[postgis-users] Converting MULTIPOLYGONS to MULTILINESTRINGS andMULTIPOINTS

[Obe, Regina]

This is just a guess (I'm not quite sure which order to collect the interior and exterior rings or if it matters).  I  don't think such a function exists, but the basic idea would be to extract all the interior rings and exterior rings and then collect them. 

Something like this

CREATE OR REPLACE FUNCTION fnpoly_to_rings(geometry)
  RETURNS geometry AS
$BODY$SELECT ST_Collect(the_line)  as multiline
FROM (SELECT ST_ExteriorRing(the_poly) as the_line
	FROM (SELECT ST_GeometryN($1, g.n) As the_poly FROM generate_series(1, ST_NumGeometries($1)) As g(n) ) As polys
	UNION 
	SELECT ST_InteriorRingN(the_poly,generate_series(1, ST_NumInteriorRings(the_poly))) as the_line
	FROM (SELECT ST_GeometryN($1, g.n) As the_poly FROM generate_series(1, ST_NumGeometries($1)) As g(n) ) As polys
	) As all_lines

		$BODY$
  LANGUAGE 'sql' STABLE;
ALTER FUNCTION "fnpoly_to_rings"(geometry) OWNER TO postgres;

---
Then you would do this

SELECT fnpoly_to_rings(the_geom)
FROM sometable

and that would return a multiline string for each record where the_geom is a multipolygon.

For the multipoint case you would go one step further 

1) by taking your output from fnpoly_to_rings or any set of linestrings - 
2) extract them to single linestrings -  
3) then use St_PointN to separate into individual points
4) then collect the points.

Its pretty much a mirror image of the above function (except without a union) and using different functions that deal with linestrings instead of polygons.

Hope that helps,
Regina

 

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Andreas Neumann
Sent: Tuesday, October 16, 2007 7:34 AM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] Converting MULTIPOLYGONS to MULTILINESTRINGS andMULTIPOINTS

Hi again,

I am wondering if there is an easy way to convert MULTIPOLYGON geometries
to MULTILINESTRING or MULTIPOINT geometries?

Thanks for any ideas.

Andreas


-- 
Andreas Neumann
Böschacherstrasse 6, CH-8624 Grüt/Gossau, Switzerland
Email: a.neumann at carto.net, Web:
* http://www.carto.net/ (Carto and SVG resources)
* http://www.carto.net/neumann/ (personal page)
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