The Curvimeter – or opisometer – is a device that measures the length of curved lines and helps to calculate the distance between the points on a map more precisely than a ruler or a compass. The mechanism has a measuring wheel, a clock face, and a hand. When the wheel moves along the line on a map that you need to measure, the hand moves around the clock face marked with measuring units – centimeters, inches, etc. All you need to do in the end is to check the map scale and to multiply your data by the denominator in the scale.