A Discussion of Various Measures of Altitude

It is common in dealing with airborne research data to encounter many different altitude terms. These include geometric altitude, GNSS altitude, INS altitude, pressure altitude, geopotential height, and so on. Despite the nomenclature, there are only two altitude scales involved: geometric altitude and geopotential altitude or height. Geometric altitude is the scale we are most familiar with; it is what we would measure with a tape measure. Radiosondes, on the other hand, generally report geopotential height – a scale which relates height to gravitational equipotentials, or surfaces of constant gravitational potential energy per unit mass. Although geopotential height approximates geometric height, they are not the same. An important type of geopotential height is pressure altitude, which is based on a standard atmospheric model for temperature as a function of pressure. One particular model, the International Standard Atmosphere (ISA), is what all aircraft altimeters use to relate static pressure measurements on an aircraft to a corresponding pressure altitude scale. There are also a number of additional altitude terms related to flying airplanes such as true altitude, indicated altitude, absolute altitude and density altitude, which are discussed elsewhere.