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

[Alexandre Neto]

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<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<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<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<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
>>
>>
>>
>>
>
> __________ Information from ESET Mail Security, version of virus signature
> database 8301 (20130506) __________
>
> The message was checked by ESET Mail Security.
> http://www.eset.com
>
>
>
> ______________________________**_________________
> Qgis-user mailing list
> Qgis-user at lists.osgeo.org
> http://lists.osgeo.org/**mailman/listinfo/qgis-user<http://lists.osgeo.org/mailman/listinfo/qgis-user>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20130506/56140cb2/attachment.html>