PRINCIPLES OF OPERATION
Atmospheric Pressure-Altimeter Relationship
a. The basic principle of altimetry is that the pressure caused by the weight of the column of air above the
observer decreases as the observer rises in altitude. The relationship between pressure and altitude is not constant since
air is compressible. Furthermore, changes in air density caused by variations in temperature, relative humidity, and gravity
will change the pressure versus altitude ratio.
b. In order to eliminate the need for converting pressure readings to altitude, the altimeter described herein is
calibrated in meters; however to calibrate an altimeter a standard pressure-altitude relationship must be used. Because
altitude does not have a constant relationship to pressure, the altimeter can be used only to measure differences in
elevation with respect to some base or reference station, preferably a known benchmark.
c. The pressure-altitude relationship holds goo(d only for certain standard conditions. If these conditions do not
exist while a survey is being made. then corrections must be applied. Air temperature and relative humidity have the
greatest effect on: the density of the air and are therefore the most significant. Application of these corrections is
The air temperature mentioned above must not be confused with the instrument temperature