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For small aircraft flying below 2,500 ft. AGL, is it appropriate to compare the landed ADS-B altitude (pressure calibrated AMSL) to the elevation of the runway in order to compute a reference offset for flight altitudes?

For example, if an aircraft's ADS-B reports a landed altitude of 700 ft. and the known runway elevation is 500 ft. above sea level, is it appropriate to apply that -200 ft. offset to reported flight altitudes? Thus a reported ADS-B altitude of 1,500 ft. would be approximately 1,300 ft. for that aircraft/flight?

This question is regarding aircraft flying in the general vicinity of the airport and with relatively short flight durations (less than 20 minutes).

Edit: For clarity, this question is related to data from flight tracking websites such as FlightAware.

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For an after-the-fact analysis, for example looking at flight data recorded by an EFB or displayed on a flight-tracking website, what you describe sounds very reasonable.

For use during a flight this is absolutely unacceptable; even for a flight conducted under Day VFR conditions, an aircraft must be equipped with a pressure-sensitive altimeter. In fact this pressure altimeter is what provides the transponder (ADS-B) with its altitude information in the first place (unless the parameter you are looking at is explicitly labeled "GPS altitude").
In an emergency situation where your altimeter becomes inoperative but the pitot-static system as a whole is still reliable, you might be able to justify the use of ADS-B altitude information to provide situational awareness until you land, which should be as soon as practicable—but the mental math required to correct the ADS-B altitude means it should be considered an emergency procedure only.

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  • $\begingroup$ My guess is that OP wants to look at data from FlightAware (or similar) and try to apply a correction to the observed altitudes. $\endgroup$
    – Jim
    Jun 2, 2022 at 2:30
  • $\begingroup$ @Jim is correct that my question is related to sites like FlightAware (I've edited for clarity) but the additional information is helpful nonetheless. Thank you. So does this mean that the aircraft's altimeter should be calibrated for local pressure which would then make ADS-B out more accurate? $\endgroup$
    – bhamjman
    Jun 2, 2022 at 2:57
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    $\begingroup$ @bhamjman: See the question (and associated resources) linked by QF for more information. The upshot is: the altimeter in the aircraft is indeed calibrated as you say, but the altitude encoder which transmits information to ATC is never calibrated. Because the ATC radar scope knows the local altimeter setting, it automatically applies the correction on the ground. This ensures that ATC sees accurate information even if the pilot mis-calibrates their altimeter. It has the side effect that systems which do not calibrate the ADS-B information, like flight tracking sites, are inaccurate. $\endgroup$
    – randomhead
    Jun 2, 2022 at 3:07
  • $\begingroup$ Thanks @randomhead. So if I'm understanding correctly, the pressure altimeter can be calibrated for local pressure, and then the altitude value is fed to the ADS-B transponder for broadcast. And I'm guessing this explains why multiple planes can all broadcast different altitudes (100-200 ft. variances) when landed? QF's link has also been very helpful. $\endgroup$
    – bhamjman
    Jun 2, 2022 at 3:33
  • $\begingroup$ Not quite @bhamjman. The altimeter is a device which shows the pilot what altitude they are at, and the pilot can adjust this device to account for the local pressure. But the altitude sent to ATC, via the transponder, does not account for local pressure. It assumes that the local pressure is equal to the standard pressure and sends that information. The ATC systems on the ground then take that number and adjust for the actual pressure, just as the pilot adjusts the altimeter in the aircraft. It is possible that ATC and the pilot could use different adjustment factors. $\endgroup$
    – randomhead
    Jun 2, 2022 at 3:42

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