[Qgis-user] Split features tool behavior

[Bernhard Ströbl]

Andreas,
when loading your shape file into a new project the project is set to 
EPSG:4326 because your data are in EPSG:4326
Your screenshots indicate that you are working in another projection. In 
EPSG:4326 split parts works flawlessly, when I use a projection I get 
the invalid geometries error.
In my experience you should edit geometries in a project matching the 
projection of the data, although editing projected geometries is not 
prohibited by QGIS.

Bernhard

Am 31.05.2017 um 09:49 schrieb 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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