1, 2 5, 1 5, 1 1))