I trying to convert a raster layer into polygons with QGIS. At first, it seems there is no problem with doing that, but when I'm checking the output for invalid geometries, it always gives me some invalid geometries (Ring Self-Intersections as shown in the picture). Does anybody know, how to avoid this problem or a workaround for it?
You can use the clean function from GRASS gis that is available in Qgis via processing. In the processing toolbox you may use v.clean with the "break" cleaning tool. It will solve self intersections by adding nodes and splitting the polygon into two. You can also read the help inside the tool for the other cleaning methods. But for this particular problem break will work.