ST_Boundary

Calculates the boundary of the specified GEOMETRY object. An object's boundary is the set of points that define the limit of the object.

For a linestring, the boundary is the start and end points. For a polygon, the boundary is a linestring that begins and ends at the same point.

Behavior type

Syntax

ST_Boundary( g )

Arguments

g

Spatial object for which you want the boundary, type GEOMETRY

Returns

GEOMETRY

Supported data types

Examples

The following examples show how to use ST_Boundary.

Returns a linestring that represents the boundary:

=> SELECT ST_AsText(ST_Boundary(ST_GeomFromText('POLYGON((-1 -1,2 2,
   0 1,-1 -1))')));
 ST_AsText
--------------
LINESTRING(-1 -1, 2 2, 0 1, -1 -1)
(1 row)

Returns a multilinestring that contains the boundaries of both polygons:

=> SELECT ST_AsText(ST_Boundary(ST_GeomFromText('POLYGON((2 2,5 5,8 2,2 2),
   (4 3,5 4,6 3,4 3))')));
                            ST_AsText
------------------------------------------------------------------
 MULTILINESTRING ((2 2, 5 5, 8 2, 2 2), (4 3, 5 4, 6 3, 4 3))
(1 row)

The boundary of a linestring is its start and end points:

=> SELECT ST_AsText(ST_Boundary(ST_GeomFromText(
   'LINESTRING(1 1,2 2,3 3,4 4)')));
       ST_AsText
-----------------------
 MULTIPOINT (1 1, 4 4)
(1 row)

A closed linestring has no boundary because it has no start and end points:

=> SELECT ST_AsText(ST_Boundary(ST_GeomFromText(
   'LINESTRING(1 1,2 2,3 3,4 4,1 1)')));
    ST_AsText
------------------
 MULTIPOINT EMPTY
(1 row)