: 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.

How did an Opisometer get its name?

Mid 19th century. From ancient Greek ὀπίσω backwards (from ὀπι- + a suffix forming adverbs indicating direction) + -meter.

What is the unit of Opisometer?

Opisometer – Measure to nearest mm, when used with ruler/metre stick. They measure the length of curved lines.