An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water.
Digital barometric pressure sensor for altitude measurement in consumer electronic applications
The altimeter on this Piper PA-28 is seen on the top row of instruments, second from right
Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context. Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.
A Boeing 737-800 cruising in the stratosphere, where airliners typically cruise to avoid turbulence rampant in the troposphere. The blue layer is the ozone layer, fading further to the mesosphere. The ozone heats the stratosphere, making conditions stable. The stratosphere is also the altitude limit of jet aircraft and weather balloons, as the air density there is roughly 1⁄1000 of that in the troposphere.
