Originally Posted by
wtsmg
If GPS ALT ...... is in the ballpark with ... altitudes on your PFDs and ISIS
Extending the above a little:
GPS and Baro
altitudes need to be pretty much
exactly the same - within 100 ft let's say. Adjusted for cold temps as needed.
That familiar "looks kind of similar" feeling pilots normally have in mind comes (incorrectly) when the SIM exercise is played above transition ALT/LVL, using STD as the reference for ADIRSes.
The observed difference is not any inherent inaccuracy, but rather the delta between QNE and QNH instead.