An opisometer, also called a curvimeter, meilograph or map measurer, is an instrument for measuring

Measurement

Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

 the length

Length

In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...

s of arbitrary curved lines.
A simple opisometer consists of a toothed wheel of known circumference

Circumference

The circumference is the distance around a closed curve. Circumference is a special perimeter.-Circumference of a circle:The circumference of a circle is the length around it....

 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 circumference × 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 map

Map

A map is a visual representation of an area—a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes....

s. Opisometers designed for this purpose provide scales reading the measured distance in kilometers and miles.