At my rowing club here in Hamburg, we enter every trip we make into the computerised logbook of the club. At any time, you can extract the total distance travelled by any member, and, naturally, it is fun to beat your friends by one or two kilometers. (That should really be nautical miles, but rowers are hopeless landlubbers. Those here in Hamburg anyway.)
After a number of (not altogether unpleasant) discussions over the actual distance covered, I manufactured this gadget. I am perfectly aware of the existence of GPS and other technical marvels that would have solved the problem more accurately, but accuracy (alone) was not the only design goal considered here.
The solution that I finally chose does work nicely when the number of possible routes are restricted, as is the case with the canals of Hamburg (within 1 hour rowing distance from our club).
I hope you will forgive me, but I did not take any pictures during the actual manufacture. Don't worry. Everything is quite obvious to build, once you have seen the close-ups of the finished thing.
Step 1: Functional principle: colour-coded chord is wrapped around nails
In my case, the route is restricted to the waterways of Hamburg. The nails are placed at strategic points: where the canal turns, at good landmarks such as bridges, at other watersport clubs.