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 <[email protected]> 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 <[email protected]> wrote:
On 28 May 2017 at 14:43, James Wood <[email protected]> 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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