<Dd> line length = wheel circumference × teeth counted / teeth on wheel . </Dd> <P> In more sophisticated models, sometimes called a chartometer, the wheel is connected via gearing to a rotary dial from which the line length can be directly read . </P> <P> 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 . </P> <P> Early versions of this instrument were patented in 1873 by the English engineer Edward Russell Morris . The instruments he produced were initially described as a Patent Chartometer although later versions were sold under the curious name of Wealemefna . Writing in 1881, Morris described how he had created a wholly original name in an attempt to outwit his imitators; he also refused to disclose the origin of the word . </P>

Tool used to measure distance on a map