[Qgis-user] Union Tool - Ftools/Sextante

[Ivan Santiago]

Hello all: 

I've ran into the same problem.  nobody gave an adequate response.  I'll (and many) will appreciate if somebody take a look at the code in that union tool.  

It is strange that this union also does not work well using sextante or even grass (inside qgis interface).
I have not tried using GRASS outside of qgis.  

I wrote a qgis 1.8 tutorial in Spanish 
http://www2.pr.gov/agencias/gis/seccioneducativa/Pages/Tutorial_Quantum_GIS.aspx 
and I had to leave the union geoprocessing example blank.


---------------------------
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
------------------------------

Message: 7
Date: Mon, 6 May 2013 17:25:28 +0100
From: Alexandre Neto <senhor.neto at gmail.com>
To: Bernhard Str?bl <bernhard.stroebl at jena.de>
Cc: qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] Union Tool - Ftools/Sextante - How should the
	real result be
Message-ID:
	<CA+H0G_GCmkZN_a8j3FgOejbZ6Fjw1MfARUPL2Qxt59XtfLfyDA at mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"

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 Str?bl <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
>>
>>
>>
>>
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