Hi Karl
I believe the best way to achieve this would be by including image processing techniques in your project; I am suggesting so becuase we do not have a predefined path to follow in this case, we need the mower to adjust itself and come up with a new and efficient path, everytime, to do its task. Using any analog sensor won't give desired results.
Although I know nothing about image processing but yes there are some contributions on EG, related to image preocessing, which will help you a lot.
Good Luck 