[Qgis-user] Split features tool behavior

[Bernhard Ströbl]

Hi Andreas,
I dug deeper and it seems that I can always split the part once. Any 
further splitting of any part results in the invalid geometry error.
https://issues.qgis.org/issues/12799 describes why.
IMHO splitting a part once is ok because you might want to delete this 
newly created part or edit its nodes in order to create a gap between 
the parts (BTW if that is done you can split parts again).
However it does not make sense to have a multipart polygon with adjacent 
parts (they could be one part then), so QGIS correctly detects this as 
an error (self-intersection).
Back to what you try to achieve: you could use "Split feature" to create 
a new feature, manipulate what you need, even split this feature again 
and then use "merge feature" with those polygons that should form the 
multipolygon. Your first mail indicates that you are puzzled because you 
get three features if you apply "split features" to a part of a 
multipart polygon (1 = split part1, 2 = split part2, 3 = all the
other parts). This seems a logical approach to me because how should 
QGIS know which of the to halves is supposed to stay with the original 
multipart polygon and which is to become a new feature? You can merge 
any of the new halves with the original multipart feature in the next step.

I use QGIS 2.14.15 on Ubuntu

Bernhard

Am 31.05.2017 um 11:15 schrieb Andreas Wicht:
> Hi Bernhard
>
> On 31 May 2017 at 10:53, Bernhard Ströbl <bernhard.stroebl at jena.de> wrote:
>> Andreas,
>> when loading your shape file into a new project the project is set to
>> EPSG:4326 because your data are in EPSG:4326
>> Your screenshots indicate that you are working in another projection.In
>
> That was just for a better visualisation for the screenshots (OTF reprojecting).
>
>> EPSG:4326 split parts works flawlessly, when I use a projection I get the
>> invalid geometries error.
>
> I can reproduce the same result as before working natively in EPSG:4326.
>
>> In my experience you should edit geometries in a project matching the
>> projection of the data, although editing projected geometries is not
>> prohibited by QGIS.
>
> Which exact steps do you follow to cut off an area like that with the
> split parts tool?
> I can reproduce the invalid geometry no matter if topological editing
> is enable or disabled.
> So this setting can be excluded from the list of scapegoats.
>
> Andreas
>
>
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