Changes between Initial Version and Version 1 of UsersWikiSplitPolygonWithLineString

Timestamp:

04/14/09 09:23:23 (16 years ago)

Author:

pierre

Comment:

--

UsersWikiSplitPolygonWithLineString

@@ +1 @@
+= 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 astext(geom )
+FROM dump ((
+
+SELECT polygonize(geomunion(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:
+
+
+{{{
+              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 dump ((
+   SELECT polygonize(geomunion(boundary(poly.geom), line.geom))
+   FROM poly, line
+));
+
+DELETE FROM split_polys a
+USING poly b
+WHERE NOT contains(b.geom, pointonsurface(a.geom));
+
+SELECT astext(geom) FROM split_polys;
+
+}}}
+
+Result:
+
+{{{
+                                 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 polygonize(g) as g FROM (
+                SELECT geomunion(boundary(geom)) as g FROM geoms
+        ) r
+)t) a WHERE EXISTS (SELECT geom FROM geoms b WHERE contains(b.geom, pointonsurface(a.geom)));
+
+}}}
+
+Yo!Zik