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

[Victor Olaya]

I have made a fix to the SEXTANTE union tool. Please check that it now
behaves correctly

Thanks!

Victor

2013/5/6 Alexandre Neto <senhor.neto at gmail.com>:
> In my experience, there are some useful uses for the example 1 given by
> Vinayan.
>
> For example, I have, in the same layer, overlapping service areas for urban
> parks. In order to calculate available park area to the population in the
> overlapping areas, I need to break polygons where they overlap, and later
> perform a sum of each available park area (eliminating the duplicated
> polygons).
>
> https://www.dropbox.com/s/xowepuzd7jklh2h/Figura%206.4.5..jpg
>
> Yet, I do understand that this operation originates duplicate polygons that
> must be fixed later, and this might be a bad principle.
>
> Alexandre Neto
>
>
> On Mon, May 6, 2013 at 4:17 PM, Bernhard Ströbl <bernhard.stroebl at jena.de>
> wrote:
>>
>> Hi Vinayan,
>>
>> just some comments:
>> IMHO features in layers to union should not overlap within one layer (e.g.
>> landuse is either A or B), so your examples are theoretical only. Example 1
>> does not make sense in this respect. Example 2: If we assume that the
>> circles in layer 2 do not overlap and the overlapping area is only part of
>> the left hand circle (1) the union operation should result in what you show
>> (except the overlapping area would be only one polygon with A=1 and B=X)
>>
>> Bernhard
>>
>> Am 06.05.2013 17:05, schrieb vinayan:
>>
>>> Hi,
>>>
>>> At present the union tool does not give correct results in
>>> sextante/ftools.
>>> After looking at some bug reports, I feel that even in QGIS 1.8, the
>>> union
>>> tool was not producing the correct output either attribute-wise or
>>> geometry-wise.
>>>
>>> My question is how should the real output be? I have two cases here.
>>>
>>> 1. Union of a single layer against itself  -
>>>
>>> https://www.dropbox.com/sh/asicnvr71vxeaz2/TKVUpdleIX#f:singleLayerInput.png
>>> 2. Union between two different layers -
>>>
>>> https://www.dropbox.com/sh/asicnvr71vxeaz2/TKVUpdleIX?lst#f:twoLayerInput.png
>>>
>>>
>>> It looks to me that for case 1, the result could be like below(11
>>> geometries
>>> in total)
>>>
>>> https://www.dropbox.com/sh/asicnvr71vxeaz2/TKVUpdleIX?lst#f:singleLayerResult1.png
>>>
>>> and for case 2, the result could be like below
>>>
>>> https://www.dropbox.com/sh/asicnvr71vxeaz2/TKVUpdleIX?lst#f:twoLayerResult1.png
>>>
>>> what do you think? Anybody knows how it looked in 1.8?
>>>
>>> Thanks
>>> Vinayan
>>>
>>>
>>>
>>
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