Hi Bernhard,
Ah - now I understand it - before feeding the polygons into the
"Eliminate selected polygons" I should only select the overlapping ones
detected by the Union (e.g. by using the "select by attribute" algorihm)
- and not all polygons in the layer. Makes sense. The algorithm will
then merge these subselects to their neighbours matching either largest
or smalllest area or common boundary. Will give it a try.
Otherwise, the "eliminate selected polygons" sounds interesting in more
"interactive", semi-automated workflow.
Thanks,
Andreas
On 2023-07-26 15:22, Bernhard Ströbl via QGIS-User wrote:
Hi Andreas,
I would like to give some explanations on your comment concerning the
eliminate algorithm: During development the goal was to have a method
that can get rid of sliver polygons (I explained my workflow for
detecting them in my previous post). The algorithm simply merges the
selected polygons with their specified neighbours. The attribute values
of the neighbour stay as they are. What would you expect attributewise?
Furthermore no definition of the polygons to eliminate is defined in
the code thus it is up to the user to decide which polygons should go,
hence it works on the selection. Making a tool that merely eliminates
all overlaps would be a different task. The eliminate tool as it is has
a broader use range, examples: You can only eliminate certain overlaps
while keeping others. You can use it to generalize by getting rid of
polygons smaller than a certain size. You have landuse data that
contains polygons for a river but you want to use lines for the river,
so get rid of all river polygons....
regards
Bernhard
Andreas Neumann via QGIS-User <[email protected]> hat am
26.07.2023 14:15 CEST geschrieben:
Hi,
Thank you all for your feedback!
@Nyall - the GEOS/JTS capabilities for cleaning and validating
geometries
look definitely interesting. Something for an upcoming QGIS-CH grant
perhaps ...
@Bernhard - I also had a look at and tested the "Eliminate selected
polygons" algorithm - but it doesn't have a way to specify attribute
handling nor grouping - and it is also strange that I have to select
the
polygons first and cannot run the algorithm on the whole data set as is
About SAGA: I couldn't get the SAGA stuff to work anymore
About GRASS: it could be an option, but a native solution in QGIS would
be
preferred
So in summary: there are several options around for cleaning overlaps -
but
it isn't as straight-forward and user friendly yet as it could/should
be.
I haven't looked at the gaps yet ...
Thank you all and greetings,
Andreas
On Tue, 25 Jul 2023 at 13:13, Bernhard Ströbl via QGIS-User <
[email protected]> wrote:
Hi Andreas,
the Algorithm "Eliminate selected polygons" was originally created to
address these questions. I usually imported the data into GRASS ran a
clean
there and reexported the results into a non topolgical dataset. Thus
overlapping areas and gaps (only if closed) are identified and can be
eliminated using the algorithm named above (selecting only a certain
area-circumference relation) by merging it with the adjacent polygon
with
the smallest/largest area or the longest common boundary.
For your case you could have used this algorithm after the union to
identify overlaps.
As for an algorithm that does all this in one go: You would need
something
that detects holes between polygons without the help of a toplogical
data
format.
regards
Bernhard
Andreas Neumann via QGIS-User <[email protected]> hat am
25.07.2023 12:01 CEST geschrieben:
Hi,
A friend of mine has a dirty input data set with lots of overlapping
geometries (within the same layer) and asked me if there is a tool
within QGIS to automatically remove the overlaps and assign the
overlapping area to the neighbor polygon with the largest area.
The solution was surprisingly hard to find, although I am pretty sure
there are multiple combinations of algorithms that would solve the
problem. Here is the solution I came up with:
https://github.com/qgis-ch/overlap_removal/tree/main [1]- perhaps you
have better ideas - more elegant solutions?
Wouldn't it be great if QGIS had a processing tool to solve this
overlap
cleaning within the same layer "out of the box" without having to use a
graphical model or a more or less complicated sequence of algorithms in
the processing toolbox? Apparently, ArcGIS has such a tool ...
Saga and GRASS also might have such tools - but I couldn't get the SAGA
based QGIS plugin "Dissect and dissolve overlaps"
(https://plugins.qgis.org/plugins/dissect_dissolve_overlaps).
The same problem exists for automatically filling small gaps in the
polygon data set ...
Andreas
Links:
------
[1]
https://github.com/qgis-ch/overlap_removal/tree/main_______________________________________________
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