Are FlightAware ADSB altitudes accurate?

[Piper18O]

I had a trip with three legs on three different days. Each day the barometric pressure was lower than the day before. In checking FlightAware, Each day's flight showed a different altitude variation. The day with the highest Barometric pressure, the first day showed my altitude about 300 ft low. the next day with the Barometric pressure a little lower, the altitude on flight aware was still low but only about 100 ft. The third day the pressure was even lower, and actually pretty low at 29.6 and the altitude on flight aware was about 300 ft high. All three flights were with Kansas City center and I set my altimeter at what I was told. Is this unusual? The plane was certified IFR about a year ago when new avionics were installed. The altitudes were in the 7 to 8 thousand feet range, depending on direction of flight. How accurate is the adsb data from FlightAware?

 

[wiregoat]

The adsb altitudes are broadcast with the altitude from the altitude encoder, which is set at 29.92, so yes, as the barometer drops, the adsb altitude becomes closer to actual

Sent from my XT1575 using Tapatalk

 

[Piper18O]

The adsb altitudes are broadcast with the altitude from the altitude encoder, which is set at 29.92, so yes, as the barometer drops, the adsb altitude becomes closer to actual

Makes perfect sense now. A 29.6 setting would show an altitude about 300 ft higher on adsb FlightAware. I don't remember exactly the first two leg's flights, but I am pretty sure the altimeter was well over 30 on the first leg as high pressure dominated. if I was at 30.20 that would explain the 300 ft low reading on FlightAware.

 

[EdFred]

Yeah, I did some flying under the surface of lake michigan a while back according to ADS-B

 

[Piper18O]

Yeah, I did some flying under the surface of lake michigan a while back according to ADS-B

Reminds me of a time when I flew home in a very low pressure system once. Altimeter was in the 29.4 range. the next time I flew, it was extremely high, around 30.4. I was flying out of a grass strip and when I got in, it showed that I was 1000 ft in the air taxiing down the runway, until I reset the altimeter.