For a project I need to calculate the orthometric altitudes of flying aircraft.
Flight altitudes above the Transition Altitude and Transition Level as reported by ADS-B (in Plane Plotter logs) are barometric altitudes.
But... are these pressure heights expressed in geopotential or geometric feet?
And in what units are they expressed below the Transition Altitude and Transition Level, or are these already reported as orthometric below TA/TRL?
Is there some ICAO or other reference document where this is clearly stated (e.g. it's not in ICAO Doc 7488/3) ?