Inaviation,atmospheric sciences andbroadcasting, aheight above ground level (AGL[1] orHAGL) is aheight measured with respect to the underlyingground surface. This is as opposed to heightabove mean sea level (AMSL or HAMSL),height above ellipsoid (HAE, as reported by a GPS receiver), orheight above average terrain (AAT or HAAT, inbroadcast engineering). In other words, these expressions (AGL, AMSL, HAE, AAT) indicate where the "zero level" or "reference altitude" – thevertical datum – is located.
| Abbreviation | Stands for | Main usage | Zero level | Measuring devices |
|---|---|---|---|---|
| AGL, HAGL | height above ground level | aviation, atmospheric science, broadcasting | ground surface | radar altimeter |
| AMSL, HAMSL | height above mean sea level | nautic, technics, geography | sea level average | barometric altimeter |
| HAE | height above ellipsoid | navigation, science | math surface modelWGS84 | GPS receiver |
| AAT, HAAT | height above average terrain | broadcasting, cellular networks | average surrounding surface |
A pilot flying an aircraft underinstrument flight rules (typically under poorvisibility conditions) must rely on the aircraft'saltimeter to decide when to deploy theundercarriage and prepare for landing. Therefore, the pilot needs reliable information on theheight of the plane with respect to the landing area (usually an airport). The altimeter, which is usually abarometer calibrated in units of distance instead ofatmospheric pressure, can therefore be set in such a way as to indicate the height of the aircraft 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 read zero on the ground of that airport. Confusion between AGL and AMSL, or improper calibration of the altimeter, may result incontrolled flight into terrain, a crash of a fully functioning aircraft under pilot control.
While the use of a barometricaltimeter setting that provides a zero reading on the ground of the airport is a reference available to pilots, in commercial aviation it is a country-specific procedure that is not often used (it is used, e.g., in Russia, and a few other countries[which?]). Most countries (Far East, North and South America, all of Europe, Africa, Australia) use the airport's AMSL (above 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 AMSL and AGL respectively.
In general, "altitude" refers to distance above mean sea level (MSL or AMSL), "height" refers to distance above a particular point (e.g. the airport,runway threshold, or ground at present location), and "elevation" describes a feature of the terrain itself in terms of distance above MSL.[2][3]
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 andglobal climate models, the state and properties of theatmosphere are specified or computed at a number of discrete locations and heights. When thetopography of thecontinents is explicitly represented, the altitudes of these locations are set above the simulated ground level. This is often implemented using the so-calledsigma 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, altitude AGL has relatively little direct bearing on thebroadcast range of a station. Rather, it is HAAT (the height above the average terrain (in the surrounding area)) which is used to determine how far abroadcast station (or any other sort ofVHF or higherradio-frequency) transmission will travel.
From aviation safety perspective though, the more important aspect is the height of theradio tower used to support theradio antenna. In this case, height AGL is the only important measurement for aviation authorities, which require that some tall towers have properaircraft warning paint andlights to avoidcollisions.