I'm looking into an autonomous mobile unit for part of a farm/field data collection thing.
What would be the best way of detecting that it is heading towards a ditch/drain?

Thoughts:-
1) A bicycle wheel on a boom out front - if the angle or the boom changes then there is an incline/decline ahead which could be a ditch
2) Ultrasound distance measuring unit mounted at an angle - if it stops detecting then the ground ahead is falling away by more than that angle.

any other ideas?