overlay geotiffs with transparency using different colormaps

If I have one geotiff plotted (using geoshow) in color how do I overlay another geotiff in black and white and make it partially tranparent?

Answer 1

[Chad Greene]

I do this often when I overlay semitransparent ice speed over satellite images of glaciers. That image was created with these steps:

1. Plot the satellite image in grayscale.
2. Use freezeColors to allow a second color map.
3. Plot ice speed using pcolorm(lat,lon,speed,'facealpha',.3).
4. Set the second color map to jet(256).

The same can be done with geoshow instead of pcolorm as Bruno pointed out. However, if you're using Matlab pre-2014b, there's a bug that might make Matlab crash when pairing geoshow texture maps with marker data. pcolorm is not ideal because it discards a row and a column of data, and plots data offset by half a pixel, but if your data is dense enough it should not be a problem, and it's better than risking a total crash.

[Rob Comer]

You can avoid 'texturemap' by doing something like this in place of the first geoshow call in the example cited by Bruno:

   Z = zeros(size(geoid));
   geoshow(Z,geoidrefvec,'DisplayType','surface','CData',geoid);

[Ruth]

This works but I am not able to save the figure showing both layers. Only the pcolorm layer is saved. Any ideas?

[Chad Greene]

This is likely an issue with the renderer. Only the opengl renderer supports transparency, so try typing

set(gcf,'renderer','opengl')

Then save. If that doesn't work, use export_fig with the -opengl tag.

Answer 2

[Bruno Pop-Stefanov]

Calling geoshow a second time will overlay over what was previously displated. Use the 'FaceAlpha' property when calling geoshow for the second time to set the transparency.

For example, using the Display EGM96 Geoid Heights Masking Out Land Areas example in the doc page for geoshow:

    % Load geoid and display
    load geoid

    % Create a figure with an Eckert projection
    figure
    axesm eckert4;
    framem; gridm;
    axis off

    % Display the geoid as a texture map
    geoshow(geoid, geoidrefvec, 'DisplayType', 'texturemap');

    % Create a colorbar and title
    hcb = colorbar('southoutside');
    set(get(hcb,'Xlabel'),'String','EGM96 Geoid Heights in Meters.')

    % Add the land in black with 30% transparency
    geoshow('landareas.shp', 'FaceAlpha', 0.3, 'FaceColor','black');