   altitude or height is defined based on the context in which it is used (aviation, geometry, geographical survey, sport, and more). 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. 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. vertical distance measurements in the "down" direction are commonly referred to as depth. == altitude in aviation== thumb|300px|vertical distance comparison in aviation, the term altitude can have several meanings, and is always qualified by either explicitly adding a modifier (e.g. "true altitude"), or implicitly through the context of the communication. parties exchanging altitude information must be clear which definition is being used. aviation altitude is measured using either mean sea level (msl) or local ground level (above ground level, or agl) as the reference datum. pressure altitude divided by 100 feet (30&nbsp;m) as the flight level, and is used above the transition altitude ( in the us, but may be as low as  in other jurisdictions); so when the altimeter reads 18,000&nbsp;ft on the standard pressure setting the aircraft is said to be at "flight level 180". when flying at a flight level, the altimeter is always set to standard pressure (29.92&nbsp;inhg or 1013.25&nbsp;hpa). on the flight deck, the definitive instrument for measuring altitude is the pressure altimeter, which is an aneroid barometer with a front face indicating distance (feet or metres) instead of atmospheric pressure. there are several types of aviation altitude: * indicated altitude is the reading on the altimeter when the altimeter is set to the local barometric pressure at mean sea level. * absolute altitude is the height of the aircraft above the terrain over which it is flying. also referred to feet/metres above ground level (agl). * true altitude is the actual elevation above mean sea level. it is indicated altitude corrected for non-standard temperature and pressure. in uk aviation radiotelephony usage, the vertical distance of a level, a point or an object considered as a point, measured from mean sea level; this is referred to over the radio as  altitude.(see qnh){{cite book | date=1 january 1995 | title=
Aerospace Physical_geography Topography