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 mean 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 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 1013.2 hPa. 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 are 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.