The unit of pressure is defined as 1013.25Millibars. This pressure is taken as the mean sea level. Expressed as the weight of air.
Altimeter Is a device used to shoe air pressure, the distance of a point above the sea level. They are commonly used in navigation purposes and most types of altimeters are barometric.
As the density of the air is lower at the higher altitudes. As the air is usually thinner at the higher altitudes. The GPS, for instance, can also provide an altimeter and used by skydivers.
