Why GPS Altitude Doesn't Match the Map

GPS measures height above a smooth mathematical ellipsoid; maps measure elevation above mean sea level (the geoid) — and the two reference surfaces differ by −100 to +85 meters depending on where you are. Add GPS's naturally larger vertical error, and disagreement is expected, not a fault.

Two different zeroes

Same hilltop, two different zeroes: GPS counts from the smooth ellipsoid, maps from lumpy mean sea level.

“Height” needs a starting surface, and there are two in play. Satellites can't sense where sea level is, so raw GPS heights are measured from the WGS-84 ellipsoid — a smooth idealized squashed sphere. But sea level follows gravity, and gravity follows the planet's uneven mass, producing the lumpy surface called the geoid — the zero that maps, signs and intuition use. The gap between the surfaces (the geoid undulation) ranges from about −100 m south of India to +85 m near New Guinea. A raw GPS height can therefore be 'wrong' by tens of meters while being perfectly correct — it's answering a different question.

Why the phone is usually fine and the browser sometimes isn't

Modern phones apply the geoid correction before showing you altitude, so the number in a hiking app is usually mean-sea-level elevation. The browser geolocation API, though, may pass through the raw ellipsoidal height, or none at all — which is why web tools show stranger numbers than apps. On top of the reference confusion sits plain measurement error: because every GPS satellite is above your horizon, the vertical solution is weakly constrained and its error runs 1.5–3× the horizontal error. A fix that's 5 m accurate horizontally can honestly be 15 m off vertically.

Barometers and terrain models — the other two answers

Phones and watches add a barometer: superb at relative change (it feels a 3 m staircase) but adrift absolutely, since weather moves the baseline hourly — that's why fitness apps recalibrate it against GPS or known points. The third source is a terrain model (DEM): a survey-derived grid of ground elevation, like the Copernicus data behind our elevation tool. It answers 'how high is the ground at these coordinates' — rock-solid for a hilltop, but it reports the ground even when you're ten floors up.

Classic confusions, resolved

Negative altitude at the beach: usually the ellipsoid/geoid gap, occasionally plain vertical error — both normal. Altitude bouncing ±10 m while you stand still: weak vertical geometry doing its thing; watch the trend, not the ticks. Airplane 'altitude' is a third system again — pressure altitude against a standard atmosphere, which is why 10,000 m on the seat-back display shouldn't be compared with anything here. Terminology worth keeping straight: elevation is ground above sea level, altitude is you above sea level, height is you above the ground.

Which number should you trust?

Standing on the ground and want the map answer: the terrain model — it's what the elevation page shows first, with your device's GPS altitude alongside for comparison. Flying a drone or paragliding: GPS/baro fusion, since you're not on the terrain. Judging a climb on a run: the barometer's relative change. Seeing a 30 m disagreement between sources isn't a bug to fix — it's three instruments answering three different questions, and now you know which question each one answers.

Frequently asked questions

Why does my GPS show negative altitude at the beach?

Mostly the reference gap: raw GPS height is measured from the ellipsoid, which sits above or below sea level by up to ~100 m depending on region. Add GPS's larger vertical error and seaside negatives are routine.

Is a barometer more accurate than GPS for altitude?

For change, yes — it resolves a single staircase. For absolute altitude, no: weather shifts its baseline constantly, so devices calibrate the barometer against GPS or known elevations and use each for what it's good at.

Which is right when my watch, phone and your site disagree?

Possibly all three: one may show corrected GPS altitude, one barometric altitude, one terrain elevation at your coordinates. Standing on the ground, the terrain model is usually closest to the map answer.