Height above ground level
In
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
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]
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
Broadcasting
In broadcasting, altitude AGL has relatively little direct bearing on the
From aviation safety perspective though, the more important aspect is the height of the
See also
- List of aviation, avionics, aerospace and aeronautical abbreviations
- Index of aviation articles
- Minimum safe altitude
References
- ISBN 9780-11792-893-0. CAP413.
- ICAO. 2006. Archived from the original(PDF) on 2016-05-19. Retrieved 2015-08-15.
- ISBN 1-874783-18-7.
External links
- AGL in aviation Archived 2007-09-28 at the Wayback Machine