The ST_Touches() function

The ST_Touches() function returns t (TRUE) if none of the points common to both geometries intersect the interiors of both geometries; otherwise, it returns f (FALSE). At least one geometry must be an ST_LineString, ST_Polygon, ST_MultiLineString, or ST_MultiPolygon.

Syntax

ST_Touches(g1 ST_Geometry, g2 ST_Geometry)

Usage

The following figure shows various geometric objects that touch but do not intersect with each other.

This graphic shows various geometric objects that touch but do not intersect each other. — Figure 1: Touching geometries

The results of the spatial relationship of the ST_Touches() function can be understood or verified by comparing the results with a pattern matrix that represents the acceptable values for the DE-9IM. The pattern matrices state that the ST_Touches() function returns TRUE when the interiors of the geometry do not intersect, and the boundary of either geometry intersects the other's interior or boundary.

The ST_Touches() function returns TRUE if the boundary of one geometry intersects the interior of the other but the interiors do not intersect.

Table 1. Pattern matrix for the ST_Touches() function
The table is a matrix. The top row and first column identify the geometry.
	Interior (b)	Boundary (b)	Exterior (b)
Interior (a)	F	T	*
Boundary (a)	*	*	*
Exterior (a)	*	*	*

The ST_Touches() function returns TRUE if the boundary of one geometry intersects the interior of the other but the interiors do not intersect.


	Interior (b)	Boundary (b)	Exterior (b)
Interior (a)	F	*	*
Boundary (a)	T	*	*
Exterior (a)	*	*	*

The ST_Touches() function returns TRUE if the boundaries of both geometries intersect but the interiors do not.


	Interior (b)	Boundary (b)	Exterior (b)
Interior (a)	F	*	*
Boundary (a)	*	T	*
Exterior (a)	*	*	*

Return type

BOOLEAN

Example

The GIS technician is asked to provide a list of all sewer lines whose endpoints intersect another sewer line.

The sewerlines table is created with three columns. The first column, sewer_id, uniquely identifies each sewer line. The INTEGER class column identifies the type of sewer line, generally associated with the line's capacity. The sewer ST_LineString column stores the sewer line's geometry:

CREATE TABLE sewerlines (sewer_id  integer,
                         class     integer,
                         sewer     ST_LineString);

The query returns a list of sewer_ids that touch one another:

SELECT s1.sewer_id, s2.sewer_id
   FROM sewerlines s1, sewerlines s2
   WHERE ST_Touches(s1.sewer, s2.sewer);