I live out in a rural area. Yesterday, a helicopter passed by and it sure appeared that he was lower than 400'. When I have sent my Phantom up to 400', it is way up there and this helicopter looked to be below that level. I can't swear to it. He was not landing or taking off in that area, and I don't know why he was that low. This could create a dangerous situation and I would never want to get in their way. Is there a minimum for horizontal flight for aircraft? As there are many trees in my area, I can't see an aircraft coming from very far away. Any suggestions?