I am looking for a tool which helps me to smooth primarily edges/outgrowths of raster areas/zones in QGIS/GRASS/SAGA.

I particular, regarding the image below, I would like to reduce the single lines (mostly probabilistic errors derived from modelling) on the edges (dark grey for better visibility), but no/least changes within the bright grey area.

enter image description here

I tried tools like GDAL sieve, SAGA majority filter and several tools of the LecoS Plugin (which is great, btw). Infortunately these tools also change the values within my perimeter(as the bright grey area is not a zone itself). Further, I could creat a binary raster (1 inside the zone and 0 or nodata for the 0-values). I could then smooth it using a nearest neighbour/moving window analysis. Creating a model would work, but:

If possible, I'd prefer "one-step" tool.

As I am a beginner to QGIS (GIS in general), for now I try to avoid using programming language.


1 Answer 1


ArcGIS's Boundary Clean uses an expand-shrink process. Therefore I suggest trying Saga's Shrink and Expand.

There are similar functions (e.g. r.region and r.grow.shrink) in GRASS which are available in a standard install but not enabled in the QGIS GUI. To use them you would have to enable them (see here) or do it from the commandline or start GRASS in stand alone mode.

  • thank you for the inputs. Using Saga Shrink and Expand with different parameters (expand/shrink; shrink/expand), majority, different radius, any value is being changed. I used a float32 raster and converted it into Int16, too. Further, my values start at 0 and go on from 1.xx up to 100 (they express percentage, I erased everything below 1%). Do you know, what could be the problem?
    – Kevin
    Feb 23, 2017 at 9:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.