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. Author manuscript; available in PMC: 2011 Sep 15.
Published in final edited form as: Cartogr J. 2010 May 1;47(2):130–140. doi: 10.1179/000870409X12488753453372

Figure 5. Two approaches to making value-by-alpha maps.

Figure 5

The modifying colour is always found in the layer that does not contain the original colour assignment. For the first method, the original colour is assigned to the top data layer, making the RGB values of the bottom background layer the modifying colour. For the second method, the original colour is assigned to the bottom of the two data layers, making the RGB values of the top data layer prior to alpha manipulation the modifying colour. (recommended size: 2 column or stacked 1 column)