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I have two choropleth of the same area but with different concentration data and I need to overlap them. My aim is to highlight the areas of intersection between the points with the highest concentration (I use a quantile interval). It is probably only a matter of mergers, but I made a couple of attemps and the graphics quality has suffered (e.g., multiply, burn). Any suggestions? Thanks in advance.

closed as unclear what you're asking by Simbamangu, Fezter, Paul, BradHards, Devdatta Tengshe Apr 28 '14 at 5:04

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    What you are attempting to create is a bivariate map, which can be challenging from both a software and graphics standpoint as you've seen. You might search some on that term to look at example approaches. – Chris W Apr 25 '14 at 16:59
  • Please clarify: Do you have raster or vector data? – underdark Apr 25 '14 at 19:25
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As Chris W says, I think that you are trying to build a bivariate representation of your data (not spatially intersecting).

Personnally, I find this easier to do in raster. Use the intensity in red for one layer and the intensity in cyan for the other layer. Black will mean low everywhere, white large everywhere, and if you can see a color it indicates that one of the band has larger values than the other band.

to convert your vector to raster (see here for details)

then create a composite image with

gdal_merge.bat -separate -of GTiff -o output.tif input1.tif input2.tif input2.tif

you can now add the image and see how it looks like.

  • The bivariate map worked! There is only a problem: when I do the legend, a 9x9 matrix with the overlapping of 3 shades of blue and 3 shades of red, is not very clear. Any suggestion for other colors? – user3322469 May 2 '14 at 15:51
  • @user3322469 The best suggestion I can give is use a high contrast value between your lightest and darkest colors on both variables. You could use individual colors for each value, but then you lose the sense of a range for each axis as opposed to nine unique values. Label methods could help - label two sides as an axis range or label all four sides with the min/max value or range of that cell. – Chris W May 10 '14 at 21:07

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