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matlabcolorbarcontourf

Colormap three colors

I have a netcdf file with positive and negative currents (upwelling and downwelling respectively). I want to create a contourf where downwelling is green, upwelling is red and 0 is black. So far this is my code including some code from the Mathworks website https://nl.mathworks.com/matlabcentral/answers/81352-colormap-with-both-positive-and-negative-values : 

%open netcdf file
ncdisp('20110810_061103.nc');
ncdisp('grid.nc');

latu=ncread('grid.nc','latitude_u');
lonu=ncread('grid.nc','longitude_u');
latv=ncread('grid.nc','latitude_v');
lonv=ncread('grid.nc','longitude_v');
u = ncread('20110810_061103.nc','vel_u'); %x axes velocity of water
v  = ncread('20110810_061103.nc','vel_v');%y axes
w  = ncread('20110810_061103.nc','w');%z axes 

Minu=min(min(min(u)))
Minv=min(min(min(v)))
Minw=min(min(min(w)))

Maxu=max(max(max(u)))
Maxv=max(max(max(v)))
Maxw=max(max(max(w)))

figure

contourf(lonu(1:681,1:711),latu(1:681,1:711),w(1:681,1:711,20))
%code I copied from mathworks    
greenColorMap = [zeros(1, 132), linspace(0, 1, 124)];
redColorMap = [linspace(1, 0, 124), zeros(1, 132)];
colorMap = [redColorMap; greenColorMap; zeros(1, 256)]';

% Apply the colormap.
colormap(colorMap);
colorbar

enter image description here

As you can see, the black is not at 0. How do I make sure that black is zero, downwelling (-) is red and upwelling(+) is green? I'm sorry if this is a duplicate. I checked question MATLAB: generating a colormap given three colors but I don't understand how you set the colors and it creates something similar to the above picture. The following has nothing to do with my question although it has the same title Matplotlib: Custom colormap with three colors . Thank you in advance for any answers.

Solution

  • The issue is not with the colormap being wrong, but with the way the data values are mapped to the colormap. This is achieved by changing the CLim property of the underlying axes object with something symmetric with respect to zero, e.g. set(gca,'CLim',[-1e-2 1e-2])

    This can also be changed interactively through the UI: Edit -> Colormap... in the figure menu. Then edit color data min/max.