creases... I don't think I'll ever get them out now...
Get a board bigger than the map, soak the map in water, with that old fashion parcel tape, the brown paper gummed kind, wet and tape down the edges. The water will expand the paper, when it dries it will shrink and become tight like a drum, this could work, I've done this for water colour painting so that the paper doesn't wrinkle. You will lose some of the edge when you cut it off the board.