An opisometer or map measurer is an instrument for measuring the lengths of arbitrary curved lines.

An opisometer

A simple opisometer consists of a toothed wheel of known diameter 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 passing a mark on the handle while this is done, the length of the line can be ascertained:

line length = wheel diameter × teeth counted/teeth on wheel.

In more sophisticated models, the wheel is connected via gearing to a rotary dial from which the line length can be directly read.

The instrument is most commonly used to measure the lengths of roads, rivers and other line features on maps. Opisometers designed for this purpose provide scales reading the measured distance in kilometers and miles.