What is QNH?
QNH is a Q code indicating the atmospheric pressure adjusted to sea level. It is a pressure setting used by pilots, air traffic control (ATC), and low frequency weather beacons to refer to the barometric setting which, when set on an aircraft's altimeter, will cause the altimeter to read altitude above mean sea level within a certain defined region.
The barometric pressure sensor in Oudie gives an absolute reading.
Oudie will typically assume the altitude should be calculated relative to the aviation standard atmosphere pressure at mean sea level of 1013.25 hPa.