[postgis-users] Split a polygon to N linestrings (Solved)

[Simon Greener]

Nicolas,

A more general solution that handles linestrings, multilinestrings etc via the use of a
function (which doesn't return a geometry rather a record of coordinates) can be
found here: http://www.spatialdbadvisor.com/postgis_tips_tricks/128/exploding-a-linestring-or-polygon-into-individual-vectors-in-postgis/

Simon
On Tue, 02 Feb 2010 18:56:13 +1100, Nicolas Gillet - MARKET-IP <nicolas.gillet at market-ip.com> wrote:

> Hello Kevin,
>
> Thank you a lot for this perfectly clear query !
>
> Nicolas
>
> -----Message d'origine-----
> De : postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] De la part de Kevin
> Neufeld
> Envoyé : vendredi 29 janvier 2010 17:44
> À : PostGIS Users Discussion
> Objet : Re: [postgis-users] Split a polygon to N linestrings
>
> CREATE TEMP TABLE mypolygontable AS
> SELECT
>    'MULTIPOLYGON(((0 0, 0 4, 4 4, 4 0, 0 0),
>                   (1 1, 1 2, 2 2, 2 1, 1 1)),
>                  ((5 5, 5 6, 6 6, 6 5, 5 5))
>                 )'::geometry geom;
>
>
> -- make line segments from every startpoint and endpoint
> SELECT ST_AsText( ST_MakeLine(sp,ep) )
> FROM
>    -- extract the endpoints for every 2-point line segment for each
> linestring
>    (SELECT
>       ST_PointN(geom, generate_series(1, ST_NPoints(geom)-1)) as sp,
>       ST_PointN(geom, generate_series(2, ST_NPoints(geom)  )) as ep
>     FROM
>        -- extract the individual linestrings
>       (SELECT (ST_Dump(ST_Boundary(geom))).geom
>        FROM mypolygontable
>        ) AS linestrings
>     ) AS segments;
>
>        st_astext
> ---------------------
>   LINESTRING(0 0,0 4)
>   LINESTRING(0 4,4 4)
>   LINESTRING(4 4,4 0)
>   LINESTRING(4 0,0 0)
>   LINESTRING(1 1,1 2)
>   LINESTRING(1 2,2 2)
>   LINESTRING(2 2,2 1)
>   LINESTRING(2 1,1 1)
>   LINESTRING(5 5,5 6)
>   LINESTRING(5 6,6 6)
>   LINESTRING(6 6,6 5)
>   LINESTRING(6 5,5 5)
> (12 rows)
>
>
> Cheers,
> Kevin
>
> On 1/29/2010 5:16 AM, Nicolas Gillet - MARKET-IP wrote:
>> Hello,
>>
>> I am looking for some tips to split a polygon to its simplest linestrings.
>>
>> An example worth better than words :
>>
>> POLYGON((0 0, 0 1, 1 1, 1 0, 0 0))
>>
>> Would become :
>>
>> MULTINLINESTRING((0 0, 0 1), (0 1, 1 1), (1 1, 1 0), (1 0, 0 0))
>>
>> Every single linestring has to have two and only two points.
>>
>> Is there a way to do this with the existing postgis functions ?
>>
>> I found several ways to turn a polygon to a (multi)linestring but not to
>> split it then into shortest linestrings.
>>
>> Thanks for any input.
>>
>> Nicolas
>>
>> Gillet
>>
>>
>>
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