[Qgis-user] Split features tool behavior

[Andreas Wicht]

On 31 May 2017 at 12:23, Bernhard Ströbl <bernhard.stroebl at jena.de> wrote:
> 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.

Exactly. So when you split the polygon and save the edits you will
have said self-intersection in your data. That's why the second split
will not work (correct behaviour).

> 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).

Creating gaps contradicts topologically clean editing in my case.
Deleting the new part is the only way to solve that problem here.

> 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

Split Features is not an option here, because in this example the
island would also be separated from the polygon which is not
necessarily wanted.

> 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.

Yes, I totally get that. But as I said before, other GIS handle it
more intuitively.
I don't know in detail how they technically do it (closed source).
- ArcGIS's "Cut Polygon" tool
- MapInfo's "Split" tool

I think logically one would have to chain splitting the part and
automatically converting the new part as a separate polygon in one
tool (given the robust automatic identification of the new part).

Probably it would have to be a separate tool which mixes "Split Parts"
and the "Split off one part and add it as a new feature" Tool in the
DigitizingTools plugin.