ST_Intersection — (T) 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.
Geography: For geography this is really a thin wrapper around the geometry implementation. It first determines the best SRID that fits the bounding box of the 2 geography objects (if geography objects are within one half zone UTM but not same UTM will pick one of those) (favoring UTM or Lambert Azimuthal Equal Area (LAEA) north/south pole, and falling back on mercator in worst case scenario) and then intersection in that best fit planar spatial ref and retransforms back to WGS84 geography.
This function will drop the M coordinate values if present.
If working with 3D geometries, you may want to use SFGCAL based ST_3DIntersection which does a proper 3D intersection for 3D geometries. Although this function works with Z-coordinate, it does an averaging of Z-Coordinate.
Performed by the GEOS module
Availability: 1.5 support for geography data type was introduced.
Changed: 3.0.0 does not depend on SFCGAL.
This method implements the OpenGIS Simple Features Implementation Specification for SQL 1.1. s220.127.116.11
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 SELECT ST_AsText(ST_Intersection('POINT(0 0)'::geometry, 'LINESTRING ( 0 0, 0 2 )'::geometry)); st_astext --------------- POINT(0 0)
Clip all lines (trails) by country. Hhere 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.geom, trails.geom))).geom clipped_geom from country inner join trails on ST_Intersects(country.geom, trails.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.geom, poly.geom), 0.0 ) ) clipped_geom from country inner join poly on ST_Intersects(country.geom, poly.geom) where not ST_IsEmpty(ST_Buffer(ST_Intersection(country.geom, poly.geom), 0.0));
Note this is not a true intersection, compare to the same example using ST_3DIntersection.
select ST_AsText(ST_Intersection(linestring, polygon)) As wkt from ST_GeomFromText('LINESTRING Z (2 2 6,1.5 1.5 7,1 1 8,0.5 0.5 8,0 0 10)') AS linestring CROSS JOIN ST_GeomFromText('POLYGON((0 0 8, 0 1 8, 1 1 8, 1 0 8, 0 0 8))') AS polygon; st_astext --------------------------------------- LINESTRING Z (1 1 8,0.5 0.5 8,0 0 10)