Cheetah; Tiger; Traveler AA5 Series
Flight Crew Instrument
ATC Issue All Types; Aircraft Equipment Problem Less Severe
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Aircraft
ATC reported me as traffic to another aircraft; telling him I was at 9000 ft; when I was at 8500 ft. I had the correct altimeter setting; and cross checked against the Mode C report display on my transponder; and it was reporting 8500 in corrected PA. (Altimeter setting was 29.91 so corrected and uncorrected should be close). Communicating to ATC; he was unconvinced there was something unusual happening; but I further cross checked against the pressure altimeter in my iPad mini4; and it agreed that I was at about 8500 ft. The GPS altitude was about 9000 ft; though; and I have ADS-B out. After frequency change to LA center; a very similar discrepancy seemed to occur between a different aircraft and ATC. Is it possible that ATC displays are showing altitudes that are sometimes derived from ADS-B and sometimes from Mode-C?  That seems really dangerous under conditions where GPS altitude differs from corrected pressure altitude.
AA5 pilot at 8500 feet reported being informed by ZAB that his Mode C showed 9000 feet. The reporter believed that the problem was with ZAB.