Hi Graeme,

There is an issue open about it:
https://hub.qgis.org/issues/14846

I wanted to have a look into it in the bugfixing sprint for 2.18 but ran
out of time after looking into the intersection algorithm.

Having a small set of features as input with a known-good expected
result would have helped me to work on the issue. Would it be ok for you
to prepare this and attach it to the issue above to help fixing it?

Thanks in advance
Matthias

On 10/26/2016 12:43 AM, SEGGIE Graeme wrote:
> Thanks Luigi,
> 
> 
> That doesn't seem to help unfortunately.
> 
> 
> I tried with the same two shapefiles in 2.14 and in both cases get 463
> zones unioned to 463 resulting in over 4000 features with nulls in
> attributes and lots of features which seem to not highlight any polygon
> when selected.
> 
> 
> Simplifying things, I created a new polygon shapefile with just three
> polygons  - one overlapping each of the other two. Result of unioning
> this with itself is 17 features. Doing same on two copies of same
> shapefile also results in 17 features. However, two different
> shapefiles, same 3 features in one and a single feature in another,
> which overlaps two of the 3, results in 6 features when unioning which
> makes some sense.
> 
> 
> Graeme
> 
> ------------------------------------------------------------------------
> *From:* Luigi Pirelli <lui...@gmail.com>
> *Sent:* 25 October 2016 20:46:42
> *To:* SEGGIE Graeme
> *Cc:* Qgis-user@lists.osgeo.org
> *Subject:* Re: [Qgis-user] Question about results of Union in QGIS
>  
> may you test using 2.14? if I well remember that version suffered a
> severe regression on ftools geoprocessing modules due the introduction
> of the new 3d geometry. During the hackmeeting in Gran Canaria we
> seriously though to announce the deprecation of that version, but a
> fix was found => and a new qgis version + processing tests where
> introduced.
> 
> regards
> Luigi Pirelli
> 
> **************************************************************************************************
> * Boundless QGIS Support/Development: lpirelli AT boundlessgeo DOT com
> * LinkedIn: https://www.linkedin.com/in/luigipirelli
> * Stackexchange: http://gis.stackexchange.com/users/19667/luigi-pirelli
> * GitHub: https://github.com/luipir
> * Mastering QGIS 2nd Edition:
> *
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> **************************************************************************************************
> 
> 
> On 25 October 2016 at 10:44, SEGGIE Graeme <gseg...@systra.com> wrote:
>> Hello,
>>
>> I was doing some analysis on UNION against the same feature (on itself) to 
>> get an understanding of what happens with slivers and overlaps. I started in 
>> ArcGIS and was able to understand what happened with the combinations 
>> available there as follows:
>>
>> Zone system used as input features composed of 463 zones including some 
>> accidental overlap.
>>
>> File with accidental overlaps:
>> Case 1 - self union, 1 ref to source, no gaps      - Generates 525 - due to 
>> lakes (3) and slivers (16) and overlaps (43)
>> -- overlaps times number of contributors i.e. two zones overlap, two overlap 
>> features with same area but different source zone
>> -- slivers -1 as source (since there is none) and single feature.
>> Case 2 - self union, 1 gaps                              - 506               
>>  - due to overlaps
>> Case 3 - self union, 2 refs to source, no gaps      - 577
>> Case 4 - self union, 2 gaps                              - 558
>>
>> Having done all this in Arc, I now wanted to check that QGIS produced the 
>> same thing. I could not check for gaps / no gaps as there is no setting to 
>> select regarding this. Also, the two tools I used via  (1) menu Vector --> 
>> Geoprocessing --> Union and (2) Toolbox Geoalgorithms --> Vector --> 
>> Polygons --> Union do not allow
> for a single reference to the source feature set.
>>
>> What puzzles me though is that instead of feature counts of the order listed 
>> above, I get much more features and many, but not enough with nulls and 0 
>> for source ID fields etc. I cannot see an obvious way to get the resultant 
>> 4,139 features (with tool 2) down to approx 500 or so.  Tool 1, returns 
>> 3,950 features on the same
> input feature set.
>>
>> I also when selecting features in the area with overlaps return normal 
>> counts of features i.e. 2, not multiple orders higher as I'd expect given 
>> the count of features in the attribute table. Do some (lots) of them have no 
>> geometry? - Actually having just added a field of area, I see this is not 
>> the case either as too many
> have geometry on that basis.
>>
>> Am I missing a key difference employed in the processing of QGIS union and a 
>> way to get to the relevant features which approximate to the 500+ I expect?
>>
>> Am using 2.12 on Windows if that has a bearing on this.
>>
>> Graeme.
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