Add me to the "ziplock bag" set of responses.

Mostly just a standard gallon ziplock. I don't know what's "normal" anymore in the hiking world, but I don't buy maps, it's easy to print them.

8-1/2" x 11" paper is just a little too long to fit in a gallon ziplock, but I pretty much always find I can trim enough on the long margins to fit it in. I backpack often enough and print enough volume of maps that I found it worth while to buy one of those paper cutters, you know, a square flat board with a hinge at one corner holding a blade with the handle on the other, you slice down guillotine style to get a consistent straight and easy cut of multiple sheets at once.

Makes it easy to fit maps and other trail data. Sometimes there's other stuff, "data book" info, "town guide" etc that fits better (again sometimes with a little help) into a quart sized ziplock.

I have also at times literally cut the spine off of trail guide books to be able to, on any given day along the trail, have just the most current few pages of that ready to hand. I'm fortunate in that I have a band saw at home to make that easy. Book pages are unfortunately typically too big for a quart sized ziplock and kind of small for a gallon sized; careful trimming sometimes makes the quart size work, other times I make do with a single fold. Quart size is particularly handy to hold and use if you can make it work.

The thing that wears out the ziplocks is repeatedly folding and unfolding them over time. I keep my ready-to-hand maps/data in one or both cargo pockets (on the thighs of) my hiking pants, and at least the gallon size and more typically the quart size too get folded this way. On a long trip (weeks or months) that folding compromises the ziplock, and if I don't replace the ziplock then at some point I'll be hiking in heavy rain or wading through tall wet grass or the like and find my maps wetting out somewhat inside.

So I periodically swap 'em for other ziplocks, either sent by mail or purchased along the way or with something I'm carrying inside the pack that doesn't need to be so water proof.

This might not be an issue for shorter trips, so --- maybe non-essential info there ... (?)

Note that if you have a fairly large map it's possible to buy 2-gallon sized ziplocks; check a big box hardware store or look online. I find them to be a little unwieldly mostly, but I've used them on occasion.

On a trip in Europe once on a lark I bought a formal, somewhat expensive German branded map case. I still have it, but just never use it and in fact rarely used it on that trip. It's very "nice", and that's part of the problem --- I don't want to try to fold it, and it's heavy. If I recall correctly, they envisioned it either being rolled up (and thus I guess stored in my pack somewhere, not too available) or hung around my neck.

For me, there are two sets of "paper" that I'm carrying (not counting t.p. ...). A small amount of very-ready-to-hand maps and trail data for the proximate day or two. And then, stored also in a ziplock deep in my pack, a combination of "maps & data for future days" as well as "other maps or reference info that I don't carry ready to hand" (but can reference in my tent or at need along the way).

Sorry for perhaps an overly long response to what's really very simple --- ziplocks are a great choice, readily available, easy to replace, so long as it's not too difficult to size your source material appropriately.
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Brian Lewis
http://postholer.com/brianle