I've been working on a prototype that easily and cheaply locates a marker on a paper map.  The following video demonstrates a method of measuring distance with a pulley (basically a cheap string pot).  It makes use of a retracting key chain, a multi-turn pot that's connected to the .NET gadgeteer and laser cut wheels.

I'm surprised how accurate it is even with a wonky wheel.  The end game is to have two of these on either corner of a map allowing the triangulation of a markers exact position on a map. More to follow. In the video each ruler is 30cm long.

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