The fact that atmospheric pressure, and hence the reading of a barometer, decreases as the altitude increases is utilized in determining differences of elevation.  The aneroid barometer is less accurate than the mercurial barometer, but is far more portable and convenient, and therefore was used almost exclusively in surveying.

It consists of a metallic box from which the air has been exhausted. The varying pressure of the atmosphere acting upon the thin corrugated top of the vacuum-chamber moves it in or out, and this motion is transmitted by multiplying levers, chains and springs to a pointer moving around a dial which is graduated so that atmospheric pressure is recorded in inches to correspond to varying heights of a column of mercury.

In addition the altitudes in feet, corresponding to the various pressures, are usually given by a second set of graduations - less accurate than the first being dependent upon barometric formulas for reducing pressure to altitude.