[Qgis-user] Split features tool behavior

[Andreas Wicht]

Hi James,
hi Bernhard,

attached you can find a small sample data set and three screenshots.
I used the Split Parts tool to cut off the lower part as shown in the
screenshots.

This is in a PostGIS layer
(POSTGIS="2.3.2 r15302" GEOS="3.5.0-CAPI-1.9.0 r4084" PROJ="Rel.
4.9.2, 08 September 2015" GDAL="GDAL 1.11.3, released 2015/09/16"
LIBXML="2.9.3" LIBJSON="0.11.99" TOPOLOGY RASTER)

geometrytype
--------------
MULTIPOLYGON

I tested with QGIS 2.18.9 in Windows 7 and Ubuntu 16.04 (ubuntugis deps).
ST_IsValidReason yields the following after editing (depicted in third
screenshot):

NOTICE:  Self-intersection at or near point -19.203503991278154
65.808265481598852
                   st_isvalidreason
-------------------------------------------------------
 Self-intersection[-19.2035039912782 65.8082654815989]

I can reproduce this behaviour with simple hand drawn polygons in a
memory layer as well.
My described workflow in the earlier email was aiming at the comment
(#16) of Jürgen Fischer in the mentioned issue
(https://issues.qgis.org/issues/12799#note-16). Here he describes that
further editing of the geometry is essential to use this function
(please correct me, if I understood that wrong).

By current status of development Bernhard's approach seems to be the
most efficient one.

Yet I think that the function should be designed in a way that it does
not require further validation/editing to get a clean geometry. That
is not obvious to a less experienced user.

cheers
Andreas


On 31 May 2017 at 02:47, James Wood <jwood911 at gmail.com> wrote:
> Hey Andreas,
> I haven't experienced the invalid geometry as you describe, but I did find that my QGIS install had some issues. I have upgraded to 2.18.9 on Win10. If you have a sample of your data available, I would be happy to try to corroborate.
>
> James
>> On May 29, 2017, at 06:16, Andreas Wicht <a.wicht at gmail.com> wrote:
>>
>>> On 28 May 2017 at 14:43, James Wood <jwood911 at gmail.com> wrote:
>>> On the digitizing toolbar, try "Split Parts" instead of "Split Features" on multipart polygons.
>>
>> Thank you for the hint, James. Somehow I always thought that this
>> function is only suitable for separating an existing part from a
>> feature (never used it though).
>> Yet, if you cut a polygon with the "Split Parts" tool, the output will
>> be an invalid geometry (I observed self-intersections). This as well
>> can not be the intended behavior. I could observe those errors in
>> simple scratch layers as well as in PostGIS layers.
>>
>> Has anybody else experienced this behavior?
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