aviation Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot air ...

atmospheric sciences Atmospheric science is the study of the Earth's atmosphere and its various inner-working physical processes. Meteorology includes atmospheric chemistry and atmospheric physics with a major focus on weather forecasting. Climatology is the study of ...

and

broadcasting Broadcasting is the distribution (business), distribution of sound, audio or video content to a dispersed audience via any electronic medium (communication), mass communications medium, but typically one using the electromagnetic spectrum (radio ...

, a height above ground level (AGL or HAGL) is a

height Height is measure of vertical distance, either vertical extent (how "tall" something or someone is) or vertical position (how "high" a point is). For example, "The height of that building is 50 m" or "The height of an airplane in-flight is abou ...

measured with respect to the underlying ground surface. This is as opposed to height

above mean sea level Height above mean sea level is a measure of the vertical distance (height, elevation or altitude) of a location in reference to a historic mean sea level taken as a vertical datum. In geodesy, it is formalized as ''orthometric heights''. The comb ...

(AMSL or HAMSL), height above ellipsoid (HAE, as reported by a GPS receiver), or

height above average terrain Height above average terrain (HAAT), or (less popularly) effective height above average terrain (EHAAT), is the vertical position of an antenna site is above the surrounding landscape. HAAT is used extensively in FM radio and television, as it is ...

(AAT or HAAT, in

broadcast engineering Broadcast engineering is the field of electrical engineering, and now to some extent computer engineering and information technology, which deals with radio and television broadcasting. Audio engineering and RF engineering are also essential par ...

). In other words, these expressions (AGL, AMSL, HAE, AAT) indicate where the "zero level" or "reference altitude" - the

vertical datum In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum, is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built stru ...

- is located.

Aviation

A pilot flying an aircraft under

instrument flight rules In aviation, instrument flight rules (IFR) is one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other is visual flight rules (VFR). The U.S. Federal Aviation Administration's (FAA) ''Instrument Fly ...

(typically under poor

visibility The visibility is the measure of the distance at which an object or light can be clearly discerned. In meteorology it depends on the transparency of the surrounding air and as such, it is unchanging no matter the ambient light level or time of ...

conditions) must rely on the aircraft's

altimeter An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...

to decide when to deploy the

undercarriage Undercarriage is the part of a moving vehicle that is underneath the main body of the vehicle. The term originally applied to this part of a horse-drawn carriage, and usage has since broadened to include: *The landing gear of an aircraft. *The ch ...

and prepare for landing. Therefore, the pilot needs reliable information on the

of the plane with respect to the landing area (usually an airport). The altimeter, which is usually a

barometer A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis ...

calibrated in units of distance instead of

atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...

, 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 in

controlled flight into terrain In aviation, a controlled flight into terrain (CFIT; usually ) is an aviation accidents and incidents, accident in which an airworthy aircraft, under aircraft pilot, pilot control, is unintentionally flown into the ground, a mountain, a body of ...

, a crash of a fully functioning aircraft under pilot control. While the use of a barometric

altimeter setting Altimeter setting is the value of the atmospheric pressure used to adjust the sub-scale of a pressure altimeter so that it indicates the height of an aircraft above a known reference surface. This reference can be the mean sea level pressure ( QNH); ...

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). 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. One mnemonic that can be used is: 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 hits the ground.

Atmospheric sciences

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...

and

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...

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 model A general circulation model (GCM) is a type of climate model. It employs a mathematical model of the general circulation of a planetary atmosphere or ocean. It uses the Navier–Stokes equations on a rotating sphere with thermodynamic terms ...

s, the state and properties of the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

are specified or computed at a number of discrete locations and heights. When the

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

of the

continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...

s 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 The sigma coordinate system is a common coordinate system used in computational models for oceanography, meteorology and other fields where fluid dynamics are relevant. This coordinate system receives its name from the independent variable \sig ...

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).

Broadcasting

In broadcasting, altitude AGL has relatively little direct bearing on the

broadcast range A broadcast range (also listening range or listening area for radio, or viewing range or viewing area for television) is the service area that a broadcast station or other transmission covers via radio waves (or possibly infrared light, which ...

of a station. Rather, it is HAAT (the height above the average terrain (in the surrounding area)) which is used to determine how far a broadcast station (or any other sort of VHF or higher

radio-frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...

) transmission will travel. From aviation safety perspective though, the more important aspect is the height of the

radio tower Radio masts and towers are typically tall structures designed to support antennas for telecommunications and broadcasting, including television. There are two main types: guyed and self-supporting structures. They are among the tallest human-made ...

used to support the

radio antenna In radio engineering, an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In transmission, a radio transmitter supplies an ...

. In this case, height AGL is the only important measurement for aviation authorities, which require that some tall towers have proper

Aircraft warning paint Aviation obstruction lighting is used to enhance the visibility of structures or fixed obstacles which may conflict with the safe navigation of aircraft. Obstruction lighting is commonly installed on towers, buildings, and even fences located i ...

and

lights Light is an electromagnetic radiation, part of which stimulates the sense of vision. Light or Lights may also refer to: Illumination * Light bulb * Traffic light Arts and entertainment Music * Lights (musician) (born 1987), Canadian singer-son ...

to avoid

collision In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...

References

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External links

AGL in aviation
Altitudes in aviation Aviation meteorology Clouds Broadcast engineering

Aviation

Atmospheric sciences

Broadcasting

See also

References

External links