Altitude above ground level
In aviation and atmospheric sciences, an altitude above ground level (or AGL altitude) is an altitude measured with respect to the underlying ground surface. This is as opposed to altitude/elevation above mean sea level (AMSL), or (in broadcast engineering) height above average terrain (HAAT). In other words, these expressions (AGL, AMSL, or HAAT) indicate where the "zero level" or "reference altitude" is located.
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 undercarriage 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 improper calibration of the altimeter may result in controlled flight into terrain, a crash under pilot control.
While the use of a barometric altimeter setting that provides a null altitude AGL on the ground of the airport is a reference available to pilots, in commercial aviation it is a country-specific (Russia, a few others) procedure that is not often used. Most countries (Far East, North and South America, all of Europe, Africa, Australia, etc.) use the airport's MSL (Mean Sea Level) elevation as a reference. During approaches to landing, there are several other references that are used, including AFE (Above Field Elevation) which is height referencing the highest point on the airfield, TDZE (TouchDown Zone Elevation) or TH (Threshold Height) which both refer to the elevation of the landing end of the runway measured above MSL, and AGL (Above Ground Level) which is height above the ground directly below the aircraft.
In general, "Altitude" refers to distance above sea level or MSL, "height" refers to distance above a particular feature of the terrain (airport, runway or ground at present location), and "elevation" describes a feature of the terrain itself in terms of distance above MSL. Think of it this way; if it's an altitude you can fly there, if it's an elevation you can walk there, and if it's height that's how far a rock will fall before it lands.
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).
In broadcasting, AGL has relatively little direct bearing on the broadcast range of a station. Rather, it is HAAT (the height above the surrounding area) which is used to determine how far a broadcast station (or any other sort of VHF or higher radio-frequency) transmission will travel.
More important is the height of the radio tower used to support the radio antenna. In this case, AGL is the only important measurement for aviation authorities, which require that some tall towers have proper painting and lighting to avoid collisions.