I recently watched a helicopter do a short 20 minute circuit around my site (361 ft elevation according to heywhatsthat.com) and land at the nearby Oaksey Park Airfield (EGTW) about 8.5 miles distant (299 ft)
The thing I am curious about was from my SkyAware GUI I watched the reported altitude descend to minus 200 feet Barometric (no reading shown for Geometric) when it touched down.
This got me thinking about the reliability of the altitudes reported in the aircraft.json or at least how to interpret the values.
When both “alt_baro” and “alt_geom” are present in the aircraft.json is the latter definitively accurate and the former just a backup?
Also, do GA pilots do any form of regular calibration of the barometric sensors prior to a flight - and would this account for the negative value reported?
Or can I just assume I’ve had a decoding error in the dump1090 that is manifesting itself as a rogue height?
The tracklog naturally indicates that all the heights were a (not unexpected!) positive number of metres above ground level.
Any pointers much appreciated - still learning about such things and seem to have time on my hands of late