Altitude Measurement

Types of Altitude used in Enroute Flight Navigation

Enroute Flight Navigation uses several notions of altitude.

True Altitudes

  • Geometric Altitude or True Altitude AMSL is the vertical distance between your aircraft and the main sea level. To avoid confusion, geometric altitudes are typically indicated in the form “T.ALT 6500 AMSL”.

  • Absolute Altitude or True Altitude AGL is the vertical distance between your aircraft and the terrain. To avoid confusion, absolute altitudes are typically indicated in the form “T.ALT 6500 AGL”.

Enroute Flight Navigation computes the absolute altitude is computed using satellite navigation data and a terrain elevation database.

Barometric Altitudes

  • Pressure altitude is the value shown by your altimeter when set to the standard value 1013.2hPa. To avoid confusion, pressure altitudes are typically indicated in the form “FL 65”.

  • Altitude is the value shown by your altimeter when set to QNH. To avoid confusion, altitudes are typically indicated in the form “6500”.

  • Cabin altitude is the altitude at which an aircraft flying in the ICAO standard atmosphere experiences a static pressure equal to the pressure in the cabin of your aircraft.

Altitude and pressure altitude is used to define airspace boundaries and vertical aircraft positions.

Warning

In Central Europe, true altitudes and barometric altitudes typically differ by 5-10%, but we have seen differences of 15% on warm summer days in Germany. Never use true altitudes to judge vertical distances to airspaces!

Enroute Flight Navigation computes altitude and pressure altitude from static pressure data reported by traffic data receivers and from QNH data reported by nearby airfields. If Enroute Flight Navigation is not connected to a traffic data receiver or if the traffic data receiver does not report static pressure, then no pressure altitude is available. Enroute Flight Navigation measures cabin altitude using the pressure sensor installed in your mobile device.

Note

As a rule of thumb, any traffic data receiver that handles ADS-B signals will report static pressure, as static pressure is required to interpret ADS-B data. In particular, all PowerFLARM devices use and report static pressure.

Density Altitude

Density altitude is the altitude at which an aircraft flying in the ICAO standard atmosphere experiences an air density equal to the density measured by the ambient pressure/temperature sensors of your device.

Enroute Flight Navigation computes the density altitude from METAR data, but only for airfields reporting the necessary data.