# ST_Intersection

## Name

ST_Intersection — (T) Returns a geometry that represents the shared portion of geomA and geomB.

## Synopsis

``` geometry ST_Intersection ( ``` geometry geomA , geometry geomB ``` ) ``` ;

``` geography ST_Intersection ( ``` geography geogA , geography geogB ``` ) ``` ;

## Description

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.
 Do not call with a ``` GEOMETRYCOLLECTION ``` as an argument
 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 values when ``` postgis.backend=geos ``` . ``` postgis.backend=sfcgal ``` , it will return a 2D geometry regardless ignoring the Z-Coordinate. Refer to postgis.backend for details.

Performed by the GEOS module

This method is also provided by SFCGAL backend.

Availability: 1.5 support for geography data type was introduced.

This method implements the OpenGIS Simple Features Implementation Specification for SQL 1.1. s2.1.1.3

This method implements the SQL/MM specification. SQL-MM 3: 5.1.18

## Examples

```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));
```

## Examples: 2.5Dish

Geos is the default backend if not set. Note this is not a true intersection, compare to the same example using ST_3DIntersection .

```set postgis.backend=geos;
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)
```

If your PostGIS is compiled with sfcgal support, have option of using sfcgal, but note if basically cases down both geometries to 2D before doing intersection and returns the ST_Force2D equivalent result which is a 2D geometry

```set postgis.backend=sfcgal;
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;

wkt
----------------------------------------------
MULTILINESTRING((0.5 0.5,0 0),(1 1,0.5 0.5))
```