[postgis-users] unioning a buffer - bug?

Wed Sep 15 13:46:37 PDT 2010

  Hi Lee,

Unfortunately, I can't duplicate your error.  I can 
st_union(st_buffer(geom, 0.1)) no problem ...
POLYGON ((
         586801.773149238 4951692.01936019,
         586801.773148314 4951692.01935961,
         586773.455669237 4951674.0093102,
         ....

What version of GEOS are you running? As I've said, it works fine for me 
running 3.2.2.  Perhaps you need to upgrade?

Cheers,
Kevin

On 9/15/2010 12:09 PM, Lee wrote:
> okay so I plotted those points that were in the topology exception 
> error, and narrowed it down to two adjacent polygons.  So now I have:
> select st_union(st_buffer(the_geom,0.1)) from 
> base.current_assessment_parcel where gid in (36,37)
> (returns the topology exception)
>
> Checking out those two parcels, they seem legit in qgis / arcview. (no 
> weird nodes that I can see).
>
> Attached is the output from the following query: select gid, 
> st_asewkt(the_geom) from base.current_assessment_parcel where gid in 
> (36,37)
> I'm relatively newbie to postgis, so can't see what might be causing 
> this, if someone could look at this for me?
> unfortunately, I am on vacation starting tonight, and won't be 
> available to provide further details. If the answer is "your polygons 
> are just messed up" then so be it. These are from someone elses 
> shapefile. Just want to help if this is indeed a bug.
>
> Thanks for your help Kevin.
> Lee.
>
>
> ------------------------------------------------------------------------
> *From:* Kevin Neufeld <kneufeld at refractions.net>
> *To:* PostGIS Users Discussion <postgis-users at postgis.refractions.net>
> *Sent:* Tue, September 14, 2010 10:49:25 PM
> *Subject:* Re: [postgis-users] unioning a buffer - bug?
>
> Interesting.  Something sure sounds amiss.  Are the results null?  Is 
> there anything usual occurring at the TopologyException location?  Is 
> your dataset large?  Can you winnow it down to the few polygons that 
> might be causing the problem (this may be just the polygons around the 
> TopologyException location)?  If so, you could paste the 
> representation of the geometries in PasteBin or something so we can 
> help you isolate the problem.
>
> Also, what version of GEOS do you have installed?
> SELECT PostGIS_Full_Version();
>
> Cheers,
> Kevin
>
> On 9/14/2010 6:03 AM, Lee wrote:
>> Hi Kevin,
>>
>> I assumed I have no results because when I load one of the working 
>> queries as a view (eg. select st_buffer(the_geom,0.1)) in QGIS, or in 
>> ArcGIS, I see all my results as I would expect, but when I modify the 
>> query to st_union(st_buffer()), all my parcels disappear and I have a 
>> blank canvas.
>>
>> Also, with st_isvalid(st_union(st_buffer(the_geom,0.1))) returning 
>> null (neither t nor f) I guess I just assumed.  Anyways, results for 
>> your suggestions are inline, below.
>>
>> Thanks for your help. Any further ideas?
>>
>> Lee.
>> ------------------------------------------------------------------------
>> *From:* Kevin Neufeld <kneufeld at refractions.net>
>> *To:* postgis-users at postgis.refractions.net
>> *Sent:* Tue, September 14, 2010 12:12:46 AM
>> *Subject:* Re: [postgis-users] unioning a buffer - bug?
>>
>> How are you determining you have no results?
>>
>> Try:
>> CREATE TABLE results AS
>> SELECT st_union(st_buffer(the_geom,0.1)) as the_geom
>> FROM base.current_assessment_parcel;
>>
>> NOTICE:  TopologyException: found non-noded intersection between 
>> 586714 4.95189e+006, 586714 4.95189e+006 and 586714 4.95189e+006, 
>> 586739 4.9519e+006 586714 4.95189e+006
>> Query returned successfully with no result in 1078 ms.
>>
>>
>>
>> SELECT count(*) FROM results;
>>
>> 1
>>
>> SELECT ST_Summary(the_geom) FROM results;
>>
>> This returns blank.
>>
>>
>> (snip)
>>
>> -- Kevin
>>
>> On 9/13/2010 7:07 AM, Lee wrote:
>>> So after some googling I see similar problems, but no solutions posted.
>>>
>>> I am trying to union a buffer, but the query is returning no 
>>> results.  Here is ultimately what I would like to achieve:
>>>
>>> select st_union(st_buffer(the_geom,0.1)) as the_geom from 
>>> base.current_assessment_parcel
>>>
>>> Here are some troubleshooting steps I have taken to try narrow it down:
>>>
>>> select st_isvalid(the_geom) a from base.current_assessment_parcel 
>>> group by a
>>> TRUE
>>>
>>> select st_isvalid(st_buffer(the_geom,0.1)) a from 
>>> base.current_assessment_parcel group by a
>>> TRUE
>>>
>>> select st_isvalid(st_union(the_geom)) from 
>>> base.current_assessment_parcel
>>> TRUE
>>>
>>> select st_isvalid(st_union(st_buffer(the_geom,0.1))) from 
>>> base.current_assessment_parcel
>>> NOTICE:  TopologyException: found non-noded intersection between 
>>> 586714 4.95189e+006, 586714 4.95189e+006 and 586714 4.95189e+006, 
>>> 586739 4.9519e+006 586714 4.95189e+006
>>> Total query runtime: 859 ms.
>>> 1 row retrieved.
>>> (returns blank record)
>>>
>>>  And just for fun
>>> select st_isvalid(st_buffer(st_union(the_geom),0.1)) a from 
>>> base.current_assessment_parcel
>>> TRUE
>>>
>>> I guess my next step would be to try the snap to grid functions, but 
>>> the first query above should work, shouldn't it?
>>>
>>> Any help appreciated. Thanks.
>>> Lee
>>>
>>>
>>> _______________________________________________
>>> postgis-users mailing list
>>> postgis-users at postgis.refractions.net
>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>    
>>
>>
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