Find how high above sea level you are by combining GPS with a digital elevation model, with estimated pressure and boiling point.
Everyone knows GPS pins your position on a map well. What almost nobody knows is that it does much worse vertically: altitude error is typically two to three times the horizontal error, with long tails. This happens because every satellite the receiver can see is above it, never below, and that awkward geometry leaves the vertical coordinate poorly conditioned. A single reading can be tens of metres off. There is a second, sneakier problem: which reference the altitude uses. Two surfaces are possible. The WGS84 ellipsoid is a smooth mathematical model of the Earth, while the geoid is the surface of equal gravitational potential and corresponds to mean sea level. They do not coincide, and the difference between them, the geoid undulation, exceeds thirty metres in parts of the world. Because Android and iOS apply different conventions, two phones side by side can show noticeably different altitudes, each consistent with its own definition. This tool works around both difficulties using a third source. The headline reading comes from a digital elevation model, the Copernicus DEM, which gives terrain height above sea level in ninety-metre cells. Because that value is orthometric by definition and free of GPS vertical noise, it answers the question people are actually asking. The GPS number stays visible alongside it, filtered through the median of several readings and clearly labelled. One warning that saves frustration: there is no barometer on the web. Native altimeter apps often use the device pressure sensor to refine altitude, but no browser exposes that sensor, and the proposed API for it never advanced. The pressure shown here is derived from the altitude through the standard atmosphere, in the opposite direction, and serves as context rather than measurement.
These references help contextualize formulas, standards, APIs and limitations used on this page. They do not replace professional validation when a result has legal, financial, medical or operational impact.
Geometry. The receiver computes position from the intersection of spheres centred on the satellites, and every visible satellite sits above the horizon, never below. That makes the vertical component far more sensitive to small timing errors than the horizontal ones. In practice vertical error runs two to three times the horizontal, so a fix with five metres of accuracy on the map can be fifteen metres off in height.
Tap to turn on GPS. Your location is used only to compute the result on this screen and is never sent to our servers.
These three values are computed from the altitude using the International Standard Atmosphere, not measured. No browser exposes a barometer, so the real pressure right now can differ by tens of hPa as weather fronts pass.
The geolocation specification defines altitude as height above the WGS84 ellipsoid, but operating systems disagree. Android passes through the raw ellipsoidal height, while iOS already converts to height above mean sea level. The difference between the two is the geoid undulation, which across Brazil runs roughly 10 to 30 metres negative. That is why this tool leads with terrain elevation, which is orthometric by definition, rather than the GPS number.
Your position is used only on this screen. Coordinates are rounded before the elevation lookup, and that request goes straight from your browser to the public API without passing through our servers. Nothing is stored.