qgisqgis-2.18ringself-intersection
When I polygonize a raster, I very often have 'pixels' holes/interior ring that touche exterior ring.
Example :
POLYGON((5 0, 10 0, 10 10, 0 10, 0 0, 5 0, 3 3, 5 6, 7 3, 5 0))
It's defined as an invalid geometry in the OGC geometry specifications.
In QGIS, how to fix this ?
I can see to ways of doing this:
Method 1:
Use add ring to cut the holes you need;
In the Settings > snapping options check the avoid intersection for that particular layer;
Use add feature, and draw a polygon around your ring or rings;
The new feature will take the shape of the hole.
Method 2:
This tool will automatically cut and fill the holes.
When I get errors like this I usually find applying a zero distance buffer can fix the geometries. So use the buffer tool in QGIS and set the buffer distance to 0 units. Then try and use this buffered layer as the mask.
Best Answer
I've found the solution here but for Postgis : https://gis.stackexchange.com/a/15301/93097 and I've test it in Spatialite/Virtual layers, and it works !
Bellow the code I use with a shapefile 'my_layer' with one field 'DN' :