Why is GPS altitude less accurate than GPS position?

A receiver surrounded by satellites on all horizontal sides can triangulate position precisely. Vertically it only ever sees satellites above it, so errors in the vertical direction cannot be cancelled out as effectively. This effect is known as vertical dilution of precision.

Is a barometric altimeter more accurate than GPS?

A barometric altimeter can be more precise over short periods, often within a few metres, but it drifts as air pressure changes with the weather and must be recalibrated at a known elevation. GPS does not drift. The most accurate devices combine both.

How Accurate Is GPS Altitude? What to Expect and How to Improve It

Q: How accurate is GPS altitude?

Under open sky, GPS altitude is typically accurate to within 10 to 20 metres. Vertical accuracy is roughly two to three times worse than horizontal accuracy because satellites are only visible above the horizon, never below it, which weakens the geometry in the vertical direction.

Why vertical accuracy is worse

Your receiver works out where it is by measuring its distance from several satellites at once. How well that works depends heavily on where those satellites sit in the sky.

Horizontally you are in a good position: satellites appear all around you — north, south, east, west — so errors pull in opposing directions and largely cancel out.

Vertically you are not. Every satellite you can see is above you. None can ever be below, because the planet is in the way. All the measurements push from the same general direction, so vertical errors reinforce rather than cancel. Engineers call this vertical dilution of precision, and it is the single reason your altitude figure is shakier than your position on the map.

Rule of thumb: whatever horizontal accuracy your device reports, expect vertical error to be roughly two to three times larger.

What makes it worse

Condition	Effect on altitude
Open ridge or summit	Best case — 5 to 15 m
Open flat ground	10 to 20 m
Dense forest canopy	20 to 40 m — leaves and wet wood absorb signal
Narrow valley or gorge	25 to 50 m — walls hide half the sky
City streets between towers	30 to 100 m — signals bounce before reaching you
Inside a building or vehicle	Unreliable
First 10 seconds after switching on	Poor until the receiver locks on
Heavy rain or wet snow	Slightly degraded

Five things that genuinely help

Go outside and stay in the open. Nothing else on this list comes close in impact.
Wait 30 to 60 seconds. Receivers improve substantially as they lock onto more satellites and average successive fixes.
Stand still. Moving forces the receiver to keep re-solving instead of refining one position.
Keep clear of walls, cliffs and vehicles. Reflected signals travel further than direct ones and quietly corrupt the solution.
Take the reading at the top. On a summit you have the most open sky you will get all day.

GPS versus barometric altimeters

Many sports watches include a barometric altimeter, which infers height from air pressure. The two approaches fail in opposite ways.

	GPS	Barometric
Typical error	10–20 m	3–10 m when freshly calibrated
Drifts over time	No	Yes — weather moves the pressure
Needs calibration	No	Yes, at a known elevation
Works indoors	No	Yes
Affected by tree cover	Yes	No
Affected by a storm front	No	Yes — can shift readings tens of metres

This is why dedicated hiking watches fuse the two: the barometer supplies fine short-term detail, and GPS keeps it honest over the course of a day.

Why your reading disagrees with the summit sign

You stand on a marked 2,847 m peak and your phone says 2,861 m. Nothing is broken. Three ordinary reasons:

Normal GPS error. A 14 m difference sits comfortably inside the expected range.
Different reference surfaces. Official heights are surveyed against a specific geoid model; your receiver may apply a slightly different correction.
You are not exactly on the marker. Standing three metres away on a rocky summit is genuinely a different elevation.

When 20 metres does not matter — and when it does

For hiking, cycling, trail running and logging the peaks you have climbed, a 10–20 m uncertainty is irrelevant. Your summit is your summit.

It matters for aviation, surveying, and construction — all of which use survey-grade equipment with correction services, not a phone.

Record it while you are up there

Zirvə reads your GPS altitude and turns it into a summit card with your elevation, coordinates, location and date — free, instant, and no account required.

Common questions

Does an expensive phone give better altitude accuracy?

Slightly. Newer phones support dual-frequency GNSS, which reduces atmospheric error meaningfully. But standing in the open matters far more than which phone is in your hand.

Does GPS work without a mobile signal?

Yes. GPS is receive-only from satellites and needs no mobile network. A data connection only speeds up the initial lock.

Why does my altitude change while I stand still?

The receiver keeps recalculating as satellites move and signal quality shifts. A drift of a few metres while stationary is completely normal.

Can I improve accuracy by taking several readings?

Yes. Taking a few readings a minute apart and using the middle value removes most of the random noise.

Try it on your next climb

Free GPS altitude reading, plus a summit card worth keeping.

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