You bring up some very good points End User. These designs are incredibly complex, extend accross huge areas of the land and contain little to no mistakes.
I find it hard to believe that a group of people, in the middle of the night, would be able to construct such elaborate figures without error. Even if it were possible, why don't these incredible artists lay claim to thier work? I know that if I were able to make such a thing, I would be showing it off to as many people as possible.
Along with the biological anomolies, many of these figures contain geometrically significant features.
This one was discovered June 7th 2003
http://www.lovely.clara.net/hawkinsb.html