Nevertheless in the 8 years I have been flying behind the old KLN94, I have never seen it display an altitude more than about 30ft (i.e. 1 millibar) different from the published airfield elevation.
Well, post May 2000 you shouldn't expect see differences much bigger than those, and if you do see them it's probably a misprint on the airfield chart or the surveyor got the height wrong (it happens
).
And if still on the tarmac and you have set your altimeter to the QNH provided by the tower, everything should more or less agree.
But what I understand is being discussed here is what happens when you take off, climb to say FL180 and then compare the GPS against the altimeter reading. In that case you can expect to see
quite a bit more than 30ft discrepancy between altimeter and GPS. More like 1000ft-1500ft on a random day over Greece
