opisometer
noun
op·i·som·e·ter
ˌäpəˈsämətə(r)
: an instrument used for measuring curved lines (as on a map) and consisting essentially of a screw with a wheel-shaped nut that is made to rotate forward along the curved lines and then backward to its original position on the screw along a straight scale
Word History
Etymology
from Greek opisō backwards + English -meter; akin to Greek opisthen