Altimeter essentials

QNH, QNE and QFE: three altitude references

A barometric altimeter measures air pressure. The selected reference pressure determines what its number means. The paraglider stays in exactly the same place – only the meaning of the indication changes.

QNH = altitude above sea level QFE = height above the airfield QNE / STD = pressure altitude; as FL in 100 ft

Where is zero?

QNH and QFE are altimeter pressure settings. For QNE, the altimeter is set to STD 1013.25 hPa. All three indications use the same pressure sensor but different vertical references.

QNH references altitude to mean sea level A paraglider pilot hangs beneath a red canopy above an elevated airfield. A blue arrow measures from mean sea level to the pilot. MSL / sea Altitude above MSL Airfield
QNH

Mean sea level reference

Local pressure reduced mathematically to mean sea level. At a launch site or airfield, the altimeter indicates approximately its published elevation.

Remember: “How high am I above the sea?” – barometric altitude above MSL.

QFE sets the airfield to zero A paraglider pilot hangs beneath a red canopy. A green arrow measures from the elevated airfield to the pilot; the airfield datum is labelled zero metres. Height above airfield 0 m at the datum
QFE

Airfield reference

Measured pressure at a defined site datum, normally an airfield or runway threshold. A vario can similarly use a launch site as its zero point.

Remember: “How high am I above this site?” – not above the changing terrain beneath me.

QNE uses the fixed standard pressure A paraglider pilot hangs beneath a red canopy on one of several schematic pressure surfaces above a mountain landscape. STD 1013.25 hPa shared pressure reference Flight level
QNE / STD

Standard-pressure reference

With the fixed setting STD 1013.25 hPa, the altimeter indicates QNE pressure altitude. For example, an indication of 6,500 ft corresponds to FL 65.

Remember: “Which common pressure surface am I using?” – not automatically altitude above sea level or ground.

QNH → Altitudeabove MSL
QFE → Heightabove the airfield datum
QNE → Flight levelpressure altitude at STD

Same airfield, three indications

The airfield elevation is 600 m MSL and the local airfield QNH is 1000 hPa. The instrument remains on the ground; only the pressure setting changes.

Airfield: 600 m MSL Simplified ISA example
Three altimeter indications at the same airfield With QNH set the instrument indicates about 600 metres, with QFE zero metres, and with standard pressure approximately 710 metres pressure altitude. QNH 600 m 1000 hPa QFE 0 m ≈ 931 hPa QNE / STD ≈ 710 m 1013.25 hPa
Swipe sideways for QFE and QNE
The paraglider has not moved. A higher pressure setting produces a higher indication.
QNH 993 hPa ≈ 832 m QNH from 1000 m QNE
Starting value 1000 m QNE STD 1013.25
QNH 1033 hPa ≈ 1164 m QNH from 1000 m QNE
h(QNH) ≈ h(QNE) + (QNH − 1013.25) × 8.3 m/hPa

Rule of thumb for the change in altimeter indication. The exact ISA conversion varies slightly with altitude. Actual temperature affects the difference between indicated barometric altitude and true geometric altitude.

Why QNE/STD matters for vertical separation

Local QNH changes with weather, time and location. Above the transition layer, everyone therefore uses STD 1013.25 hPa: FL 70, FL 80 or FL 90 then identify the same standardised pressure surfaces for every aircraft. This common scale is what makes planned vertical spacing possible.

QNH, the transition layer and QNE with paragliders Below the transition altitude, altitudes are flown using local QNH. The pressure setting changes in the transition layer. From the transition level upward, everyone uses standard pressure and therefore common flight levels. QNE / STD STD: 1013.25 hPa FL 110 FL 100 Transition Level (TL) Descent: STD → QNH Transition layer Buffer between altitudes and Flight Levels Transition Altitude (TA) Climb: QNH → STD QNH Local pressure · altitude above MSL MSL
Two paragliders on separate Flight Levels: both use the same pressure scale, so their vertical spacing remains unambiguous.

Climbing: QNH → STD

Up to the transition altitude (TA), use QNH and express the level as an altitude. When climbing through the TA, set 1013.25 hPa; above it, use Flight Levels.

The transition layer

It lies between the TA and the transition level (TL), the lowest usable Flight Level. The TL follows current QNH. Under EU procedures, at least a nominal 1,000 ft remains between traffic at the TA and TL.

Descending: STD → QNH

Use standard pressure and Flight Levels down to the TL. When descending through the TL, set current QNH; below it, use altitudes.

Not collision protection by itself: QNE/STD provides the same vertical scale. Safe separation also requires correct settings and serviceable instruments, compliance with clearances and rules, lookout and – where provided – ATC and collision-warning systems.

Published transition altitudes and local procedures differ. Current charts, the AIP and clearances always take precedence.

What is stored in an IGC file?

An IGC B record has separate fields for GNSS altitude and ICAO-ISA pressure altitude. Under the FAI specification, every fix remains referenced to 1013.25 hPa – even if the cockpit vario displays QNH or a different altimeter setting is logged separately.

One fixed raw datum

QNH changes continuously. The fixed ISA datum makes pressure altitudes from different places and times nominally comparable and calibratable.

Independent of the display

The vario setting can change its display, but it must not alter the standard-pressure altitude logged with every fix.

Reproducible later

Starting from the unchanged measurement datum, analysis can later model a QNH line using weather data matched in time and space.

In flight operations a common scale for Flight Levels and vertical separation
In the IGC file a common recording and analysis datum
In the analyzer: The QNE line shows the logged raw standard-pressure altitude. The QNH line is modelled from weather data and, where available, additionally aligned to a plausibility-checked terrain or GNSS altitude reference. QFE is a local cockpit reference and is not needed for IGC airspace analysis. The IGC pressure-altitude field does not prove that the pilot selected STD on the display.
Why do the GPS and QNH lines not always agree?

GNSS and the pressure sensor measure in different ways and use different references. Sensor offsets, the weather model, temperature and the GNSS altitude datum also matter. Small or noticeable differences are therefore possible without either line simply being “wrong”.

Which setting matches which published level?

The label attached to the level is what matters. Actual national procedures and published aeronautical information always take precedence.

AMSL / MSL → QNH

For altitude above mean sea level and terrain or airspace levels published with that reference.

Above airfield → QFE

When height above the airfield or threshold datum is explicitly required. QFE is not the continuously changing height above ground.

FL → QNE / STD

For flight levels using standard pressure 1013.25 hPa, normally above the transition level.

Important with QFE: An indication of 500 m means 500 m above the selected airfield datum. It does not automatically mean 500 m above the terrain directly below you.
Is QNH altitude the true geometric altitude?

Not exactly. Pressure altimeters use a standard-atmosphere model. In air colder than ISA, true altitude above the QNH datum is generally lower than indicated. QNH provides a common barometric MSL reference, but it does not remove temperature, instrument or weather-model errors.

Sources and further reading

This guide explains display and post-flight analysis. It does not replace flight preparation, the AIP, NOTAMs, weather briefing or mandatory altimeter-setting procedures.