It has always been within 60 ft, usually within 30 ft.
The altimeter should also be within 60 ft! If it is not, then there is something wrong.
That was my point. GPS seems to always comply with that requirement to the altimeter.
A barometric altimeter will be quite accurate at the airport elevation with the correct local altimeter setting set into the altimeter. To pass the IFR check, the tolerance is +/- 20 feet for altitudes at and below 1000 feet. The tolerance grows to +/- 40 feet at 6000 feet, +/- 120 feet at 18,000 MSL.
Baro altitude will be in close agreement with a typical GPS altitude that is corrected for the local geoid. Interestingly, the GNSS altitude emitted by ADS-B Out is referenced to WGS84 and is not corrected for geoid differences.
As one climbs in altitude, the GPS and Baro altitude diverge with temperature. It is not uncommon to see differences that are +/- 500 feet during colder and warmer than ISA conditions. GPS altitude is better suited for terrain warning systems as the error is not temperature dependent. Mixing the two systems of altitude measurement would be dangerous for vertical separation between aircraft, so any change would have to affect all users of the airspace. RVSM airspace requires even tighter altimetry and it is not clear that GPS altitude could meet the standards. GPS altitude has greater error than lateral position on the order of an extra 50% due to geometry of the satellites because the best resolution would be achieved if satellites below the horizon could be used. Most of the earth is not covered by SBAS correction and GPS position is not as accurate as SBAS position inside the various SBAS service volumes.
A barometric altimeter will be quite accurate at the airport elevation with the correct local altimeter setting set into the altimeter. To pass the IFR check, the tolerance is +/- 20 feet for altitudes at and below 1000 feet. The tolerance grows to +/- 40 feet at 6000 feet, +/- 120 feet at 18,000 MSL.
That sounds like US regs. In Europe, at least, we’re supposed to check the altimeter for 60 feet accuracy on the ground. (For altimeters used up to FL300) Given that QNH roundoff may give close to 30 feet error, it is sufficient for the altimeter itself to be accurate within 30 feet.
