Under normal circumstances I wouldn't want to use my map as a tarp. It just increases the chances of a piece of critical gear being lost or destroyed. However, the army does make survival maps that are printed with local edible plants and a few other pieces of information. They're printed on tyvek and are meant to be used as a tarp in a pinch. I don't know of any source for them, since I've only seen one as a wall decoration.