It's An Opisometer

Day 30: People will think you're being vulgar if you call this device by its proper name. Yep, it's an "uh-PIS-uh-meet-er." I call it my "map-walker," a term which may raise questions but not necessarily eyebrows. By rolling it along a trail on a topographic map and using different conversions depending on the map's scale, it's actually fairly accurate for measuring distance. I've compared it against actual mileages on a 7.5' map often enough to trust it to give me an error factor of no more than a tenth of a mile over a five-mile run. Of course a lot depends on how many switchback corners there are and how carefully the operator can trace them, but it's a good sight better than "as the crow flies," a functional tool for telling this Crow how far she'll have to leg it.