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Barometric Altimeter vs GPS Altitude: Which Is Right?

Your phone has two ways to know your height: the GPS, and a tiny air-pressure sensor. The barometer is far better at changes in height; the GPS is better at absolute height without a reference. Used together, they are good at both.

Updated 1 October 2026

How a barometric altimeter works

Air pressure falls as you climb, because there is less air above you. Near sea level it falls by about 1 hectopascal (hPa) for every 8.3 m of height, a little under 1 hPa per 30 ft. Higher up the air is thinner and each hPa is worth more height, about 9.6 m at 1,500 m. An altimeter measures the pressure and converts it to height using the International Standard Atmosphere, which assumes 1013.25 hPa at sea level.

Nearly every iPhone since the iPhone 6 has a barometer; the original iPhone SE is the exception. The sensor is sensitive enough to notice you climbing a flight of stairs.

Its weakness: the weather

Pressure also changes with the weather. When a low-pressure system moves in, pressure can fall 10 hPa in a day, and an uncalibrated altimeter will show you about 80 m higher without your having moved. That is why pilots set their altimeters to the local pressure before take-off and as they fly between regions.

So a barometric altimeter is excellent at relative height: how much you climbed on a hike, the vertical of a ski run, the floor you are on. It is only as good at absolute height as its last calibration, and it drifts as the weather changes.

GPS altitude: absolute but noisy

GPS needs no calibration, but its height is its weakest measurement. All the satellites are above you, so vertical errors run about one and a half times the horizontal ones: tens of metres when the sky is partly blocked. It also jitters from fix to fix, so a GPS-only climb total adds up noise and comes out too high on a flat walk.

There is a second subtlety. GPS measures height above the WGS 84 ellipsoid, a smooth mathematical shape. Sea level, the geoid, sits above or below it by up to about 100 m depending on where you are. Phones convert to height above sea level with a geoid model, and iOS gives both values to apps.

BarometerGPS
Change in heightExcellent, a metre or lessNoisy, several metres
Absolute heightGood right after calibration, then drifts with the weatherNo drift, but roughly ±10 m or worse
Indoors, underground, in a planeWorks (in a plane it reads the cabin)Weak or no signal
NeedsA reference height or the local pressureA clear view of the sky

Getting accurate elevation

  1. Calibrate at a known height: a summit marker, a trailhead sign, a benchmark, or an airport's published elevation. Enter it and the altimeter works out the matching sea-level pressure.
  2. Or enter the local sea-level pressure (QNH) from a nearby airport's weather report or a forecast.
  3. Recalibrate when you can, especially if the weather is changing, and on any multi-day trip.
  4. Use the barometer for totals: climb and descent from the barometer are far more realistic than from GPS.

Some phones help with this automatically: on iPhone 12 and later, Apple offers apps an absolute altitude reading that pairs the barometer with the phone's location data.

In a plane

An airliner's cabin is pressurised, so the phone's barometer shows the cabin altitude, not the plane's. US rules require it to be no higher than 8,000 ft (about 2,440 m) in normal operation. The Boeing 787 and Airbus A350 are pressurised to about 6,000 ft. Watching the barometric altitude climb after take-off and settle at cruise is a direct readout of the cabin pressure schedule. See plane speed and altitude on iPhone.

Sources

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