A device for measuring accurate distance on a map - a bit like a mini version of those trundle wheels that surveyors use.
I used to have an analogue one when I used to do Orienteering (when I was young and fit) a Sliva one - you can't beat Silva I think for that kind of thing (and compasses as well).
Have a look at this one - a newer version of the one I used to use which never gave me a moments problem. I do like the analogue ones best - to be honest unless you want accuracy to the cm and to be honest if you are walking or driving that kind of accuracy is irrelevant, then an analogue one is easier to read and use.
I do need to be reasonably accurate as the trip meter will measure to the meter. with a good wheel I should be able to find those dam tracks that we seem to keep on having trouble in finding on the plain