The altimeter is essentially a barometer, a device that measures the difference between two pressure levels.

It’s better known as “that thing in a plane that tells you how high up you are”. Exactly what that means varies, but in aviation there are basically two ways of expressing it:

Altitude, or QNH, is your height in feet above mean sea level. Sea level is very often lower than the ground below you, but QNH is useful because it gives a common level to work with between airfields, and constantly updating your setting isn’t really feasible.

Height, or QFE, is your height above the ground level at a certain location, generally an airfield. Hence a good way to remember the two: QNH is Nautical, QFE is airField. The uses of calibrating your altimeter to the ground level are obvious (especially when landing), but of course the ground doesn’t stay at that level and away from the airfield the reading isn’t always useful.

(There are also Flight Levels used by airliners but shhhh that’s just everyone using 1013 millibars as a standard pressure setting)

There is no magical device that tells you how far the piece of ground directly below you is. Seriously! Not unless you have some very expensive laser/sonar setup of some kind. Regardless, that’s not how the altimeter works. This is important for new pilots, because it means air pressure & calibration has a big effect on your displayed altitude.

As stated earlier, the altimeter compares air pressure outside the plane to something. That something is called your ‘pressure setting’. When you switch from QFE to QNH for example, you are switching your ‘datum’, or point of reference: or which air pressure you will be measuring against.

Confused? It’s simpler than it sounds. Take a plane flying at 2000ft. In general, air pressure is less by about 1 millibar for every 30ft you ascend. So say the pressure at sea level is 1015 mbar. At 2000ft, it should probably be about 949 mbar. So an airfield on a mountainside at 2000ft would in this case report a QFE of 949, while QNH is 1015. If a pilot were to be flying in the area at 2500ft (on that QNH ), then swtich to QFE, their altimeter would now display 500ft instead. They are now using a point of closer pressure to measure the vertical distance (pressure outside the plane being about 933), hence the closer reading!

There’s more to it than that, of course; pressure at sea level varys over time and from place to place, temperature alters the accuracy, etc. But this is the crux of altimeters.

And now you know!