[postgis-users] Transform overlapping polygons to non-overlapping?

[Obe, Regina]

Brent,
 
I guess it really depends on how exactly you want to achieve non-overlapping.
If for example you are basing it on some sort of attribute and all your overlapping polygons are valid
 
Then a simple
 
SELECT ST_Union(the_geom) As newgeom, field1
FROM sometable
GROUP BY field1
 
I think will guarantee non-overlapping polygons because as part of the process of ST_Union - it would irradicate the overlapping regions to just create one.  That is part of the reason why its so much slower than ST_Collect for example.
 
For your exact case below - you would union all the overlapping polygons together which could be really slow depending on how many overlap. The query I would write to achieve that would be something like this
 
SELECT MAX(a.gid) As newgid, ST_Union(a.the_geom) As the_geom
FROM poly a
GROUP BY (SELECT MAX(r.gid) FROM poly r 
   WHERE (a.gid = r.gid OR ST_Overlaps(r.the_geom, a.the_geom)));
 
Hope that helps,
Regina


________________________________

From: postgis-users-bounces at postgis.refractions.net on behalf of Brent Fraser
Sent: Fri 7/11/2008 12:14 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Transform overlapping polygons to non-overlapping?



To All,

  There doesn't seem to be an obvious answer to the problem given below (aka cleaning polygons, creating planar polygons, etc).  I did see a note on the PostGIS wiki wishlist to "Add a geometry cleaner". There is also a suggestion to convert to linestrings, node, then polygonize (while that may work for a small set of polygons, I've got 1.1 million to clean).  JTS, Geos, etc will likely fail due to the large number of polygons so I'll need a different approach.

  I'm considering writing some code to iterate through my table of polygons, cleaning a small subset at a time.  I think using PostGIS for the geometry storage and spatial query/selection makes sense.  Any suggestions on which API to use?
        GDAL's OGR
        PostgreSQL's libpq
        other?

Thanks!
Brent Fraser

Brent Fraser wrote:
> PostGIS'ers,
>
>  I've got a table of overlapping polygons.  How can I make it a table of
> non-overlapping polygons?
>
> For example, if table "polys2" contains two polygons A1 and B1 which
> overlap.  I'd like to create table "polys3" with polygons A2, B2, C2,
> where C2 is the overlap region of A1 and B1, and A2 = A1 - C2, and B2 =
> B1 - C2.
>
> Looking at the overlay operations in the JTS doc it looks like doing an
> Intersection (to get only the overlapping area) then adding the
> Symmetric Difference (to get the non-overlapping areas) might work.
>
> Am I on the right track or is there an easier way (since all the
> polygons are in one table)?
>
> Thanks!
> Brent Fraser
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> postgis-users at postgis.refractions.net
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