[Qgis-user] Union Tool - Ftools/Sextante - How should the real result be

[Bernhard Ströbl]

Hi

Am 07.05.2013 16:21, schrieb Ivan Santiago:
> In this bug report:
> http://hub.qgis.org/issues/4567
> there is a link with the data provided:
> http://hub.qgis.org/attachments/5563/shapes_and_fake_polys.zip
>
> Giovanni Manghi updated the report:
> ...the tool is not wrong, in fact if you see in detail (see attached screenshot) the two polygons that seems wrong after the union are not identical in the input layers, there is a very small difference, something like 0.4 mm... but this makes them two non identical, overlapping polygons...
>
> It would be  n i c e  if the use can provide a tolerance to the tool.  In this way, this would avoid some of the spurious data that appear when doing overlays.

IMHO this would be dangerous because normally you would not know 
beforehand the result of your overlay operation. A better approach is to 
do your overlay and then examine the result. The overlay operation must 
of course work correctly, which fTools - unfortunately - currently does 
not (I used intersect yesterday and the resulting polygons had wrong 
attributes). If you want to get rid of sliver polygons after the 
overlay, use the eliminate tool in fTools. It is then up to you what 
qualifies as sliver, not up to an algorithm.

Bernhard

>
> ---------------------------
> Iván Santiago
> GIS Specialist
> Information Technologies
> Office of Management and Budget
> 787.725.9420 x 2378
> Calle Cruz 254
> PO Box 9023228
> San Juan, PR 00902-3228
>
> -----Original Message-----
> From: Victor Olaya [mailto:volayaf at gmail.com]
> Sent: Tuesday, May 07, 2013 10:02 AM
> To: Ivan Santiago
> Cc: senhor.neto at gmail.com; qgis-user at lists.osgeo.org
> Subject: Re: [Qgis-user] Union Tool - Ftools/Sextante - How should the real result be
>
> Ivan
>
> could you provide some small dataset where you are seeing that?
> Information about SEXTANTE version is also welcome (I assume you are using the latest QGIS master, but just in case...There is no need to reinstall SEXTANTE, just reinstall the latest QGIS dev, which includes
> SEXTANTE)
>
> Thanks!
>
> 2013/5/7 Ivan Santiago<isantiago at ogp.pr.gov>:
>> Hello:
>>
>>
>>
>> I downloaded (now) and reinstalled Sextante.
>>
>>
>>
>> Unfortunately the results were the same, holding duplicate polygons
>> when using Union tool.
>>
>>
>>
>> It is easy to see the dups applying transparency.
>>
>>
>>
>>
>>
>> ---------------------------
>> Iván Santiago
>> GIS Specialist
>> Information Technologies
>> Office of Management and Budget
>> 787.725.9420 x 2378
>> Calle Cruz 254
>> PO Box 9023228
>> San Juan, PR 00902-3228
>>
>>


__________ Information from ESET Mail Security, version of virus signature database 8307 (20130507) __________

The message was checked by ESET Mail Security.
http://www.eset.com