Ignore them! LOL Make an ortho camera looking down from above. Apply a gradient texture into a diffuse slot (black to white). Apply said material to your land mass. UV the whole thing from the side, using Fit, to autosize the texture. Render camera.
I wouldn't bother with a heightmap as a base for specular, that doesn't really make any sense... it's not like "higher" things are always more shiny than "low" things... for example imagine you have a metal plane covered with some thick cloth, the metal is "lower" in the heightmap but more shiny than the cloth... like I…