The Aircraft Altimeter is an Instrument that is used to measure the Altitude of the surface of the land, or for that matter any object that can fly, like an air plane. There are 2 main varieties of altimeters that are used and they are the following:

Pressure Altimeter, or Aneroid barometer and

Radio Altimeter

The Pressure Altimeter or aneroid barometer helps approximate the Altitude of an object above the mean sea level. The task of the Radio Altimeter is to measure the absolute value of the altitude that is based on the time that is needed for the signal of a radio wave for traveling from an air craft from the ground and back. (Altitude refers to the distance of the land above the sea level).