Above ground level

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In aviation and atmospheric sciences, an altitude is said to be above ground level (AGL) when it is measured with respect to the underlying ground surface. This is by opposition to above mean sea level (AMSL). In other words, these expressions (AGL or AMSL) indicate where the 'zero level' or 'reference altitude' is located.

[edit] Relevance to aviation

A pilot flying a plane under instrument flight rules (typically under poor visibility conditions) must rely on its altimeter to decide when to deploy the aircraft carriage and prepare for landing. Therefore the pilot needs reliable information on the altitude of the plane with respect to the air strip. The altimeter, which is normally a barometer calibrated in units of distance instead of atmospheric pressure, must therefore be set in such a way as to indicate the altitude of the craft above ground. This is done by communicating with the control tower of the airport (to get the current surface pressure) and setting the altimeter so as to report a null altitude AGL on the ground of that airport. Confusing AGL with AMSL, or improperly calibration of the altimeter may result in controlled flight into terrain, a crash under pilot control.

[edit] Relevance to atmospheric sciences

In weather and climate studies, measurements or simulations often need to refer to a specific height or altitude, which is naturally AGL. However, the values of geophysical variables measured in various places on the natural (ground) surface may not be easily compared in hilly or mountainous terrain, because part of the observed variability is due to changes in the altitude of the surface. For this reason, variables such as pressure or temperature are sometimes 'reduced' to mean sea level.

In general circulation models and global climate models, the state and properties of the atmosphere are specified or computed at a number of discrete locations and heights. When the topography of the continents is explicitly represented, the altitudes of these locations are set above the simulated ground level. This is often implemented using the so-called sigma coordinate system, which is the ratio of the pressure at a location (latitude, longitude, altitude) divided by the pressure at the nadir of that location on ground surface (same latitude, same longitude, altitude AGL =0).