Originally Posted by
silverstrata
As far as I know, FR24 is simply picking up ADS-B 1013 pressure altitudes, so you will need to know the QNH of the day, and the altitude of the airport, to calculate the true height of the aircraft. Transponders were designed for seperation on airways, not for separation with terrain, so the older units all used 1013 baro, and ADS-B follows suit.
No, as stated up-thread ADS-B emits BOTH barometric and GNSS ellipsoid altitude when both have onboard sources. Which for the 737-8 is true.
FR24 don't export one or the other in their downloadable CSV data, I haven't determined which, but both are visible in the web UI during interactive usage.