Above ground level
Encyclopedia
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 as opposed to 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.
(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 improperly calibration of the altimeter may result in controlled flight into terrain
, a crash under pilot control.
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 model
s, the state and properties of the atmosphere
are specified or computed at a number of discrete locations and heights. When the topography
of the continent
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 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).
(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.
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
and atmospheric sciences
Atmospheric sciences
Atmospheric sciences is an umbrella term for the study of the atmosphere, its processes, the effects other systems have on the atmosphere, and the effects of the atmosphere on these other systems. Meteorology includes atmospheric chemistry and atmospheric physics with a major focus on weather...
, an altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
is said to be above ground level (AGL) when it is measured with respect to the underlying ground surface. This is as opposed to above mean sea level
Above mean sea level
The term above mean sea level refers to the elevation or altitude of any object, relative to the average sea level datum. AMSL is used extensively in radio by engineers to determine the coverage area a station will be able to reach...
(AMSL), or in broadcast engineering
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...
, height above average terrain
Height above average terrain
Height above average terrain is used extensively in FM radio and television, as it is actually much more important than effective radiated power in determining the range of broadcasts...
(HAAT). In other words, these expressions (AGL, AMSL, or HAAT) indicate where the "zero level" or "reference altitude" is located.
Aviation
A pilot flying a plane under instrument flight rulesInstrument flight rules
Instrument flight rules are one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other are visual flight rules ....
(typically under poor visibility
Visibility
In meteorology, visibility is a measure of the distance at which an object or light can be clearly discerned. It is reported within surface weather observations and METAR code either in meters or statute miles, depending upon the country. Visibility affects all forms of traffic: roads, sailing...
conditions) must rely on its altimeter
Altimeter
An altimeter 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 underwater.-Pressure altimeter:...
to decide when to deploy the aircraft undercarriage
Undercarriage
The undercarriage or landing gear in aviation, is the structure that supports an aircraft on the ground and allows it to taxi, takeoff and land...
and prepare for landing. Therefore the pilot needs reliable information on the altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
of the plane with respect to the air strip. The altimeter, which is normally a barometer
Barometer
A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather...
calibrated in units of distance instead of atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
, 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
Controlled flight into terrain
Controlled flight into terrain describes an accident in which an airworthy aircraft, under pilot control, is unintentionally flown into the ground, a mountain, water, or an obstacle. The term was coined by engineers at Boeing in the late 1970s...
, a crash under pilot control.
Atmospheric sciences
In weatherWeather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
and climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
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
Global climate model
A General Circulation Model is a mathematical model of the general circulation of a planetary atmosphere or ocean and based on the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources . These equations are the basis for complex computer programs commonly...
s, the state and properties of the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
are specified or computed at a number of discrete locations and heights. When the topography
Topography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...
of the continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
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 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, 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 stationBroadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
(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
Painting
Painting is the practice of applying paint, pigment, color or other medium to a surface . The application of the medium is commonly applied to the base with a brush but other objects can be used. In art, the term painting describes both the act and the result of the action. However, painting is...
and lighting
Lighting
Lighting or illumination is the deliberate application of light to achieve some practical or aesthetic effect. Lighting includes the use of both artificial light sources such as lamps and light fixtures, as well as natural illumination by capturing daylight...
to avoid collisions.