If you've work in surveying, go into the back equipment room. They'll be there in the "cool, but never used" bin buried under a bunch of stuff. I'd take pacing as a more accurate measure than these. A "dialed in" pace can be pretty accurate. At least, given my experience with the optical rangefinder in my car trunk.
A bit more reading on them: optical rangefinder - Everything2.com Landscape Surveying - Harry L. Field - Google Books