[GRASS-user] Precipitation color table?

[Luigi Ponti]

>
> From: Hamish <hamish_b at yahoo.com>
> Subject: Re: [GRASS-user] Precipitation color table?
> To: GRASS user list <grass-user at lists.osgeo.org>,	Markus Neteler
> 	<neteler at osgeo.org>
> Message-ID: <570989.41209.qm at web110014.mail.gq1.yahoo.com>
> Content-Type: text/plain; charset=us-ascii
>
> Markus wrote:
>   
>> > anyone having a nice precipitation color table (e.g., range from 0mm
>> > to 2000mm)? 
> you might find some nice ones here:
>   http://sview01.wiredworkplace.net/pub/cpt-city/
>
> convert those .cpt (ie GMT) color rules to something suitable for r.colors
> with the r.cpt2grass addon script.
>   

Sorry, I omitted that in my reply to Markus because it had been 
mentioned several times on the list -- my fault.
>
> or maybe one of the color scales here do the trick:
>   http://oceancolor.gsfc.nasa.gov/PRODUCTS/colorbars.html
>
> I have converted a couple of those to GRASS r.colors rules files, here:
>   http://trac.osgeo.org/grass/browser/grass-addons/raster/r.colors.tools/palettes
>
> conversion script are given in the SVN log messages of those if you would
> like to convert the euphotic depth rules. By the look of it their version
> of NDVI is staged and would best be converted by hand.
>   
The SVN conversion script is something I have tried to figure out for a 
while: great! It always challenges me to find ways to dynamically 
generate raster color maps.

I have understood that the algorithm is able to scale a 0-255 color map 
to the 0-65535 range; I am not sure how it could be adapted to scale to 
any range of data. Questions:

- Why do you use 65535?
- Slope and Intercept: are they parameter of a linear regression to 
convert 0-255 values to 0-65535?
- In the line:
        10((Slope * (($1 +1)2 -1)) + Intercept), $2, $3, $4)}'
I am not sure what (($1 +1)2 -1)) does.

The goal would be to develop a script which would take the data range of 
any raster and scale a 0-255 color rule to it.

Thanks and regards,

Luigi



# scale 0-255 to 0-65535 and then convert to chlor-a values