Most if not all low fly-bys I have ever seen, Joe Nall, Florida Jets, Bay of Quinte, etc, etc, there has always been something in the background as a positive reference, and you could clearly see the aircraft go by without question to altitude.

The amazing crash by Shulman was pretty darn low, and you could tell by the background.

Simple, when I first saw this one, I couldn't see any fixed reference.
An eye witness is the only real reference.

Quite a rare, and daring shot, very good indeed. I certainly wouldn't want to be out there.