An instrument that determines the altitude of an object with respect to a fixed level.

The most common type is an aneroid barometer calibrated to show the drop in atmospheric pressure in terms of linear elevation as an airplane, balloon, or mountain climber rises.

It shows height above sea level, but not above such land features as hills, mountains, and valleys.

The radio altimeter, or terrain-clearance indicator, is an absolute altimeter which indicates the actual altitude over water or over terrain, regardless of how uneven it is.

It functions by first sending either continuous or pulse radio signals from a transmitter in an aircraft to the earth's surface.