In reply to Jenny C:
I agree to buying new waterproof maps, though sometimes if you have lost a map, without realising until in the area where you are going to walk, you may find that local shops do not sell them (they only sell the cheaper non-waterproof maps).
More to the point, is that I have non-waterproof maps which are 30-40 years old, which I still use, and those do need a map case, otherwise in constant drizzle/cloud/rain the paper details begin to rub off, and that may be a real problem out on the hill.
Water proof maps are harder to mark than the old paper maps, or maps printed off at home.