Like many useful gadgets, Aspen Industries’ Topotrek ruler was born out of reallife necessity. It happened during a bear hunt when a couple of hunters had only a map and a pen knife to help them determine the distance between them and a good shot at a bear. Back home (with the bear) the hunters used this experience to create a ruler that instandly converts horizontal distances on USGS topographic maps(or any map with a 1:24,000 scale) into yards and fractions of a mile. You can also use Topotrek as a protractor to draw arcs or circles the desired distance around a specific point on a map. About $6.
Aspen Industries’ Topotrek Ruler
Aspen Industries' Topotrek Ruler