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I've got a huge database of depth points for the whole Irish sea which I'd like to be a nice smooth surface, or at least have a depth value associated with the entire area (i.e. include the inter-point spaces). I've tried interpolation but can't get it to work well, so I thought to make Voronoi polygons from the points, and then graduated colour them.

Problem: In 2.1 master, a blank column is created in the Voronoi polygons shapefile, and switching from single symbol: single symbol towards graduated results in a complete blank: complete blank. This even happens with the csv file, from dots: dots towards blank: blank. Andre Joost got around a similar problem by using semicolon delimited files, but it feels to me like this is a genuine bug in master, especially given the same files don't blank the Voronoi shapefile in DuFour: don't blank the Voronoi shapefile in DuFour. Has anyone else experienced this? I'm about to raise it on QGis hub.

Question: can I colour the Voronoi polygons using the depth value? The Voronoi shapefile has depth as a field but it doesn't do anything - possibly it doesn't know how to deal with the fact that each Voronoi polygon has a number of depths equal to its number of edges (usually 4 in my case here). Can I colour them using the depth value from the points file somehow?

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  • you seem to have a regular grid, so why not convert it to raster ? Have you checked that the field you want to symbolize is of numeric type ?
    – radouxju
    Jan 19, 2014 at 19:04
  • seems that it is a regular grid all over (for some reason zooming out adds weird overlaps which make it look otherwise). Raster here we come, thanks!
    – dez93_2000
    Jan 20, 2014 at 13:38

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you can give the value of your points to your polygons using the spatial join (vector > management tools). Then you will have a depth value for all your polygons.

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  • I'll try this, then the raster idea above. The benefit of this method is that it fills odd-shaped the coastal voronois with values, which the raster will only partially do. Thanks!
    – dez93_2000
    Jan 20, 2014 at 13:40

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