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I'm following the online qgis manual available on qgis website. I'm doing the step 21.11.3. (http://docs.qgis.org/2.14/en/docs/training_manual/answers/answers.html#complete-analysis-3).

To make it simple : I have a raster converted to vector, so now it is a polygon (several polygons). I'm doing an interior buffer exactly how it is described in the manual. I didn't obtain the same result. I can see the interior buffer just in some polygons, not in all of them ->

enter image description here

The interior buffers appear just for those polygons, that don't contain a hole. I use qgis 2.18.3.

This is how i created the interior buffer ->

enter image description here

In green you see the input layer : suitable_terrain, in blue the output layer, the interior buffer.

It is not a problem of a projection. I use WGS84 UTM Zone 34S where the units are in meters. I did also the Check validity and i obtain the same result.

  • Have you tried checking if they are valid geometries? – lpd2 Mar 5 '17 at 21:34
  • @Ipdudley You mean the CRS ? Both, the input layer and the interior buffer have the same one - WGS84 UTM Zone 34S – Janita Mar 5 '17 at 21:36
  • No, with the qgis 'validate geometries' tool – lpd2 Mar 5 '17 at 21:37
  • I don't know that tool. I'll do some research and try it. Thanx for the tip – Janita Mar 5 '17 at 21:40
  • OK I found the tool : Check validity under Vector / Geometry tools. I did the check for both layers, the input = suitable-terrain, and the output = suitable-terrain-continuous-100m. The valid outputs of both layers are the same, so apparently there is no problem of geometry. I tried to take a very simple polygon and I created the interior buffer with the same methodology and again it's not working – Janita Mar 5 '17 at 22:05
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I tested the Processing toolbox -> QGIS geoalgorithms -> Vector geometry tools -> -> Fixed distance buffer on a polygon shapefile that has holes, as in your case, and I have got the same result that polygons with holes are not considered for interior buffers of -100 m distance.

enter image description here

Here is the output:

enter image description here

However, when I used the buffer vectors located under Processing toolbox -> GDAL/OGR -> [OGR] Geoprocessing -> Buffer vectors, I got the correct result that you want of distace of -100 m, as you can see below:

enter image description here

Here is the output:

enter image description here

I noticed that the buffer shape is different between the two results, but it is more accurate in [OGR] Buffer vectorstool than in QGIS Fixed distance buffer tool.

You need to use Processing toolbox -> GDAL/OGR -> [OGR] Geoprocessing -> Buffer vectors in your analysis.

  • Perfect! It's working, It's magic! thanx a lot – Janita Mar 6 '17 at 9:01
  • Glad it worked. – ahmadhanb Mar 6 '17 at 9:03

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