It depends. . .
. . on whether your altimeter is more accurate than the Stevensons screen of the weather man. . .also, the QNH is the pressure setting of mean sea level (as you know) for a specific period and is an average of that specific area be it the Chatham ASR or the EGLL QNH . . . whereas, your reading of Zero feet on the ground is real time.
Last edited by Upper Air; 7th March 2011 at 14:16.