Several years ago there was a series of articles in Radio-Electronics (I
think?!?!?) which was a do-it-yourself autonomous lawn mower.  It used a
series of split IR emitter/detector pairs to sense the "length" of the grass
blades.  The steering mechanism attempted to keep half of the IR pairs in the
uncut grass.

Using this method, you mow around all of the obsticles in the yard, then go to
auto mode and let the mower do it's thing.  I'm not sure what it did when it
got done...


Regards,
Jack Warren