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it is also necessary to review the resulting attributes -> it would necessary to give a look to all the discussions in particular #4567

If this is not going to be tagged as blocker (and #7708 closed because this ticket is replacing it) then I would say that the union tool has to be removed from qgis because it misleads the users with wrong results, this is very bad.

But honestly missing a union tool in a major release it would be bad too (I'm not asking anything, just making a point).

cheers!

I will work on this bug. (just for avoid double work)

I will take into account your considerations Giovanni.

Thanks in advance

Daniel Vaz wrote:

I will work on this bug. (just for avoid double work)

I will take into account your considerations Giovanni.

Thanks in advance

Thanks Daniel! very good news indeed!

Please Giovanni, download the .tar.gz files and give it a try.

Turn edit mode on and move the features of results layer.

Thanks

Ok, I already have been understood it.

Is it the expected behaviour if the three geometries are inside the same layer, right?

What is expected if the two circles resides in one layer and the rectangle resides in another layer?

My attached results almost reach it, but the green don't repeat three times, but only two. I didn't find an elegant solution to get the green part be repeated three times.

But the current solution is worse than mine, although both are "wrong".

Thanks in advance

Hi Daniel,

Is it the expected behavior if the three geometries are inside the same layer, right?

I don't know if I understand correctly, but as far as I know, the union operation is supposed to have 2 (or more***) different inputs. I guess that eventually the same layer can be used two times, but I don't think it makes much sense.

What is expected if the two circles resides in one layer and the rectangle resides in another layer?

This is the common situation, the union tool needs two inputs. In the example the circles would be in one layer and the rectangle in another.

The result of a union operation is just one layer, so in the example you are using all the resulting 11 features are in the same layer, a few of them repeated/overlapped.

(***)In QGIS union and intersect just allow to select 2 input layers. There is already a feature request to allow choose >2 input layers (as for example Arcgis does). I believe that in this case the operations are made by unioning/intesecting the layers 2 by 2 and then the result unioned/intersected with the following one.

But the current solution is worse than mine, although both are "wrong".

I really appreciate your effort. I'm still convinced that we cannot ship a tool that we know produce a wrong result. I would be in favor to eventually remove (temporarily) the tool from qgis 2 and reinstate it later when a solution is found. But this obviously must be discussed in the dev mailing list.

Hi Giovanni,

I was thinking, could somebody download the two .tar.gz attached files and using arcgis do the union process and attach the results here?

The shapefiles are:

File: arcgis_dataset.shp (all features in same layer)

File: rectangle_only.shp and two_circles.shp

Maybe we need to discuss the requisite of a union tool instead of try to copy a arcgis tool.
I am not convinced that the requisite of the QGis union tool was completely clear. If you see the dev-list discussion you will face with opposite opinions.

See for example #4567 (a related bug):

You can see this easily applying transparency to the resultant layer. In that way you can detect overlapping polygons in a single layer. That must not exist as a result of a union function.

The user expects that a union tool just union the overlapped area not create a new polygon.

Again, I think that we need to discuss the requisite of a union tool, not get a arcgis feature and try to reproduce.

Thanks in advance

Hi,

What is "union" for rest of the world is "dissolve" for ESRI. This speed comparison deals loosely with that
http://lin-ear-th-inking.blogspot.fi/2013/02/jts-union-vs-arcgis-dissolve.html

I believe that union in QGIS should yield the same results as union in PostGIS http://postgis.refractions.net/documentation/manual-svn/ST_Union.html. Rather than using the ESRI terminology we could have a ESRI-FOSS4G lexicon about geographic operations with sample images.

I use union ofter very often for converting a bunch of polygons into a smallest possible number of polygons which cover the same total area.

Hi Raymond and jratike80,

thanks for you comments.

For me in good whatever direction we think is more appropriate. I personally suggested to try do as esri does just because it is what most people would expect. But using another approach is good to. The main issue here is having a tool that works as advertised.

cheers!