So, I've been considering ways in which a practical map can be constructed as a navigation aid in places like the Underdark, or really any other area where a normal 2D map is essentially useless.
The idea I've come up with is to use disks, or squares, or hexagons, or whatever other shape tickles your fancy, and cut out sections corresponding to caverns, or tunnels. Each "disk" or "page" would represent a set and static distance, perhaps a kilometer across and 100 feet vertically. So, as you ascend or descend you would switch to the next sheet. It would be a more or less entirely accurate scaled recreation of whatever zone it's mapping, so cut out spaces are tunnels, caverns, underground rivers and whathaveyou, and then the metal would correspond to stone and dirt, or even the materials can be mixed and matched, or otherwise marked to represent different materials.
Though, yes you would need a different set of map disks, or sheets, or whatever the crap you decide to call them, for use in sufficiently detailed areas.
I'm envisioning each metal sheet to be about the thickness of a coin, though the length and width could vary freely.
As the second necessary piece to figure out where you is a compass, though in a fantasy setting one that has multiple needles that each point to a different landmark or "magnetic" lodestone or whatnot. Possibly even two of them connected and shaped like a right angle with each side having its own set of compasses to allow for direction to be measured in both horizontal and vertical terms.
So, what say you folks? Is this a an efficient and practical (if almost guaranteed to be a pain in the ass to initially map out and construct) navigation aid? It's the simplest most complex device I can think of that requires a minimum (or possibly even none at all) of magic in order to operate. Though, at this moment it requires sight to use, which is a hurdle I've yet to think around.