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20000726: Gempak color question (resend)

>From: Michael Voss <address@hidden>
>Organization: UCAR/Unidata
>Keywords: 200007261657.e6QGveT01373

>(I'm sending this again in case the last one got lost during the power outage)
>Steve,
>
>I've been creating gifs for classroom and web viewing using Gempak. I've been 
> playing with the coltbl files trying to make my own color ramps to suit our n
> eeds. That part is going fine, my question has to do with 'absolute' color sc
> ales. Using 850 temps as an example, I'm creating a color ramp for temp, colt
> bl.temp, and I can access this fine with GDPLOT. However, my color scale alwa
> ys starts at the lowest temp found on the map, thus creating inconsistency in
>  colors and temps between maps/times. I've searched the archives but I haven'
> t found anything directly addressing this issue. I've tried playing around wi
> th setting the FINT min to various values and the beginning color = zero. I c
> an get it to be consistent if I set the FINT min value to be somewhere above 
> the actual minimum, but then I get the problem of not being able to contour t
> emps below the min, not a good solution. Is there a way to force the color sc
> ale to begin at a certain value, thus providing consistency between maps? I h
> ope this makes some sense :)
>
>Any ideas how I might address this problem?
>
>Thanks,
>
>MIke
>
>
>--------------------------
>Mike Voss                                
>Department of Meteorology              
>San Jose State University                        
>One Washington Square                             
>San Jose, CA 95192-0104   
>             
>408.924.5204 voice
>408.924.5191 fax   
>


Mike,

Generally, you can specify specific specific contour values with FINT like:
FINT=.01;.1;.25;.5;.75;1;1.25;1.5;1.75;2;2.25;2.5;2.75;3;3.25;3.5;3.75;4
FLINE=0;21-30;14-20;5

In that way, the contour values are always specified. Generally, this works well
for mag(obs) plots for instance where you always want 70knts to be the first
color value, and you know the specific value of your contours. Or in the
case of precip levels as shown above. In those cases, you generally have the 
first 
fill color as zero- but for a temp map, that may not be the case- unless you 
just want
to color the areas from -10 to 0F as potential icing regions for example. 

If you don't really know the minimum value ahead of time, then its pretty tough
as you know. If you assume any temp colder than say -30F at 850 is your base 
color,
(you don't need color=0 unless you want some area not to be colored), then
your first color is all temps colder than -30f. You can still draw contour lines
in that region even if its all one color using ctype=c/f, so you can have a -35
line contour within the area thats colder than -30F.

Steve CHiswell