You wished to convert a CBM into an overlay and I suggested you use a splat map. Here is a sample based upon you posted image.
All water is sand, the shorelines are snow, and the regular ground is rough grass. I included the splat legend in case you decide to add topo features.
To use it, start in the middle and draw any elevation above water, and when you cross your snowy shoreline as you've drawn it, obviously you stop at the sand.
In the TE, import splat, Ciaphas.bmp. It's a 32 bit bitmap.
