ST_Intersection — Returns a geometry that represents the shared portion of geomA and geomB
Returns a geometry that represents the point set intersection of the Geometries.
In other words - that portion of geometry A and geometry B that is shared between the two geometries.
If the geometries do not share any space (are disjoint), then an empty geometry collection is returned.
ST_Intersection in conjunction with ST_Intersects is very useful for clipping geometries such as in bounding box, buffer, region queries where you only want to return that portion of a geometry that sits in a country or region of interest.
Do not call with a
Performed by the GEOS module
This method implements the OpenGIS Simple Features Implementation Specification for SQL OGC SPEC s22.214.171.124
This method implements the SQL/MM specification: SQL-MM 3: 5.1.18
SELECT ST_AsText(ST_Intersection('POINT(0 0)'::geometry, 'LINESTRING ( 2 0, 0 2 )'::geometry)); st_astext --------------- GEOMETRYCOLLECTION EMPTY (1 row) SELECT ST_AsText(ST_Intersection('POINT(0 0)'::geometry, 'LINESTRING ( 0 0, 0 2 )'::geometry)); st_astext --------------- POINT(0 0) (1 row) ---Clip all lines (trails) by country (here we assume country geom are POLYGON or MULTIPOLYGONS) -- NOTE: we are only keeping intersections that result in a LINESTRING or MULTILINESTRING because we don't -- care about trails that just share a point -- the dump is needed to expand a geometry collection into individual single MULT* parts -- the below is fairly generic and will work for polys, etc. by just changing the where clause SELECT clipped.gid, clipped.f_name, clipped_geom FROM (SELECT trails.gid, trails.f_name, (ST_Dump(ST_Intersection(country.the_geom, trails.the_geom))).geom As clipped_geom FROM country INNER JOIN trails ON ST_Intersects(country.the_geom, trails.the_geom)) As clipped WHERE ST_Dimension(clipped.clipped_geom) = 1 ; --For polys e.g. polygon landmarks, you can also use the sometimes faster hack that buffering anything by 0.0 -- except a polygon results in an empty geometry collection --(so a geometry collection containing polys, lines and points) -- buffered by 0.0 would only leave the polygons and dissolve the collection shell SELECT poly.gid, ST_Multi(ST_Buffer( ST_Intersection(country.the_geom, poly.the_geom), 0.0) ) As clipped_geom FROM country INNER JOIN poly ON ST_Intersects(country.the_geom, poly.the_geom) WHERE Not ST_IsEmpty(ST_Buffer(ST_Intersection(country.the_geom, poly.the_geom),0.0));