An opisometer, also called a curvimeter, meilograph, or map measurer, is an instrument for measuring the lengths of arbitrary curved lines.

What does the word Opisometer mean?

: 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 do you use a Curvimeter?

To use the device, you roll the wheel across the item that needs measuring. As the wheel moves, the measuring hand moves as well, tracking how far the wheel has traveled. If your distance is greater than one rotation, the device also tracks how many circuits it has completed.