Modify colorbar for range of value?

Q&A
1

Hi,

It might be easier to explain on a picture :

test
test712×852 181 KB

So the topography and bathymetry are given by @earth_relief and with the coast command I overlay a semi-transparent blue colour over the sea.

Now I’d like to also add this same hue to the colorbar, only for the negative values (bathymetry) … Is there a way to achieve that ?

Thanks !

gmt begin test png 
    gmt makecpt -Cgray -T-2500/2500 -H -I > relief.cpt
    gmt grdimage -R30/110/-30/30 -JM5c @earth_relief_03m -I+d -Crelief.cpt
    gmt coast -Dh -Wdefault,black -Sdeepskyblue -t25 -Bafg -B+t"A map" --MAP_GRID_PEN=thinner,gray
    gmt colorbar -Crelief.cpt -D+e -Bxaf+l"topography" -By+l"[m]"
gmt end show
2

no known solution ?

3

Maybe create 2 CPT and combined them into one??

4

Probably something like this (add -H if in modern mode):

gmt makecpt -Cdeepskyblue -T-5000/0 -N > my.cpt
gmt makecpt -Cgray -G0.5/1 -T0/2500 -N >> my.cpt
5

It’s a good alternative solution, but it doesn’t really do the trick… With this way I’m loosing the bathymetry in the colorbar (as well as some texture in the ocean).

6

OK, See what you want. Perhaps try gmt math RGB2HSV on the graycpt for the negative z values, then change the H (hue) to that of the deepskyblue color and convert back with HSV2RGB and assemble the two pieces.

7

I’m not too familiar with this. I tried to apply

makecpt -Cgray -T-2500/2500 -N -H > relief.cpt
gmt math relief.cpt 0 LE RGB2HSV = relief_hue.cpt

But the second line already fails.
I think I need to apply math to -C1-3 only, but then I loose the depth column. Also LE output only 0 and 1 so not very useful here.
The workaround would be to use 2 cpt like you suggested earlier, but the real problem stays the same : I can’t use RGB2HSV as the cpt contains value such as “gray10”.

Thanks

8

I don’t have time to try this for you but you would probably need to use awk to just output r g b for gmt math to read since 255/45/45 is not going to work. See -Fr for numerical output instead of gray10 and white.

9

Thanks @pwessel. That could be interesting to learn how to use RGB2[…], but the application is definitely not worth the time. Would have been nice to have an easy workaround but I’ll just change the appearance.
:+1:

10

this script might be helpful although its different from what you want but oceans topography colors are in colorbar

gmt begin test png 
    gmt grdimage -R30/110/-30/30 -JM5i @earth_relief_03m -I+d -Cglobe
    gmt coast -Dh -Wdefault,black -Sdeepskyblue -t25 -Bafg -B+t"A map" --MAP_GRID_PEN=thinner,gray
    gmt colorbar -Dx6.2c/-1.19c+w10c/0.3c+jBC+h -Bxaf+l"topography" -By+l"[m]"
gmt end show
11

Hum, nice indeed. I’ve never realized colorbar could handle two information this way.