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raster bivariate map symbology

October 15 2008 | 0 comments
Categories: Cartographic Effects, Symbology

Greetings. I am trying to create a map that illustrates the combination of two raster layers, but rather than simply showing the result, I'd like to illustrate what generated the result. Consider two raster layers A & B, that when combined in some fashion yield C. Observing a low cell value in layer C, the audience doesn't know if the result is due to low values of A or B. Ideally one could symbolize a raster using a two dimensional color ramp showing both layers A & B, with C as mix of the two colors. Is a technique available to produce a layer symbology such as the example map attached? It is exactly what I need to do. It was coded in Matlab, but shouldn't it be possible in ArcGIS?

Thanks for any comments.

Mapping Center Answer:

The trick is that if the function that evaluates A and B to produce C is not simple, e.g., A + B = C, then this won't be easy.

The idea of displaying raster A with a white to red color ramp and then display raster B with a white to blue color ramp, but with 50% transparency is what I'm thinking of.  That will give you a map with four distinct color qualities:

  1. Red:  High value of raster A and very low value of Raster B
  2. Blue:  High value of raster B and very low value of Raster B
  3. Purple:  High value of rasters A and B
  4. White: Low value of rasters A and B

The concept is deceptively simple.  Confounding are the ideas that different hues behave differently in intensity across a range of lightness values--mixing them is not always adviseable if the function that produces C from A and B is simple.  You can control for some of this using the Raster layer's histogram--change the shape of the curve (by default I think it is a 45 degree angle straight line).   I just imagine that would be an exceedingly difficult task to correctly execute. 

Conventional cartographic wisdom is not to make a complicated map or use a complicated method that relies on super-human graphic communication skills to pull off (nearly all multi-variate mapping fits into the class of "hairball" for either the reader or the cartographer, if possible should be avoided if rapid clear communication is the goal). 

In this case I would strongly advise employing the idea behind small multiples, where you show A, B, and C.  You can annotate them with callouts to draw attention to critical or important characteristics.  Most of the time that will be much easier for audience to comprehend.

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