An curvimeter, also called a opisometer, meilograph, or map measurer, is an instrument for measuring the lengths of arbitrary curved lines.
To use the device, the pointer is set to zero by turning the small wheel at the base of the measurer. The tracking wheel is placed onto the map and is moved over the map following the line of your course (you may need to press firmly to avoid the wheel from slipping). As the wheel is moved over the map, the pointer rotates to record the distance. When completed, the pointer will indicate the total distance covered for the particular scale of your map.