QGIS » QGIS Application

There are different issues here, both in QGIS (and that is very unfortunate) and in your data.

The Union between your 'dissolved.shp' & 'wards.shp'fails because the "wards" layer has a lot of issue, both from a geometry point of view but also from a topology one, in fact the polygons that you probably expected to be adjacent are not, many of them have gaps, that at the end of the operation you see as "artifacts". If you give a pass trough Processing/v.clean using also the snapping parameter then the union works well.

On the other hand the union/intersection of "areaA" and "areaB" does not works because in one of the two layers you have geometry errors (auto intersections and duplicate nodes). This errors are reported in the QGIS log after the operation runs, but of course this is NOT enough for the average user, a proper warning must be thrown. This is the error

2016-12-12T07:36:28    2    GEOS geoprocessing error: One or more input features have invalid geometry.
2016-12-12T07:36:28    0    Feature geometry error: One or more output features ignored due to invalid geometry.
2016-12-12T07:36:28    2    GEOS geoprocessing error: One or more input features have invalid geometry.
2016-12-12T07:36:28    0    Feature geometry error: One or more output features ignored due to invalid geometry.

Moreover while the intersection operation seems to work ok (after fixing those issues) the union one creates an output that smells rotten fish, in fact it seems to create duplicate geometries (see attached images).

See also: #9297

I'm tagging as SEVERE all this geoprocessing issues because they MUST be fixed once for all.

Re the union, I've been having issues with unexpected duplicated polygons in the results.

Attached are two input shapefiles and the result of a QGIS union between these.
The resulting union has one duplicated polygon which I wouldn't expect to be there: FID 3 (circ Y) from Test_input_B is duplicated in it's entirety as FID 9 in the union output shapefile.
The input, expected and duplicated features are shown in the attached images.

Sorry, forgot to attach last image, screenshot of duplicated feature attached.

see also #11986 for more intersection errors!

In 2.18.7 the union operation with the provided dataset, that usually returned a wrong result, now is BROKEN

2017-05-12T19:08:56 2 Uncaught error while executing algorithm
Traceback (most recent call last):
File "C:/OSGEO4~1/apps/qgis/./python/plugins\processing\core\GeoAlgorithm.py", line 203, in execute
self.processAlgorithm(progress)
File "C:/OSGEO4~1/apps/qgis/./python/plugins\processing\algs\qgis\Union.py", line 164, in processAlgorithm
if diff_geom.isGeosEmpty() or not diff_geom.isGeosValid():
AttributeError: 'NoneType' object has no attribute 'isGeosEmpty'

I'm looking at an example sent by Giovanni Manghi, will report here what I see.

The example sent to me by Giovanni, with the instructions provided, results in an error sent to the user about the input having invalid geometries. It seems to be the correct behavior with regard to this ticket (which is about incorrect results), so this is still in a "cannot reproduce" state for me. This is as of fd7b52d5b8e668bd5db43f81eecc60818ff490ab in release-2_18 branch (which is what "Affected QGIS version" references).

I'm closing again. Please consider filing a new ticket if it helps reducing the confusion between what was fixed and what not (as this one seems to be about many different cases)

replaced by #17131