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

[Brent Fraser]

To all,

  My quest for non-overlapping polygons continues:

I started with a table (temp_polys) of 3253 polygons (with some overlap) with a "class" attribute.

To get rid of overlapping polys with the same class value:
	CREATE TABLE temp2_polys as SELECT class, ST_UNION(the_geom) from temp_polys GROUP BY class;

This created a table of 32 multi-polygons (grouped by class).  I still have to remove the overlap between polygons with different class values, so my plan is to convert to linestrings, node the linestrings, polygonize, and (re)assign the class value using StarSpan. So first:

Convert to linestrings:
	INSERT INTO temp3_lines (the_geom) SELECT ST_ExteriorRing( ST_GeometryN(the_geom, generate_series(1, ST_NumGeometries(the_geom)))) AS the_geom FROM temp2_polys;

This produced 1768 linestring records.  Attempting to node the linestrings:
	INSERT INTO temp4_lines (the_geom) SELECT St_Union(the_geom) AS the_geom FROM temp3_lines;

Yikes!  This query ran for 4.5 hours and crashed Postgres (1.8 gHz Windows XP, Postgres 8.3.3, PostGIS 1.3.3).

I dumped the temp3_lines table into a shapefile and asked OpenJump to node AND polygonize.  That took 24 seconds.

Since the above data is a small sub-set of my 1.2 million polygons, OpenJump is not really a solution for cleaning the data all at once.  Looks like some scripting is in order...

Brent


Brent Fraser wrote:
> Regina,
> 
>  The "SELECT MAX..." query didn't work on my sub-set of 12800 polygons.  
> It created 12643 polygons some of which overlap (I expected more, not 
> less, than the original).
> 
>  I may try converting to linestrings, creating one "minimum bounding 
> rectangle" for the entire dataset, then doing an intersect of the lines 
> with the MBR.  In my case this would be ok as there are not attributes 
> on the polygons yet.
> 
> Thanks!
> Brent
> 
> Obe, Regina wrote:
>> 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
>>>  _______________________________________________
>>>  postgis-users mailing list
>>>  postgis-users at postgis.refractions.net
>>>  http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>
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