Originally Posted by
treadigraph
But if this helicopter was at 400' AGL, should he not be at, say 1500' ASL to meet the regs? Most of the other helicopters and light aircraft I see are visibly at a sensible height (I've seen a couple of other aircraft which appear to have been way below 1000' and the area definitely conforms to "built up" though close to the edges) - and this guy was low.
Not having been there, I can't comment on what height the guy was flying at. I'm simply trying to explain why, most of the time, height reported by ADS-B will be higher or lower than the true height.
Edit: Had it been a recent event, we could have looked up the local QNH and applied that to get a reasonably accurate height AMSL, but I don't know of any online METAR source that goes back more than 5 days.