The Opisometer Measures Map Lengths
An opisometer is a device that measures lines, particularly curved lines on a map. It contains a screw with a wheel-shaped nut that rotates along the curve and then back to its original position. So, anyone curious about the distance from one end of the road or location to another can roll this device along a map. Think of it as a precursor to today’s GPS. But it won’t tell you which way to turn or when a road is closed.