"i didnt know computer mouses had balls???"
That is how you tell the difference, male or female... You really are a NEWBIE

The ball and optos could mount on a spring loaded platform in the front or back.

This is an interesting problem, in my work we use doppler radar along with gps for applicators that move across the field at 20 mph. Even then things could end up depending on plowed or disked - satellite coverage - etc- 50 cm off in a couple of km.

Run a "female" mouse over paper with lines and you will see the problems with that. I bet the RF idea with beacons would be the most accurate.

If I think of something very small and cheap will let you know.