[postgis-users] Convert multipolygons to separate polygons

[Paragon Corporation]

Bruce,
Yes you can achieve that with this if you are trying to do what I think you
are trying to do.  

ST_Dump also makes available another variable called path, which is an array
position of the location of the geometry in the master geometry.  For multi
geometries, the path is one dimensional, for goemetrycollections its
ndimensional and I think the dimensionality depends on the complexity of
your geometry collection - e.g if you have nested collections the path info
would get quite interesting.  

For Simple shapes like POLYGON, LINESTRING, POINT the path is empty.

So to take advantage of path info - its easiest to do a subselect something
like this

SELECT gid As parent_id, true as is_child, foo.geom As the_geom, foo.path[1]
As position_in_parent 
FROM (SELECT sometable.gid, sometable.field1, sometable.field2, 
(ST_Dump(the_geom)).* 
FROM sometable) As foo

Even for simple geometries where there is no path information - PostgreSQL
would silently ignore you are trying to access an array element that doesn't
exist and will just return null as I recall.

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 Bruce
Raup
Sent: Wednesday, August 20, 2008 12:06 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Convert multipolygons to separate polygons

It's good to learn about this PostGIS solution to this.  I had need for the
same functionality, where I could control how the polygon sub-parts
inherited attributes (IDs and so on) from the "parent".  In particular, I
wanted the sub-parts to inherit all attibutes except for one, where the
sub-parts would get an attribute identifying them as a "child".  I ended up
writing a program that does this in perl, using the Geo::Shapelib module.  I
put up a project-specific web service so that others can "flatten" their
shapefiles.

It looks like this PostGIS method also causes sub-parts to inherit
attributes.  Does it?  (I'll have to upgrade before I can test this.)

Thanks,
Bruce Raup

On Tue, Aug 19, 2008 at 9:38 PM, Paragon Corporation <lr at pcorp.us> wrote:
> As Mike mentioned below - ST_Dump is probably the fastest way to do 
> this (faster than ST_GeometryN if you need to dump all polygons out) 
> and  then to get Point on surface - would look something like this
>
> SELECT sometable.field1, sometable.field2,
> ST_PointOnSurface((ST_Dump(the_geom)).geom) As pos
>         FROM somestatetable
>
>
> Would dump out a point on surface for each polygon in each 
> multipolygon as separate records for each poly.
>
> 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 
> Bresnahan, Mike
> Sent: Tuesday, August 19, 2008 6:59 PM
> To: PostGIS Users Discussion
> Subject: RE: [postgis-users] Convert multipolygons to separate 
> polygons
>
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA256
>
> Simon O'Malley said:
>> Is there a way to split multipolygons into separate polygon features?
>>
>> Or is there a way to step through all the polygons in a multipolygon 
>> and use pointonsurface to create a point for each polygon in the 
>> multipolygon.
>
> ST_Dump()
> ST_NumGeometries()
> ST_GeometryN()
>
> Reference:
> http://postgis.refractions.net/documentation/manual-1.3/ch06.html
>
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--
Bruce Raup
http://cires.colorado.edu/~braup/
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