A buddy of mine strapped a Casio watch with an altimeter on it once. It would hold the highest altitude reading. He set the watch so ground level was 0 feet. He had a spotter with binoculars to watch the plane for him and would tell him left or right, etc. He took off and started climbing in circles. The model, which was a trainer finally ran out of fuel and started it's decent. When he landed the altimeter read 6,000 + feet. The field is at 2400 ft so he was better than 8,000 ft. above sea level.