Irrespective of whether a fence, stone wall, etc are deemed to be structures always be aware of the lone farmworker bent over in a field of beet pulling weeds.
I can assure you that the look of terror on their face as they straighten up and realise that the approaching noise is accompanied by a lump of metal intent on decapitating them, can only be matched by the look on the pilot's face when he sees what he's just about to hit.
In this (real) scenario, only the quick reactions of the farmworker throwing himself to the ground, saved his neck, literally.
The moral of this story is that 10 to 15 feet above the level a crop is better than 2 to 3 feet!