[postgis-users] The "bow tie" error

[Hugues François]

Hello,

Thanks for your answer Stephen. Of course dumping the multipolygons works well with qgis. But for our users we need to keep the unique id attached to each object, simple or multipolygon. They might need to export data to their gis software for their work and all our database is built based on this uniqueness constraint .

In fact, I really don't understand why qgis see these as an error. they seem topologically correct to me, especially since unioning the dumped part in qgis create the same error ! Definitely, it sounds like the error comes from qgis and the error seems impossible to solve.

Hugues.

-----Message d'origine-----
De : postgis-users-bounces at lists.osgeo.org [mailto:postgis-users-bounces at lists.osgeo.org] De la part de Stephen Woodbridge
Envoyé : jeudi 21 mars 2013 22:11
À : postgis-users at lists.osgeo.org
Objet : Re: [postgis-users] The "bow tie" error

On 3/21/2013 5:06 PM, Stephen Woodbridge wrote:
> Francois,
>
> Can you dump the polygons in the multiplyogon and create separate 
> records for them. So in you example graphic, you would end up with two 
> separate polygons the touch at a point. I think this would be OK for qgis.

This might be helpful:

http://www.bostongis.com/postgis_dump.snippet
http://postgis.17.n6.nabble.com/converting-multipolygon-to-polygon-td3530291.html

> -Steve W
>
> On 3/21/2013 4:57 PM, Francois Hugues wrote:
>> Hello,
>>
>> I have to deal with a dataset with a lot of errors. I ran a 
>> st_makevalid which seems to give me good results (all polygons then 
>> appear to be valid, closed and simple). But, when I open this table 
>> as a layer in qgis it still throws me a lot of errors. The image 
>> attached to this message provide you with an example and the error 
>> said : Le segment 0 de l'anneau 16 du polygone 6 est intersecté au 
>> segment 0 de l'anneau 1 du polygone 7 à 933828.8196, 6261164.472 (in 
>> english this may be something like : line 0 of ring 16 is intersected 
>> by line 0 of ring 1 of polygon 7 at point...).
>>
>> As the rings of the multipolygon share one point, I tried to dump it 
>> and then to union the rings to build a single polygon but I got a 
>> multipolygon back and still an error.
>>
>> The dataset have a lot of "bow ties" and it seems that st_makevalid 
>> change them to multipolygon but that do not correct all errors in 
>> qgis.
>>
>> Does anyone have an idea to find any solution ?
>>
>> Hugues
>>
>> --
>>
>>
>>
>> Hugues FRANÇOIS
>>
>> hugues.francois at irstea.fr
>>
>>
>>
>> _______________________________________________ postgis-users mailing 
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>> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
>>
>
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