A device for measuring the length in cm of a path. The input wheel is marked in cm, the output wheel in tens of cm. Simply zero the device and then roll it along the path.

A 1:50000 map is two cm per km.

For axles I used some 1.75mm filament. The holes are undersized and will need to be opened to final size with a drill.

Also, this is version 2 of the case with slightly increased clearance on gear one, which had a tendency to jam. I apologize to the one person who downloaded version one before I caught that error. I hope that they have not printed it yet.