Measurement of height above a given reference point is made using either an altimeter or a GPS.

Air pressure reduces with height at a known rate - the pressure lapse rate - of approximately 1 millibar for every 30 feet (up to about 10,000 feet). An altimeter uses a barometer to measure the air pressure and it is displayed as a height reading. In order for it to read accurately the air pressure at a given reference point must be used to calibrate the instrument.

Different types of reading will be displayed according to the reference point used.

QFE

FIELD ELEVATION - Current air pressure at the take off site. When an altimeter is set at this pressure the instrument will read height above the ground. This setting is used during take off and landing and when flying in the circuit.

On the ground your altimeter will read zero.

QNH

NAUTICAL HEIGHT The current mean air pressure over a given region at sea level (if there is no sea, this is a virtual value by adjusting the value at the ground for its elevation), the ‘regional pressure setting’. When an altimeter is set at this pressure the instrument will read the actual flying height or altitude with respect to mean sea level - this level is marked on navigational charts.

On the ground your altimeter will read the elevation of your field above mean sea level.

QNE

PRESSURE ALTITUDE The indicated altitude when an altimeter is set to 1013 millibars. It is used to determine flight levels. A flight level of 40 is equivalent to a pressure altitude of 4000 ft.

On the ground your altimeter will read the elevation of your field above (or even below) the level at which the pressure is 1013 millibars. On a high pressure day the indicated reading will be lower than on a low pressure day because the flight levels have moved upwards ie the reference level of 1013 millibars has moved up but the field has remained at the same level.

Example

Your take-off field is 90 feet above mean sea level. The pressure at sea level is 1023mb - a high pressure day.

The actual pressure on the field will be 1020mb (90 divided by 3 mb lower than sea level)

If you calibrate your altimeter with a pressure of 1020mb it will read zero - QFE

If you calibrate your altimeter with a pressure of 1023mb it will read 90 feet - QNH

If you calibrate your altimeter with a pressure of 1013mb it will read -210 feet QNE

Why? - because the reference level of 1013mb is actually 210 feet above you -

Why do we use this? - well Flight Levels allow vertical separation of aircraft which are flying high and well clear of most ground obstacles, independent of the actual conditions on the ground. In the UK flight levels are usually used above 3000 feet. Constant recalibration of altimeters is not required and their use ensures that aircraft are using the same reference point.

Why are we bothered? - well flight levels are not fixed in the sky - they go up and down according to the pressure variations at sea level so on a low pressure day a flight level of say FL40 - that is 4000 feet QNE - could actually be a lot lower than 4000 feet above sea level and well within our flying capabilities.

On a low pressure day say 1003mb - the reference level will be 10 x 30 = 300 feet lower than sea level so FL40 will actually be at 3600 feet QNH. The lower the pressure the lower the flight level so beware of infringing these upper air lanes - particularly on low pressure days.