The short answer
Elevation
How high the ground is. A property of a place: Salzburg's elevation is 424 m whether or not anyone is standing there.
Altitude
How high a thing is. A property of an object: yours changes when you take the lift, the aircraft's changes constantly.
In everyday use they are swapped freely and nobody is confused. Where it starts to matter is the next question down — above what? Because your phone, your watch, your map and your car satnav can each answer honestly and still give you four different numbers.
The four heights your devices might mean
1. Above the ellipsoid
Satellites do not know where the sea is. They compute positions against a smooth mathematical shape — the WGS 84 ellipsoid — that approximates the planet without matching it anywhere in particular. Raw GNSS height is measured from that shape.
2. Above mean sea level
What people actually mean by height. The reference is the geoid: the surface the oceans would settle into under gravity alone, extended under the continents. Gravity is not uniform, so the geoid is lumpy — and the gap between it and the smooth ellipsoid is the correction nobody mentions.
Outside Salzburg, where this app was written, that gap is 45.65 metres. Worldwide it runs from roughly −105 m to +85 m.
So a device reporting ellipsoidal height here reads about 46 m higher than one reporting height above sea level — while both are working perfectly.
3. From a terrain model
Web maps and elevation lookups do not measure anything. They take your coordinates and read a height out of a digital elevation model built from satellite radar. These come in grids — 30 metres per cell for SRTM, 90 for Copernicus GLO-90 — and every cell holds one number for the whole square. On a flat plateau that is fine. On a ridge line or in a narrow gully, one number for a 90-metre square is a considerable compromise, and models built from different missions disagree with each other by a few metres even on easy ground.
4. From a barometer
Your phone's pressure sensor infers height from air pressure. It is by far the best of the four at noticing change — it will see a single flight of stairs — and the worst at absolute position, because the weather keeps moving its zero. Left uncalibrated overnight while a front passes, it can be tens of metres out. How that works, and how to correct it →
Why an iPhone and an Android phone can differ by 46 metres
This one catches people out, and it is not a bug in either. The two platforms document different reference surfaces for the same property:
- Apple documents
CLLocation.altitudeas height above mean sea level, with the ellipsoidal value offered separately. - Android documents
Location.getAltitude()as metres above the WGS 84 ellipsoid, with a sea-level variant added later.
Two phones side by side, both correct, one reading about 46 m higher than the other here — and further apart than that in parts of the world where the geoid is steeper. If an app or a web page passes the platform value straight through without saying which one it is, that difference lands in front of you unexplained.
Worth doing before trusting any app on this: stand two phones side by side and compare. It takes a minute, and it tells you more about an altimeter app than its screenshots do.
How to check your elevation on an iPhone
You already have it. Open Compass — the elevation is shown at the bottom of the screen, under the bearing. It needs location access, and on first use you may have to let it settle for a few seconds.
What the built-in reading does not tell you is how sure it is, where the number came from, or whether it moved because you climbed or because the weather did. That is the gap between a readout and an instrument.
Or check it here, in the browser, alongside what your phone's GPS thinks — and the gap between the two.
Check my elevationSo which number should you use?
- How high is this place? A terrain model. It knows the ground, and it does not care about the weather.
- How high am I, right now, in a building or on a lift? Your device's own altitude — the terrain model has no idea you left the ground.
- How much did I climb today? The barometer, and it is not close. Satellite height is two to three times worse vertically than horizontally, so summing GPS heights over a day accumulates noise into hundreds of phantom metres.
- Something exact to the centimetre? None of these. Surveying is a different discipline with different equipment.
The honest version
Every height is a measurement plus a choice of what to measure it from. An app that shows you one number and no ± has made that choice for you and declined to mention it.
VERT reads the barometer, shows the uncertainty the system actually reports, separates what you climbed from what you descended, excludes lifts, and writes the day to disk as you walk. It will not tell you your height to the centimetre. Nothing honest will.