If you have the CD version, open the Extended Map Editor. Use the "Strip" button at the bottom, thus revealing the Venhola contours on the real map, not just the minimap. Very simple.
Otherwise, use the top button (circled), which resides on page 2 of the button-interface. Set a huge fill area by using the "Fill Range" button (just above the blue "play" button). And then you can right click on the map to start manually filling the whole map with that clear terrain.