The vertical distance of a point above a datum. The vertical datum is usually an estimate of mean sea level. Older measurements were often determined in a local coordinate system and were not tied to
a global reference frame. Some were made not with surveying instruments but with barometers, in reliance on the decrease of atmospheric pressure with altitude. It is now usual to measure altitude or elevation using the Global Positioning System or an equivalent global navigation satellite system. Altitude and elevation are synonyms in common usage, although altitude is less ambiguous. The unqualified word 'elevation' can also refer, for example, to the act of elevating or to angular distance above a horizontal plane.
The lowest altitude at which Mountain Permafrost occurs in a given area downslope of the general Permafrost region. The altitudinal limit of Mountain Permafrost rises progressively with decreasing lat
itude in both hemispheres.
The vertical subdivision of an area of Mountain Permafrost into Permafrost zones, based on the proportion of the ground that is perennially cryotic. As mean annual temperatures decrease with increasin
g elevation, Mountain Permafrost can be expected to be more extensive, thicker and colder at higher elevations, although aspect and the extent of vegetation and Snow Cover will moderate this effect.