altitude

Altitude is the vertical distance of a point or object above a reference level. The choice of reference depends on context: mean sea level (MSL) is common in geography and aviation; ground level near the object is used in terrain-aware measurements; in geodesy the Earth’s ellipsoid serves as a mathematical reference surface.

Altitude can be described as altitude above mean sea level (AMSL), also called AGL for above ground level. In aviation, several altitudes exist: indicated altitude (what an altimeter shows with local pressure setting), true altitude (geometric height above MSL), pressure altitude (altitude corresponding to a given pressure in standard atmosphere), and density altitude (pressure altitude corrected for temperature).

Measurement: Altimeters measure pressure; GPS provides geometric height; radar altimeters measure height above terrain. The standard atmosphere defines temperature and pressure profiles that relate altitude to pressure and temperature, with sea level pressure about 1013.25 hPa and a -6.5 C per km lapse rate in the troposphere.

Applications: In aviation, altitude ensures safe separation and performance. In meteorology, geopotential height is used for weather maps. In geography and mountaineering, elevation indicates terrain height. In astronomy, altitude refers to the angle of a celestial object above the horizon (altitude angle).

Impact: Air density decreases with altitude, affecting breathing, engine performance, and boiling points of liquids. Humans experience physiological effects at high altitudes and acclimatization is often required. Understanding altitude is essential in planning flights, mountain expeditions, and climate studies.