[Qgis-user] Split features tool behavior

[Bernhard Ströbl]

Hi Andreas,
my comments below

Am 31.05.2017 um 13:04 schrieb 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).

Still you can save it and have thus an invalid geometry
>
>> 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.

Not necessarily because you could fill a new feature or part into the 
gap in the next step.

> Deleting the new part is the only way to solve that problem here.

Which would create a gap, wouldn't it?
>
>> 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.

I do not get, what you mean, anyways you can merge several features into 
one at anytime.

>
>> 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 do not have access to these packages. Could you describe what they do 
differently?
>
> 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).

But that is exactly what split feature does if you apply it to a part: 
It splits the part and adds both "halves" as new features. I would not 
expect any of the halves becoming a new feature and the other staying a 
part of the multipart feature arbitrarily. What should I do if the wrong 
half stays?
Therefore for me the workflow is clear: 1) Split features 2) merge the 
feature that should stay with the multipart feature. 3) Done

BTW: We should agree on common terms; these are the terms QGIS uses 
AFAIK: a feature is a dataset, i.e. one row in the table; it can either 
have one geometry or several geometries as spatial representation (or 
none but that's of no interest here). If it has several geometries it is 
called a multi-part feature, each individual geometry is called a part 
then. A polygon is a type of geometry; if it is a multi-part polygon, 
each part is a polygon in itself (therefore I tried to avoid "polygon" 
altogether and used "feature" and "part" instead to be clear). An island 
is a hole in a single polygon, QGIS calls it "ring". In DigitizingTools 
I use "gap" for a space surrounded by polygon features but without being 
part of a polygon itself.

>
> 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.

The idea behind "Split off one part and add it as a new feature" is that 
you cannot use "Split features" to extract a whole part (workaround 
would be to apply "Split features" to the part and merge the two 
resulting new features into one, but that's not intuitive :)

Bernhard


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