2

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

sirk
  • 33
  • 4
  • 1
    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? – Michael Stimson Aug 22 '16 at 03:24
  • I used raster-extract-contour to get my contour lines. Is there another method I can use that would output simpler polygons? – sirk Aug 22 '16 at 13:19
  • That sounds like the link to GDAL_Contour http://www.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 http://gis.stackexchange.com/questions/100251/qgis-contours-smoothing-and-cleaning/100303#100303 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. – Michael Stimson Aug 22 '16 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 Aug 24 '16 at 22:23
  • There is a tool in the toolbox https://docs.qgis.org/2.6/en/docs/user_manual/processing_algs/saga/grid_tools/resampling.html or you can use Translate (GDAL_Translate http://www.gdal.org/gdal_translate.html) with a changed cell size, but be sure to use GDAL 2.0+ to specify -r bilinear – Michael Stimson Aug 24 '16 at 23:29
  • The thing is I have no experience with coding, I was hoping for a simple plugin or menu-bar tool that could do this for me. Im just a kid, I know nothing of c++ or GDAL. – sirk Aug 26 '16 at 12:57
  • There's no coding in those links. The first is from the toolbox http://docs.qgis.org/2.0/en/docs/user_manual/processing/toolbox.html and the second, GDAL_Translate, is accessible from the QGIS menus (Raster::Translate) but there's not a help page for that (that I know of), the options are explained in the command line (CMD) tool and are the same for the GUI. – Michael Stimson Aug 28 '16 at 21:41
  • Thanks! I used the translate method and It worked perfectly. – sirk Aug 31 '16 at 00:24

1 Answers1

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.

Community
  • 1
jbalk
  • 7,505
  • 1
  • 17
  • 39