Elevation Where I Am

Three different numbers are called “elevation”

If this page and your phone’s compass app disagree, neither is broken — they are measuring different things. It is worth knowing which one you are looking at.

Terrain elevation (what this page shows)

The height of the ground surface at a given latitude and longitude, looked up from a terrain model. It does not know where you are standing vertically — on a beach or on the tenth floor of a building on that beach, it returns the same number.

GPS altitude

Computed by your device from satellite geometry. Horizontal GPS is good; vertical GPS is roughly two to three times worse, and errors of 10–30 m are ordinary. It also needs a correction to convert from the mathematical ellipsoid the satellites use to actual sea level, which not every device applies consistently.

Barometric altitude

Inferred from air pressure by watches, aircraft instruments and some phones. Excellent at detecting change — it is what counts your stairs — but it drifts with the weather. A passing front can move the reading by tens of metres without you going anywhere.

Where this number comes from

Elevations are looked up from the Copernicus digital elevation model via the Open-Meteo elevation API. It is a global grid with roughly 90-metre spacing, built from satellite radar. When you click the map, the value returned is the model’s height for the grid cell containing that point — so clicking anywhere inside the same ~90 m square gives the same answer.

The “Use my location” button only uses your device to obtain a latitude and longitude. The elevation itself still comes from the terrain model, which is deliberate: for a fixed point on the ground, the model is considerably more stable than your device’s own altitude estimate.

What “above sea level” actually means

There is no single sea to be above. Sea level is not a sphere and not an ellipsoid — the ocean surface bulges toward concentrations of mass and sags away from them, so gravity alone makes it lumpy by tens of metres, quite apart from tides and weather.

Two reference surfaces are therefore in play. The ellipsoid is a smooth mathematical figure — the one GPS satellites compute against, because orbital mechanics needs something tidy. The geoidis the lumpy surface gravity actually produces, which is what “sea level” means and what every national survey measures heights against. The separation between them is large: roughly 85 m above the ellipsoid near Iceland, and about 106 m below it in the Indian Ocean south of Sri Lanka.

Heights here are orthometric — measured from the geoid, the same convention as a topographic map or a road sign at a mountain pass. A receiver that reports raw ellipsoidal height without applying a geoid model can be tens of metres out while being entirely correct about its own position, which accounts for a good share of the disagreements people notice between devices.

When it will be wrong

On open, gently sloping ground the model is typically accurate to within a few metres. It degrades in four fairly predictable situations:

• Steep terrain, where a 90-metre cell has to average a cliff, a ridge line or a narrow valley floor into one value.
• Dense cities, where the radar returns partly reflect rooftops rather than street level.
• Forest canopy, for the same reason — the model can sit some way above the actual ground.
• Anything built or excavated recently. The underlying survey is a snapshot, not a live feed.

None of this makes the number useless; it makes it an estimate of the terrain, which is usually exactly what people want. If you need a legally or structurally authoritative height, that is a job for a surveyor.

What the number actually changes

Elevation is rarely interesting on its own — it is interesting because air thins predictably as you climb, and a surprising number of everyday things follow from that. Using the standard atmosphere:

The oxygen in air stays at 20.9% all the way up; there is simply less air, so each breath delivers proportionally less of it. At 2,500 m you are taking in roughly three-quarters of the oxygen you would at sea level, which is why endurance performance falls off and why altitude sickness becomes a real consideration somewhere around that height.

The boiling point column is the reason high-altitude recipes exist. At 2,000 m water boils at 93°C, so anything cooked in it is being cooked cooler and needs longer — pasta, eggs, dried beans and rice all noticeably so.