Actually I wonder if a self winding tape could be put on a hefty stake beyond the eventual end of the proposed ditch. Then fasten the free end of the tape to your helmet, everybody working with heavy equipment wears a helmet of course, and just reach up once in a while to see that the tape remains straight ahead of you. It winds up as you follow it along.
For even more accuracy, fasten a stick to the digger so it points at the tape where you can reach it to check. Tom Fowle
