Originally Posted by DaveReidUK
ie height AMSL
…so altitude, then.
Originally Posted by Big Eric
…which is probably ADS-B data including Flight Level based on SPS, so would not necessarily equate to altitude, height above Heathrow, or height above Kew. If Webtrak is corrected to the relevant QNH then one would have to subtract Kew elevation to obtain height. Whatever, quite noisy...