# [postgis-users] Splitting a Polygon with a Linestring

Obe, Regina robe.dnd at cityofboston.gov
Wed May 28 12:02:21 PDT 2008

```Kevin,

that looks neat.  I'll have to remember that one.

I presume if you have a geometry with holes, this would not work.   How would you tackle,   if say you wanted to cut a donut with a line.

Thanks,
Regina

________________________________

From: postgis-users-bounces at postgis.refractions.net on behalf of Kevin Neufeld
Sent: Wed 5/28/2008 1:47 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Splitting a Polygon with a Linestring

Right.

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

));

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

James Beaudoin wrote:
> 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
> - geomunion the lines together.
> - polygonize the union-ed set
>
> Thanks,
>
> Jim
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