During a recent discussion with one of our suppliers, their programme manager suggested that 500 feet is classed as low level. He was a little surprised at the response. 500 feet - in the UK - is the lowest that a civil aircraft can fly (technically, it is, "within 500 feet of any person, structure or building"). Again in the UK, the lowest that the military can fly (in dedicated areas, such as LLTRs) is 250 feet. During wartime, however, the lowest that an aircraft can fly is generally dictated by the lowest point at the aft end of the aircraft. This is generally between 3 and 6 feet, since that is the height difference between the location of the Radar Altimeter (RADALT) and the lowest point of the engine exhaust.
The art of low flying is simply this: do not hit anything solid.
You know that you're flying low when you have to ask, "On what side of the road do we drive here?" Just see the videos of the Foreign Legion helicopters in Africa to understand what this means.
The video is a nice example of gaining speed (kinetic energy) before gaining altitude (potential energy) and is great to watch; it is not, however, low flying. When you see a jet passing over a saddle in the mountains at 450kts, 20 feet above the ground and inverted... THAT'S low flying.
The definition of a 'Pessimist': an Optimist with experience...