= Split Polygon With Line String =


This example demonstrates how to Split a Polygon into multiple polygons using a Line.

'''[http://postgis.refractions.net/pipermail/postgis-users/2008-May/019750.html James asks]:'''

I think the steps below will work but I have one question. I am creating
an application that allows a user to split a polygon by drawing a
linestring across the polygon they wish to change. The crossing
linestring has parts that are outside the polygon and I want to remove
that before I geomunion. What's the easiest way to do that?

- extract the lines that make up the polygon
- add to this your crossing linestring
- geomunion the lines together.
- polygonize the union-ed set

Thanks,

Jim

--THE BELOW solution may not work with holes ---

'''[http://postgis.refractions.net/pipermail/postgis-users/2008-May/019902.html Kevin answers]:'''

Here's an example splitting a polygon with a line.  Note that the
dangling cutlines outside the polygon are quietly dropped.


{{{

SELECT ST_AsText(geom )
FROM ST_Dump ((

SELECT ST_Polygonize(ST_Union(ST_Boundary(poly), line)) AS mpoly
FROM
   (SELECT 'POLYGON ((1 1, 1 3, 3 3, 3 1, 1 1 ))'::geometry AS poly) AS a,
   (SELECT 'LINESTRING (0 2, 4 2)'::geometry AS line) AS b

));

}}}


Result:


{{{
              st_astext
--------------------------------
  POLYGON((1 1,1 2,3 2,3 1,1 1))
  POLYGON((1 2,1 3,3 3,3 2,1 2))
(2 rows)

}}}


Cheers,
Kevin

'''[http://postgis.refractions.net/pipermail/postgis-users/2008-May/019903.html  Regina asks]:'''

This doesn't work with holes does it?

'''[http://postgis.refractions.net/pipermail/postgis-users/2008-May/019904.html Kevin answers]'''

Things would get a little bit more complicated when holes are involved,
but I don't think overly so.

The polygonizer builds polygons from all linework - this includes
building polygons for what should be holes.   What you would need to do
as a post-process is remove all polygons that don't belong.  (In the
following example, I use contains and pointonsurface to determine which
polygons I should exclude from the final result.

Here is another sample polygon splitting, this time the polygon has two
holes where one is intersected by the splitting line.


{{{
CREATE TABLE poly AS
   SELECT 'POLYGON (( 1 1, 1 7, 7 7, 7 1, 1 1 ),
                    ( 2 3, 4 3, 4 5, 2 5, 2 3 ),
                    ( 5 5, 6 5, 6 6, 5 6, 5 5 ))'::geometry AS geom;

CREATE TABLE line AS
   SELECT 'LINESTRING (0 4, 8 4)'::geometry AS geom;

CREATE TABLE split_polys AS
SELECT geom FROM ST_Dump ((
   SELECT polygonize(ST_Union(ST_Boundary(poly.geom), line.geom))
   FROM poly, line
));

DELETE FROM split_polys a
USING poly b
WHERE NOT ST_Contains(b.geom, ST_PointOnSurface(a.geom));

SELECT ST_AsText(geom) FROM split_polys;

}}}

Result:

{{{
                                 st_astext
----------------------------------------------------------------------
  POLYGON((1 1,1 4,2 4,2 3,4 3,4 4,7 4,7 1,1 1))
  POLYGON((1 4,1 7,7 7,7 4,4 4,4 5,2 5,2 4,1 4),(5 5,6 5,6 6,5 6,5 5))
(2 rows)

----------------------------------------------------------------------

}}}

You can also make mosaic from polygons from one table.
When you have for example this table


{{{
CREATE TABLE geoms
(
  id serial NOT NULL,
  geom geometry,
  CONSTRAINT geoms_pkey PRIMARY KEY (id),
  CONSTRAINT enforce_dims_geom CHECK (ndims(geom) = 2),
  CONSTRAINT enforce_srid_geom CHECK (srid(geom) = (-1))
)
WITH (OIDS=FALSE);

}}}

This is the query that returns mosaic:


{{{
SELECT geom FROM (SELECT (ST_Dump(g)).geom as geom FROM (
	SELECT ST_Polygonize(g) as g FROM (
		SELECT ST_Union(ST_Boundary(geom)) as g FROM geoms
	) r
)t) a WHERE EXISTS (SELECT geom FROM geoms b WHERE ST_Contains(b.geom, ST_PointOnSurface(a.geom)));

}}}

Yo!Zik