In Airspace Explorer, altitude of aircraft over the UK is calculated as above mean sea level (AMSL) using the ‘standard’ pressure setting of 1013.2 hPa (hectopascal), equivalent to 1013.2 mbar (millibar) or 29.92 inHg (inches of mercury). This pressure setting is universally adopted as the standard for high-level flight i.e. that above Transition Altitude (TA).
Standard pressure setting altitudes are used in the app primarily for simplicity. However, the sophisticated Air Traffic Management systems our NATS controllers use calculate altitude of flights below TA by using regional pressure settings NOT the standard pressure setting. Regional pressure at sea-level is often referred to as ‘QNH’. The letters QNH historically come from transmitting regional pressure information via Morse code, but these days are commonly associated with the mnemonic ‘Query Nautical Height’. QNH pressure settings vary over time and region and are continuously updated in our Air Traffic Management systems.
An alternative to using standard pressure and QNH, is to use ‘QFE’. QFE uses a fixed position on the ground to calculate the height of an aircraft above it. Often the ground location used within a region of airspace is an airfield. Unlike standard pressure and QNH, QFE does not use mean sea level as its baseline. Again its origin comes from Morse code, but these days the mnemonic ‘Query Field Elevation’ is often associated with it.
It should be recognised that flight tracking apps and websites often use different measures for calculating and displaying flight height and altitudes. This can explain why measures shown across them differ. It should also be noted, that whilst most general users want to know the height of an aircraft above a certain point on the ground (e.g. a QFE derived measure) most apps and websites use the standard pressure setting or QNH to display altitude.
We recognise that the current ability of NATS Airspace Explorer to show altitude is limited to standard pressure setting, so we are currently looking at ways to include QNH. However, we are not currently investigating QFE – as this isn’t a measure we typically use in Air Traffic Control.
If you are specifically interested in the height or altitude of an aircraft above a specific area, we recommend you use a flight tracking system that incorporates QFE or QNH dependent on the downloaded pressure setting from the aircraft’s ADS-B system. Many airports have flight tracking systems available via their websites that incorporate QNH and show flight tracks around their geographic vicinity. In the first instance we recommend you explore those.
More information about flight altitude can be found here:
http://www.skybrary.aero/index.php/Altimeter_Pressure_Settings
https://en.wikipedia.org/wiki/Pressure_altitude
https://en.wikipedia.org/wiki/QNH
https://en.wikipedia.org/wiki/Flight_level#Transition_altitude