[GRASS-user] Precipitation color table?

[Luigi Ponti]

Hamish wrote:
> Luigi Ponti wrote:
>   
>> I just ran into this page
>> http://geography.uoregon.edu/datagraphics/color_scales.htm
>>
>> that includes, among others, precipitation color tables. I
>> don't know if that can be useful.
>>
>> The page of this Lab also provides an interesting paper titled "End of 
>> the Rainbow" which elaborates on why one should not use continuously 
>> varying color schemes (and absolutely no rainbow color table). The way 
>> to go seems to be banded color schemes -- a color scheme with equally 
>> sized bands of constant color [1].
>>     
> ...
>   
>> [1] Borland D, Taylor II RM (2007) Rainbow color map (still) considered 
>> harmful. IEEE Computer Graphics and Applications, 27, 14-17.
>>     
>
> In matters of human perception the individual experience is not subject
> to hard rules. Of course it is important to think about how the color-
> blind will see things and how it will look printed in greyscale, etc.
>
> An interesting subject to me, thanks for the link/look forward to reading
> it. It reminds me that I still need to read "How to lie with maps":
>   http://www.eyrie.org/~eagle/reviews/books/0-226-53421-9.html
> (in the spirit of "How to lie with statistics")
>   

I found the refs I posted while browsing the net in search for good 
color schemes: it puzzled me that the majority of the maps (including 
mine) would use a rainbow, continuous color bar despite good arguments 
against this technique were available in the literature. I have no bias 
since my experience in very limited (nor do I want to lie with maps).

> It is really quite amazing/scary how much changing the colors affects
> the interpretation and is able to hide/highlight features. After spending
> a fair bit of time with the issue I personally feel it is most honest to
> use a color scale which can be described as a continuous mathematical
> function. e.g. log()+BCYR or something directly based on the univariate
> stats of the data. -> Take the biased "what looks nicest" human out of
> the picture. 

Thanks: I was about to ask your opinion on this. Intuitively, I also 
feel that continuously varying stuff should be represented as such.

> Choosing arbitrary band limits on a continuous value dataset
> just compounds the opportunities for arbitrary human influence. 

The argument I read is that color bands act as contours lines do in 
elevation maps. It is true that choosing arbitrary band limits on a 
continuous value dataset is simply... ...arbitrary. However, when you 
have a continuously varying color bar in the map, you will have label 
numbers next to the bar and you will try to associate the bar color next 
to the label value with a color in the map. The argument Borland & 
Taylor (2007) make is that if each label of the color bar is located 
between two constant color bands in the bar, you will have that value 
easily located in the map along the boundary between the two contiguous 
areas of constant color. What do you think about this?

I have no bias given my very limited experience on the matter -- just 
trying to make an informed decision. Thanks again for discussing this 
and sorry if this is way off topic for the list (please advice).

> The
> statistical equivalent is to slowly vary the number bins in a histogram
> while the peaks seemingly double and halve.
>   

Yes, you lose information that way.

Kind regards,

Luigi


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