[postgis-users] Convert multipolygons to separate polygons

Bruce Raup brauplists at gmail.com
Tue Aug 19 21:05:46 PDT 2008

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.)

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
> 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

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