# [eli5] How does an altimeter work? How does it know how high you are off the ground?

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[eli5] How does an altimeter work? How does it know how high you are off the ground?

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The altimeter measures the pressure of the atmosphere at the plane. The pressure is higher at ground level.

regular altimeter reads air pressure, and subtracts from it the air pressure at the ground level (which varies on weather). If you use this to land, somebody at the airport needs measure and tell you the air pressure there

radar altimeter indeed knows exactly how high you are, but only above a single spot on the ground directly below you. There is [at least one major plane crash](https://admiralcloudberg.medium.com/all-the-presidents-men-the-story-of-the-smolensk-air-disaster-and-the-death-of-lech-kaczyński-590a3977f) where pilots were misguided by a ravine right in front of the landing strip

finally, GPS-based systems such as [Terrain Avoidance](www.flyingmag.com/how-it-works-terrain-awareness-and-warning-system/) can tell you exactly where you are in all 3 dimensions. it also uses radar altimeter

It works based on detecting atmospheric pressure. You start by knowing the elevation of where you are. i.e. you need to know how high the land is where you are from sea level. Most mechanical altimeters have a small knob on them that allows you to make adjustments based on the current barometric pressure and this his how you calibrate it. You set the altimeter to the current barometric pressure and make sure it matches your current elevation.

The higher you go up in altitude, the lower the atmospheric pressure, this is why your ears pop when you go up in an elevator. Altimeters sense this difference in pressure through either an electronic pressure sensor or a device called a bourdon tube. It’s basically a small tube of metal that’s formed into an arc. As the pressure around the tube changes, the tube flexes and changes shape. The tube flexing pushes against a lever that turns some gears attached to a dial to show your current altitude.

The cheapest kind of altimeters use barometric pressure sensor readings to detect changes and map them as elevation. This only works well if the barometric region is somewhat static. For instance, locations along a coast like Southern California USA can have drastically different micro climates within just a few miles of the water, and the differences in water heavy coastal air and dry desert winds can get interpreted as elevation changes.

I think some older devices used a GPS location and lookup tables that contain the average elevation of the region/location.

I think newer devices use extra GPS signals from additional sats and then do the geometry calculation.

GPS sats are basically beacons that transmit/stream their location and the time. The most accurate GPS ground instruments also use the timing of the transmission signal to dial in changes. This is used by systems like the USGS earthquake monitoring sensors.

Additional question: planes are going at speed, so isn’t it hard to get a measure of the actual pressure?