These are some great suggestions, thanks to everyone.
I'm trying to measure the distance i'm running a sensor over. like the digital tape measure here:http://www.amazon.com/STRAIT-LINE-6041400CD-Rolling-Tape-300/dp/B0002VM7OO
(Except with higher resolution.)
I like the LVDT idea, and will look at how it could be adapted to rolling over a surface. But since I need to be in contact with the surface I'm traveling over, it seemed like something wheel based would be better.
Another thought I had was using something optical, but I can't be sure of the reflective properties of the surface.
I should have included this on my first post, my apologies. Any thoughts??