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I have a map of the UK imported as a raster. It was originally a JPEG and has areas that are coloured red and purple. I have a data set imported from a CSV file showing UK locations as dots. I want to select the dots that are within the coloured areas of the raster map.

I thought that in order to do this, I would need to create a new layer only containing the parts of the map that are red and purple and then use this to clip the other layer. I don't know how to just select these areas of the map without just doing it visually using a polygon (this would take ages, as there is so much detail!). Is there a way to select features by their colour?

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    Specify what software you want to use, please. Also: Is your raster georeferenced already? – Senshi Jun 7 '17 at 12:05
  • I can use QGIS or ArcMap, it doesn't matter. Yes my raster is georeferenced. – Mitchell Jun 7 '17 at 12:56
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In QGIS, try the Add raster values to points tool from the SAGA toolbox (included in the default distribution of QGIS).

That way, you can add the RGB values to your point features as attributes, and then use all the regular feature filtering on those values.

  • I'm not sure whether I didn't explain properly, or whether I am not following your suggestions correctly. When I go to Add raster values to points, there is only the option to select my data set imported from the CSV file. I then must select a Grid (my raster file is the only option given for this). When I try to run it I get this error message: "Unable to execute algorithm- Input layer cgmodifiedpoly2 has more than one band. Multiband layers are not supported by SAGA". Firstly, I don't understand what this means! Second of all, I'm not sure how this allows me to add RGB features to my raster? – Mitchell Jun 7 '17 at 13:51

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