What can be measured with opisometer?

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. The instrument is most commonly used to measure the lengths of roads, rivers and other line features on maps.