ST_Touches

Given two shapes A and B, ST_Touches(A, B) returns true if both the following are true:

• At least one point in the set of points that comprises A is also a member of the set of points that make up B.
• No points that make up the interior of A are also part of the interior of B.

In other words, A and B have a point along their boundaries in common (i.e., they “touch”), but none of their interior points intersect. This distinction between shapes touching along a boundary vs. intersecting is also made by the standard.

ST_Touches works on the following data types:

ST_Touches will attempt to use any available to speed up its operation. Use the prefixed variant _ST_Touches if you do not want any spatial indexes to be used.

Examples

The screenshots in these examples were generated using geojson.io, but they are designed to showcase the shapes, not the map. Representing GEOMETRY data in GeoJSON can lead to unexpected results if using geometries with other than 4326 (as shown below).

True

In this example, ST_Touches returns true because both of the following are true:

• At least one point in the set of Points that comprise Polygon A is a member of the set of points that make up the LineString B.
• No points from the interior of A are also part of the interior of B.

SELECT st_touches(st_geomfromtext('SRID=4326;POLYGON((-87.906471 43.038902, -95.992775 36.153980, -75.704722 36.076944, -87.906471 43.038902), (-87.623177 41.881832, -90.199402 38.627003, -82.446732 38.413651, -87.623177 41.881832))'), st_geomfromtext('SRID=4326;LINESTRING(-87.623177 41.881832, -90.199402 38.627003, -82.446732 38.413651, -87.623177 41.881832)'));

   st_touches
---------------
     true

(1 row)

False

In this example, ST_Touches returns false because:

• Some points from the interior of the LineString B are also part of the interior of the Polygon A.

SELECT st_touches(st_geomfromtext('SRID=4326;POLYGON((-87.906471 43.038902, -95.992775 36.153980, -75.704722 36.076944, -87.906471 43.038902))'), st_geomfromtext('SRID=4326;LINESTRING(-88.243385 40.116421, -87.906471 43.038902, -95.992775 36.153980, -95.235278 38.971667)'));

   st_touches
---------------
     false
(1 row)

See also

• Introducing Distributed Spatial Data in CockroachDB (blog post)