[GRASS-user] Help using color tables (differences)

[Nikos Alexandris]

On Sat, 2009-08-08 at 15:35 -0400, Thomas.Adams at noaa.gov wrote:
> Hamish,
> 
> I just tried using r.colors.stdev -z — I guess I incorrectly *assumed* it would not do what I wanted. It looks like it does what I want except for the option to invert the colors. So, adding the 'Invert' option would be great.
> 
> Regards,
> Tom

Perhaps a (very) stupid (attempt to) work-around for the time being:
"r.colors -n rast=..." on a map already colored based on "r.colors.stdev
-z" :-?

Nikos


> 
> ----- Original Message -----
> From: Hamish <hamish_b at yahoo.com>
> Date: Friday, August 7, 2009 6:39 pm
> Subject: Re: [GRASS-user] Help using color tables (differences)
> 
> > Hi Thomas,
> > > I have been using r.colors -n map=mapname color=differences
> > > to display soil moisture differences from one day to another
> > > (to identify areas that have received precipitation and
> > > areas that are drying); I would like to depict wetting areas
> > > as shades of blue to white and drying areas as shades of
> > > white to red — as the the option color=differences tries
> > > to do. The difficulty I'm running into is that unless there
> > > is a fairly even proportion of of positive & negative
> > > values I'll have wetting areas colored a shade of red
> > > instead of blue, but more often, the reverse where drying
> > > areas are colored blue, due to the shift of the color scale.
> > > Ideally, I want values close to -0- to be white and wet
> > > areas to be a shade of blue and dry areas to be a shade of
> > > red. How can I do this? Any suggestions?
> > 
> > try the 'r.colors.stdev -z' which was recently moved into the
> > main source tree from the addons repository. maybe a new option
> > could be added to do a better differences map.
> > 
> > some examples here:
> >  http://hamish.bowman.googlepages.com/grass_color_maps
> > 
> > 
> > for the color rules $GISBASE/etc/differences does:
> > 0% blue
> > 0 white
> > 100% red
> > 
> > what you really want is to have it evenly scaled like:
> > 
> > peak = the greater of abs($min) and abs($max)
> > -$peak blue
> > 0 white
> > $peak red
> > 
> > 
> > you could use awk to do that a>b logic in a script if you needed
> > to.
> > 
> > then use those rules as custom color rules with r.colors.
> > 
> > or for you entire dataset decide some static rules, then inter-
> > map differences are comparable on the same visual scale.
> > 
> > 
> > Hamish
> > 
> > 
> > 
> >      
> > 
> > 
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