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.stro...@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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