This device has a dial on each side. One dial reads in centimeters and on the other reads in inches.
On just about every map there is a legend. There will be a scale in the legend, which will say, for example, 1 inch equals 10 miles. There is a little brass wheel at the tip of the Map Distance Measuring Tool which, as you trace it over the route you will be traveling, the dial arm will move and measure. You will then be able to figure out the distance of the trail.
This tool is really handy when you are tracing over a winding trail or road which you would not be able to easily guesstimate or measure with a ruler.
Comes complete with storage case and instructions.