See forum for background.

As Paul said: "You would like to derive the intensities from one grid (say topo) and look up the colors via the CPT and another grid, and the two grids may not have the same grid spacing? It would seem this would require a modification to the grdimage -I option to allow you to pass a grid there that is not an intensity grid but a surface from which you wish to derive intensities (instead of from the main grid)."