Hi
Thanks for the replies...
So the general feeling is that it is QNH at the ATC unit not the aircraft, that makes sense, OK.... about the 28ft per mb...
Having searched the www I have found that the pressure lapse rate does vary with height quite a bit, 27ft/mb at SL, 36ft/mb at 10000ft and 48ft/mb at 18000ft are just a few examples. These are calculated from ISA values and are based on temperature and pressure using a formula:
ft/mb = (96*Temp in Kelvin)/Pressure in mb
so that gives me all the info to calculate altitude given QNH the lapse rate and the PA from the encoder...
As an aside does anyone know the highest transition altitude in use?
Cheers
smithgd