Height above ground level

Last updated July 17, 2023

In aviation, atmospheric sciences and broadcasting, a height above ground level (AGL^[1] or HAGL) is a height measured with respect to the underlying ground surface. This is as opposed to height above mean sea level (AMSL or HAMSL), height above ellipsoid (HAE, as reported by a GPS receiver), or height above average terrain (AAT or HAAT, in broadcast engineering). In other words, these expressions (AGL, AMSL, HAE, AAT) indicate where the "zero level" or "reference altitude" – the vertical 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 model WGS84	GPS receiver
AAT, HAAT	Height Above Average Terrain	broadcasting, cellular networks	average surrounding surface

Aviation

A pilot flying an aircraft under instrument flight rules (typically under poor visibility conditions) must rely on the aircraft's altimeter to decide when to deploy the undercarriage and prepare for landing. Therefore, the pilot needs reliable information on the height of the plane with respect to the landing area (usually an airport). The altimeter, which is usually a barometer calibrated in units of distance instead of atmospheric 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 in controlled flight into terrain, a crash of a fully functioning aircraft under pilot control.

While the use of a barometric altimeter 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] One mnemonic that can be used is: if it is an altitude you can fly there, if it is an elevation you can walk there, and if it is height that's how far a rock will fall before it hits the ground.

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).

Broadcasting

In broadcasting, altitude AGL has relatively little direct bearing on the broadcast 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 a broadcast station (or any other sort of VHF or higher radio-frequency) transmission will travel.

From aviation safety perspective though, the more important aspect is the height of the radio tower used to support the radio antenna. In this case, height AGL is the only important measurement for aviation authorities, which require that some tall towers have proper Aircraft warning paint and lights to avoid collisions.

Related Research Articles

In aviation, visual flight rules (VFR) are a set of regulations under which a pilot operates an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than basic VFR weather minima, i.e., in visual meteorological conditions (VMC), as specified in the rules of the relevant aviation authority. The pilot must be able to operate the aircraft with visual reference to the ground, and by visually avoiding obstructions and other aircraft.

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.

Altitude or height is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context. Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.

Mean sea level is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datum – a standardised geodetic datum – that is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is instead the midpoint between a mean low and mean high tide at a particular location.

An airfield traffic pattern is a standard path followed by aircraft when taking off or landing while maintaining visual contact with the airfield.

Airspace is the portion of the atmosphere controlled by a country above its territory, including its territorial waters or, more generally, any specific three-dimensional portion of the atmosphere. It is not the same as outer space which is the expanse or space outside the Earth and aerospace which is the general term for Earth's atmosphere and the outer space within the planet's vicinity. History:

Pressure altitude is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure as that of the part of the atmosphere in question.

In aviation, lowest safe altitude (LSALT) is an altitude that is at least 500 feet above any obstacle or terrain within a defined safety buffer region around a particular route that a pilot might fly. The safety buffer allows for errors in the air by including an additional area that a pilot might stray into by flying off track. By flying at or above this altitude a pilot complies with terrain clearance requirements on that particular flight leg.

Flight plans are documents filed by a pilot or flight dispatcher with the local Air Navigation Service Provider prior to departure which indicate the plane's planned route or flight path. Flight plan format is specified in ICAO Doc 4444. They generally include basic information such as departure and arrival points, estimated time en route, alternate airports in case of bad weather, type of flight, the pilot's information, number of people on board, and information about the aircraft itself. In most countries, flight plans are required for flights under IFR, but may be optional for flying VFR unless crossing international borders. Flight plans are highly recommended, especially when flying over inhospitable areas such as water, as they provide a way of alerting rescuers if the flight is overdue. In the United States and Canada, when an aircraft is crossing the Air Defense Identification Zone (ADIZ), either an IFR or a special type of VFR flight plan called a DVFR flight plan must be filed. For IFR flights, flight plans are used by air traffic control to initiate tracking and routing services. For VFR flights, their only purpose is to provide needed information should search and rescue operations be required, or for use by air traffic control when flying in a "Special Flight Rules Area."

In aviation, an instrument approach or instrument approach procedure (IAP) is a series of predetermined maneuvers for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach to a landing, or to a point from which a landing may be made visually. These approaches are approved in the European Union by EASA and the respective country authorities and in the United States by the FAA or the United States Department of Defense for the military. The ICAO defines an instrument approach as, "a series of predetermined maneuvers by reference to flight instruments with specific protection from obstacles from the initial approach fix, or where applicable, from the beginning of a defined arrival route to a point from which a landing can be completed and thereafter, if landing is not completed, to a position at which holding or en route obstacle clearance criteria apply."

A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to travel to ground, reflect, and return to the craft. This type of altimeter provides the distance between the antenna and the ground directly below it, in contrast to a barometric altimeter which provides the distance above a defined vertical datum, usually mean sea level.

The United States airspace system's classification scheme is intended to maximize pilot flexibility within acceptable levels of risk appropriate to the type of operation and traffic density within that class of airspace – in particular to provide separation and active control in areas of dense or high-speed flight operations.

<span class="mw-page-title-main">Pressure altimeter</span>

Altitude can be determined based on the measurement of atmospheric pressure. The greater the altitude, the lower the pressure. When a barometer is supplied with a nonlinear calibration so as to indicate altitude, the instrument is a type of altimeter called a pressure altimeter or barometric altimeter. A pressure altimeter is the altimeter found in most aircraft, and skydivers use wrist-mounted versions for similar purposes. Hikers and mountain climbers use wrist-mounted or hand-held altimeters, in addition to other navigational tools such as a map, magnetic compass, or GPS receiver.

The National Airspace System (NAS) is the airspace, navigation facilities and airports of the United States along with their associated information, services, rules, regulations, policies, procedures, personnel and equipment. It includes components shared jointly with the military. It is one of the most complex aviation systems in the world, and services air travel in the United States and over large portions of the world's oceans.

Inex-Adria Aviopromet Flight 450, JP 450, was an international charter flight from Tivat in the Socialist Federal Republic of Yugoslavia to Prague, Czechoslovakia which crashed in the Prague suburb of Suchdol on October 30, 1975, at 09:20 AM. The McDonnell Douglas DC-9-32 with 115 passengers and 5 crew on board descended, under Instrument Meteorological Conditions (IMC), below defined Minimum Descent Altitude (MDA) during the final approach to Prague Ruzyně Airport RWY 25, entered a gorge above Vltava river, and was unable to outclimb the rising terrain. 75 of the 120 occupants died during the crash itself while 4 others died in Hospital. The accident remains the worst aviation disaster on the Czech Republic soil.

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); the pressure at the nearby surface airport (QFE); or the pressure level of 1,013.25 hectopascals which gives pressure altitude and is used to maintain one of the standard flight levels.

Vertical position or vertical location is a position along a vertical direction above or below a given vertical datum . Vertical distance or vertical separation is the distance between two vertical positions. Many vertical coordinates exist for expressing vertical position: depth, height, altitude, elevation, etc. Points lying on an equigeopotential surface are said to be on the same vertical level, as in a water level.

On 29 November 1975, former Formula One champion and team owner Graham Hill died when the Piper Aztec light aircraft he was piloting crashed near Arkley in Hertfordshire, United Kingdom, while on approach to Elstree Airfield. The other five passengers on board, forming the core of his Embassy Hill Formula One team, were also killed.

Height above mean sea level is a measure of the vertical distance of a location in reference to a historic mean sea level taken as a vertical datum. In geodesy, it is formalized as orthometric heights.

References

↑ Radiotelephony Manual. UK Civil Aviation Authority. 28 May 2015. ISBN 9780-11792-893-0. CAP413.
↑ Procedures for Air Navigation Services – Aircraft Operations (PANS-OPS), Volume II: Construction of Visual and Instrument Flight Procedures (PDF) (5th ed.). ICAO. 2006. Archived from the original (PDF) on 2016-05-19. Retrieved 2015-08-15.
↑ Pratt, Jeremy M. (2003) [1996]. The Private Pilot's Licence Course: Navigation, Meteorology (3rd ed.). met22–met23. ISBN 1-874783-18-7.

External links

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[CAP413-1] Radiotelephony Manual. UK Civil Aviation Authority. 28 May 2015. ISBN 9780-11792-893-0. CAP413.

[PANS-OPS-2] Procedures for Air Navigation Services – Aircraft Operations (PANS-OPS), Volume II: Construction of Visual and Instrument Flight Procedures (PDF) (5th ed.). ICAO. 2006. Archived from the original (PDF) on 2016-05-19. Retrieved 2015-08-15.

[3] Pratt, Jeremy M. (2003) [1996]. The Private Pilot's Licence Course: Navigation, Meteorology (3rd ed.). met22–met23. ISBN 1-874783-18-7.

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