A couple of questions about Barometric vr Geometric altitude measurement

[hphillip]

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.
https://flightaware.com/live/flight/N784F/history/20200524/1408ZZ/EGTW/EGTW

Any pointers much appreciated - still learning about such things and seem to have time on my hands of late

[wiedehopf]

Transponders don’t transmit according to what the pilot has set on his altimeter.
They always transmit a value as if the pilot had set his altimeter to standard.

It’s up to the receiving party to make a correction for local pressure (which ATC does for aircraft below the flight levels).

Actually thought about implementing an input field in tar1090 to display altitudes corrected for the local altimeter, but never did.

[hphillip]

Thanks @wiedehopf that makes a lot of sense. I guess I should have done a bit more reading before I posted the questions.

I have subsequently come across Altimeter Setting Procedures | SKYbrary Aviation Safety which is now makes me realise how much more complex the procedures are, but for good reasons.
Much appreciated.