修正海平面气压(QNH)，是航空图上所标注机场数据。在机场附近进行起飞、爬升、下降以及着陆过程时都需要以修正海平面气压为标准来拨正高度表。来确保所有起飞降落的飞机都使用同一个 标准来测定飞行高度。

例如假设飞机由香港国际机场飞往成田机场， 申报高度FL350，出发地QNH为1002 目的地QNH为1011。 那么到达目的地时， 会有约300尺误差，因此飞行员需要修正气压变化所产生误差，以防止撞地/飞机相撞或者异常接近等事故的发生。

高度+ (1013.2 - QNH) x 30 = 修正海平面气压高度

QNH is one of the many Q codes. It is defined as, "barometric 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. Within United Kingdom airspace, these are known as Altimeter Setting Regions (ASRs); these regions may be large areas, or apply only to the airfield for which the QNH was given. An airfield QNH will cause the altimeter to show airfield altitude, that is, the altitude of the centre point of the main runway above sea level on landing, irrespective of the temperature.