Step 3: The Ground Layer and the Height field
In this step you will create three distinct ground layers and apply varying layers of bump to them in gimp.
Firstly create a new layer above the greyscale map as shown in the first screenshot Completely fill it with a light green colour. You may have to alter the mode of the image to RGB to get colour (Image>>Mode>>RGB). Make sure that "Fill whole selection is selected". I have used a light green texture that is not too uniform, as can be seen from the second screenshot.
Then select bump map (Filters>>Map>>Bump Map) and use the greyscale map layer as the bump map as shown in the third screenshot. In the bump map window adjust only the elevation to around 38. Then hit OK and you should get a result similar to the fourth screen shot with the rive paths clearly evident.