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I've got a composite image built up from Red Green and Blue composite colours. Each channel represents the differing concentration of a different variable in each location - with 0 being the lowest concentration and 255 the highest. I'm bringing each of these unique values together on the composite image to form a colour based on the RGB values. That bit is straight forward - although I'm trying to find a useful way in which to represent the colour scheme derived as a Legend. I've tried various RGB triangles and cubes, although nothing seems to fit quite right. I was wondering if anyone had attempted this before / could point me in the direction of some Creative Commons images that could be used to this end? Cheers, Andy

p.s. Heres a screen shot of what I'm trying to represent:

Screenshot

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You should have enough reputation to post screenshots now -- please do so for the benefit of other readers as well as yourself! –  blah238 Jan 2 '13 at 9:50
    
Reputation, really? Heres the screen shot of the main image, note my response below about the sameness of the colours. –  Andy TIce Jan 4 '13 at 4:49
    
Thanks for posting the screenshot. I embedded it for you. You could have used the image button to upload it to StackExchange's affiliated image host (imgur) and automatically embed it but this works too. You can read about reputation in the FAQ. –  blah238 Jan 4 '13 at 7:26
    
Thanks again - noted how to upload images directly in your edits. Cheers, –  Andy TIce Jan 6 '13 at 22:52
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1 Answer

up vote 6 down vote accepted

It might help to describe what you have tried, as well as how the default legend for 3-band rasters (shown below) is not sufficient for your purposes.

Default legend for 3-band rasters

One thing to think about is that human vision is not well adapted to quantitatively judge small variations in hue (source) so a composite image containing potentially millions of colors is unlikely to be an effective method of communicating the important information.

You might consider showing each band in a separate inset, perhaps alongside the composite image so that the patterns are more distinct.

Alternatively, you could bin the values into a smaller number of colors that are more visually distinguishable and create a legend similar to this one:

Massachusetts Land Cover Map

(Found in this question: How to present three spatial variables in one map?)

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+1 - Page 393 of this article has another similar example "ternary" legend (Friendly, 2007). What I have done in the past to recreate such multivariate legends in ArcMap is to actually make geographic data to mimic the legend and just treat it as a second inset map (here is an example I did for a bivariate legend). Also +1 for the limitiations, I give a similar discussion on the site in the bad maps thread. –  Andy W Jan 2 '13 at 12:50
    
Thanks for this, quite enough to be getting on with! Also thanks for the point about adjudging colours - duly noted. The main point of the map is to point out that there are substantial similarities between locations. –  Andy TIce Jan 4 '13 at 4:33
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