How Do You Measure Altitude?

Airplanes measure altitude against the known air pressure at certain heights.
Airplanes measure altitude against the known air pressure at certain heights. (Image: Thomas Northcut/Photodisc/Getty Images)

Measuring altitude shows the elevation of land or the height of an aircraft. The typical starting point for altitude measurements is sea level, though some instruments use varying points to determine the level of the sea. The barometric pressure drops as altitude increases, so aircraft height is measured with a barometric pressure reading instrument. Altitude is also shown on a Global Positioning System, but typically from a different sea level reference. Problems arise for pilots using either means of determining altitude, but air traffic controllers make corrective compensations.

Things You'll Need

  • Barometric altimeter
  • Global Positioning System

Check in with the nearest air traffic controller for a barometric pressure reading before takeoff. Small municipal airports don't always have an ATC working in an on-site tower. Often one control tower serves several nearby general aviation airports.

Dial the barometer reading into your altimeter with the setting knob. This makes up for any pressure differences in the atmosphere such as a storm carrying low pressure that would otherwise interfere with determining altitude.

Read the needle position on the altimeter dial measuring altitude in feet as the aircraft climbs and drops. Fly at the altitude prescribed by the air traffic controller for your heading to maintain a safe altitude for all other air traffic.

Set a GPS for altitude measurements. Read the elevation given for your proximity. Use the pressure altitude setting if available on the GPS when checking aircraft altitude. The pressure setting uses a barometric indicator similar to the barometric altimeter of aircraft. Check with a nearby air traffic controller for the local barometer reading to make pressure compensations for your altitude reading. Tell the ATC you're using GPS altitude measurements if pressure readings are not available.

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