I'd also question the data accuracy of the picture above too, particularly how the altitude displayed is matched to QNH and a whole host of data correlation and verification, not least of which is geographical map, to radar overlay of that specific map and recorded track correlation. Getting those correct is a long laborious process, ask anyone who's done it , and involves a bit more than buying a map from a supplier and sticking it underneath a radar track ...
Yes, it is very difficult to estimate heights, but ADS-B readouts are from the transponder, I believe, which uses 1013 as the datum, so where is the difficulty with that, on a standard day?
McD