I'm interested in the areas of my investigation area where the slope is smaller than 25%. For this, I used elevation raster data SRTM 90m Digital Elevation Database v4.1 from CGIAR-CSI (http://srtm.csi.cgiar.org/srtmdata/) in CRS 4326.

I used the following data processing steps:

  1. I clipped the data to my investigation area
  2. I used Raster > Analysis > Slope with a ratio of vertical units to horizontal units of 111120 to calculate the slope in percent
  3. I used Raster > Conversion > Polygonize to receive polygons
  4. I checked the calculated slopes and they are in reasonable magnitude and seem plausible
  5. I added a new field with the Field calculator with:
     "Slope" <= 25
  6. I deleted the field Slope

enter image description here

Thus, I have a lot of small polygons, which have the attributes 1 or 0 depending on if the slope is smaller than 25%. Now I would like to combine the polygons together, as I am merging several vector layers later on. I tried Vector > Geoprocessing Tools > Dissolve. However, a lot of geometries seem to be invalid and are skipped.

enter image description here

This approach gives me this result:

enter image description here

Is there another way of merging the polygons together without losing some areas?

  • Maybe check geometries core plugin (activate it in the menu plugins if it does not appear in the menu vector) can help fixing the geometries? What messages do you get if you run the plugin? If geometry errors were created, it happende in step 3. Did you inspect some of these defective geometries? Can you post a screenshot of one of them?
    – Babel
    Nov 30 '20 at 22:41
  • Try to select the polygons that you want to dissolve and then use Polygon Dissolve from Saga -> Vector polygon tools -> Polygon Dissolve (all polygons) and click Selected features only. Check the result.
    – ahmadhanb
    Dec 1 '20 at 0:08

Change your workflow to:

  1. Slope
  2. Raster calculator ("Slope@1">=25) which will evaluate to 1 where the slope is >=25 and 0 where it's not. I got this the other way around, I think you want ("Slope@1"<=25) for your task.
  3. Vectorize

enter image description here

Then the vectorizing should be faster and you dont need to dissolve the results since you only have two values, 0 and 1.


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