ADSB-X Showing VMBA10 FAT ALBERT doing repeated runs at Annapolis...some as low as 300 feet AGL. At one point above the intersection of Ritchie highway and route 450 the track shows they were at 175 feet. If you didn't know better, you'd think they were on strafing runs!
In most cases the displayed ADSBExchange altitudes are pressure altitudes uncorrected for local altimeter setting. Annapolis altimeter setting is about 30.33 inHg right now so pressure altitudes will read about 400 feet lower than MSL altitude.
Or, was it the other way around, I never could remember...