[postgis-users] Cleaning non valid multipolygon

[Stefan Keller]

I'd like to point out that this sounds similar to the ELIMINATE command  in
ArcGIS.

And be aware that the area parameter is missing so called '*dumb-bell*'
polygons, which means sliver polygons which are very small but also very
long. In this case, an additional parameter would be of help: The
circumference.

Any ideas?

-S.

P.S. CLEAN is another cleansing command. See my next post.

2009/2/4 Yves Moisan <yves.moisan at boreal-is.com>

> Kevin,
>
> Thanx a lot.  I ended up doing the few remaining by hand.  I could start
> from the linestring and figure out where the bad vertex was.  Not all
> the polygons I had closed and some even missed a side or two, so no hope
> for any algorithm there.  I'll study what you're proposing and see how
> many remaining non valid geometries remain.
>
> Thanx again for your helpful comments.
>
> Yves
>
> Le mardi 03 février 2009 à 14:11 -0800, Kevin Neufeld a écrit :
>  > Hi Yves,
> >
> > No, you can't set a tolerance on building an area.  Depending on your
> requirements, you can snap all the vertices in the
> > geometry to some grid so that such polygons do close.
> >
> > Using OpenJUMP, I can see in your example that your linear ring falls
> short of about 0.15 units from closing.  So, this
> > should work:
> >
> > UPDATE mytable
> > SET the_geom = ST_BuildArea(
> >         ST_Union(
> >            ST_Boundary(ST_SnapToGrid(the_geom, 0.2)),
> >            ST_Startpoint(ST_Boundary(ST_SnapToGrid(the_geom, 0.2)))
> >         )
> >      )
> > WHERE id = 122;
> >
> > If snapping the entire geometry to a grid is not preferable to you, then
> you're left with some mucky business, but I
> > think you can do it.
> > 1. extract and node the exterior ring (your geometry runs back on itself
> so you can't tell where the start and endpoints
> > of the exterior ring lie)
> > 2. merge the ring to a single linestring (making the start and end points
> obvious)
> > 3. update the startpoint to be equal to the endpoint
> > 4. repolygonize.
> >
> >
> > CREATE TABLE tmp AS
> > SELECT id,
> >    -- Create a single linestring
> >    ST_LineMerge(
> >      -- Remove overlapping segments
> >      ST_Union(
> >        ST_Boundary(the_geom),
> >        ST_StartPoint(ST_Boundary(the_geom))
> >      )
> >    ) AS the_geom
> > FROM mytable
> > WHERE id = 122;
> > -- At this point, you should only have LINESTRINGs in tmp, not
> MULTILINESTRINGs.  You should verify this to see if you
> > have other problems than just the endpoints not closing.
> >
> > UPDATE mytable a
> > SET the_geom =
> >    ST_BuildArea(
> >      ST_SetPoint(b.the_geom, 0, ST_EndPoint(b.the_geom)))
> > FROM tmp b
> > WHERE a.id = b.id;
> >
> > Yields a single polygon for id=122:
> > POLYGON ((
> >          375838.8459248 8718402.9401143,
> >          375844.0704528 8718410.3968027,
> >          375904.0280814 8718366.8994515,
> >          375898.8035535 8718359.4427696,
> >          375838.8459248 8718402.9401143
> >      ))
> >
> >
> > It would complicate things if you have holes in your non-closing
> polygons, but for starters this should do for you.
> >
> > Hope that helps,
> > Kevin
> >
> > Yves Moisan wrote:
> > > [cut] Can I either set some tolerance on the buildarea so it
> > > snaps to the closest point in the case the multilinestring does not
> > > close or enforce a constraint on the union so it produces a closed
> > > linestring ?
> > >
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