I installed QGIS 3.6.1 with GRASS 7.6.1 As I am trying to get administrative borders from old map. I did some pre-processing in GIMP and already selected the borders with treshhold functions and so on... afterwards exported it as a *.tiff and opended it in QGIS georeferenced it.
As the selection (see picture below) is not very "clean" I want to have only the middle of the line. Therefore I want to use the Raster skeletonization from SAGA (SAGA -> analysis tools -> Raster skeletonization)
Somehow this does not work... I also tried any other SAGA tools but non of them are working :( even when I use a projected raster image with correct CRS.
Does anybody know what the problem with those SAGA tools might be? Or is there any other way to "clean" the borders?