Hi
to answer your first question:
1 millibar of preassure equals roughly 30 feet.
If for example if QNH (sea level preassure) is 1012 millibars
and QFE (preassure on aerodrome) is 1009 millibars
the difference is 3 millibars (1012-1009=3)
since 1 millibar equals roughly 30ft
3 X 30 = 90ft...aerodrome elevation is therefore 90ft feet above mean sea level.
thats the answer to your first question, if i said anything incorrect feel free to correct me anyone.
P16