[postgis-users] Removing tiny polygons

[Rémi Cura]

If you are saying that dump doesn't break your multi polygon into pieces,
either
_ dump has a bug (unlikely)
_ you don't have a multipolygon but a geometry collection (try
ST_Dump(ST_CollectionExtract(geom,2)))
_you have an invalid simple polygon (try st_isvalid() )
_you have a simple polygon where interior rings should be stand alone
polygon, use St_ringextract
Cheers,

Rémi-C


2014-01-28 Hugues François <hugues.francois at irstea.fr>

> What you say sounds very weird to me. I don't understand how postgis can
> consider a multipolygon as a polygon. Could you provide us a test sample of
> your data ?
>
> Hugues.
>
>
>
>
> -----Original Message-----
> From: postgis-users-bounces at lists.osgeo.org [mailto:
> postgis-users-bounces at lists.osgeo.org] On Behalf Of Tom McCallum
> Sent: Tuesday, January 28, 2014 3:25 PM
> To: morten at sickel.net; PostGIS Users Discussion
> Subject: Re: [postgis-users] Removing tiny polygons
>
> Yes I just want the largest polygons.  The problem being that when I
> simplify the polygon it appears to need a lot more simplification due to
> these islands rather than if it just did did one mainland polygon (
> asGeoJson comes in at 437 polygons rather than 1 polygon).  Not sure why
> GeoJson can split it into multiple polygons but Dump[] just hands me one.
>
> Was hoping to use ST_Area but currently cannot break down the single
> polygon.
>
> Interestingly QGIS can, if I do single to multipart it can break down into
> main landmass and islands.  What might be an equivalent postgis method for
> doing this?
>
>
> On Mon, 27 Jan 2014 19:15:18 -0000, Morten Sickel <morten at sickel.net>
> wrote:
>
> > It depends on what you mean with "remove" - do you want to just have
> > the large polygons without the area that is covered by the small ones
> > or do you want to add the area of the small polygons onto one or more
> > of the larges? (e.g. how one typically wants to handle shiver
> > polygons)
> >
> > In the first case it is as easy as a query like
> >
> > select the_geom where ST_area(the_geom) > ...
> >
> > Morten
> >
> >
> > Tom McCallum skrev:
> >> Hi all,
> >>
> >> If I have a polygon geometry field which seems to be made up of a
> >> large number of tiny polygons and some larger ones, what is the best
> >> way to remove polygons below a certain size?  Would I need to
> >> preprocess this and save in a new table/record or can I dynamically
> >> do it?
> >>
> >> Thanks
> >>
> >> Tom
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