[Qgis-user] Split features tool behavior

[Tobias Wendorff]

Yes, Bernhard is right. Actually, this might be caused by rounding issues
(or by the limit of floating point calculations).

I often have problems with topology when working with another projection.
The direct and its invers calculation hardly is the same.


Am Mi, 31.05.2017, 10:53 schrieb 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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>
>
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