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Indicated altitude is the reading on the altimeter when it is set to the local barometric pressure at mean sea level. In UK aviation radiotelephony usage, the vertical distance of a level, a point or an object considered as a point, measured from mean sea level; this is referred to over the radio as altitude.(see QNH) [2]
The transition level (TL) is the lowest flight level above the transition altitude. The table below shows the transition level according to transition altitude and QNH. When descending below the transition level, the pilot starts to refer to altitude of the aircraft by setting the altimeter to the QNH for the region or airfield.
Height above mean sea level is a measure of a location's vertical distance (height, elevation or altitude) in reference to a vertical datum based on a historic mean sea level. In geodesy, it is formalized as orthometric height. The zero level varies in different countries due to different reference points and historic measurement periods.
The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Vertical datum).
Room air at altitude can be enriched with oxygen without introducing an unacceptable fire hazard. At an altitude of 8000 m the equivalent altitude in terms of oxygen partial pressure can be reduced to below 4000 m without increasing the fire hazard beyond that of normal sea level atmospheric air.
Height above mean sea level (AMSL) is the elevation (on the ground) or altitude (in the air) of an object, relative to a reference datum for mean sea level (MSL). It is also used in aviation, where some heights are recorded and reported with respect to mean sea level (contrast with flight level ), and in the atmospheric sciences , and in land ...
In aviation, atmospheric sciences and broadcasting, a height above ground level (AGL [1] or HAGL) is a height measured with respect to the underlying ground surface.This is as opposed to height above mean sea level (AMSL or HAMSL), height above ellipsoid (HAE, as reported by a GPS receiver), or height above average terrain (AAT or HAAT, in broadcast engineering).
Usually, the altitude of an aircraft is measured from sea level, while its height is measured from ground level. Elevation is also measured from sea level, but is most often regarded as a property of the ground. Thus, elevation plus height can equal altitude, but the term altitude has several meanings in aviation.