An opisometer, also called a curvimeter, meilograph, or map measurer, is an instrument for measuring the lengths of arbitrary curved lines. A simple opisometer consists of a toothed wheel of known circumference on a handle. The wheel is placed in contact with the curved line to be measured and run along its length. By counting the number of teeth ...
instrument for measuring curved lines
• (n.) An instrument with a revolving wheel for measuring a curved line, as on a map.
Op`i·som'e·ter noun [ Greek ... backwards + -meter .] An instrument with a revolving wheel for measuring a curved line, as on a map. 
Mechanical device for measuring non-linear distances on maps.
An opisometer is an instrument with a revolving wheel used for measuring a curved line, such as on a map.