How to create smoothly shaded colors in geoshow?

I'm making a global map of a 512x1024 grid using the geoshow function. The map looks good, but it's pixelated if you zoom into areas of, say, 1000 km by 1000 km. I would like to smooth the colors in the image out, the way that "shading interp" does with a figure created by "surf".

However, trying to apply shading interp to the map after it's created makes the map go blank, and it yields an error message: "Warning: size(CData) must equal size(ZData) for interpolated shading ".

To be specific, this is what I'm trying to do to plot the grid Z:

Rs = makerefmat(0.5*360/1024, 90-0.5*180/512, 360/1024, -180/512);
geoshow(Z,Rs,'DisplayType','texturemap');
shading interp

Everything works except for the shading interp command.

Does anyone know how to interpolate colors on a map like this, after its been created? I'd rather not interpolate the data itself and then plot it, since it takes a while for matlab to plot up the map if you interpolate to much higher grid spacings (e.g., to a 4096 x 8192 grid).