I am working with QGIS. I am creating a contour world map, I started my project with a raster image of the world and then extracted contour lines much too complex for my purposes. I simplified the polygons using a line generalization plugin. However I noticed that after simplifying some of my polygons are lapping into each other, how can I fix this. enter image description here

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    do not try to generalize contours it will only lead to this sort of mess. Resample your DEM to create more general contours. What method/tool are you using to generate your contours from your worldwide DEM? Commented Aug 22, 2016 at 3:24
  • I used raster-extract-contour to get my contour lines. Is there another method I can use that would output simpler polygons?
    – sirk
    Commented Aug 22, 2016 at 13:19
  • That sounds like the link to GDAL_Contour gdal.org/gdal_contour.html which I have used and found to be quite reasonable, but tends to be a bit more on the engineering contour side (more blocky), have a read of an answer I posted gis.stackexchange.com/questions/100251/… In your case I would resample the raster to a larger cell size using Bicubic or B-Spline method to filter out some of the worst lumps before generating your contours. Commented Aug 22, 2016 at 22:20
  • So how should I go about doing this? Is there a plugin I can download or what? Sorry I am quite new to this.
    – sirk
    Commented Aug 24, 2016 at 22:23
  • There is a tool in the toolbox docs.qgis.org/2.6/en/docs/user_manual/processing_algs/saga/… or you can use Translate (GDAL_Translate gdal.org/gdal_translate.html) with a changed cell size, but be sure to use GDAL 2.0+ to specify -r bilinear Commented Aug 24, 2016 at 23:29

1 Answer 1


You can do this using GRASS. According to this post: Generalizing polygon file while maintaining topology in QGIS?, GRASS will handle topology. So, before you generalize, set up topology and then use GRASS v.generalize to simplify the polygons.

However, as Michael Miles-Stimson pointed out, you are better off resampling your DEM at a lower resolution to create the generalize contours. This will be much more accurate that generalizing contours you have already produced.

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