How the app finds your elevation
Sea Level does not guess: it combines GPS, a barometer when the phone has one, and public elevation data as the fallback. How the three sources work, how they rank, and where the meters come from.
Published on 3 min read
Elevation is the one measurement that does not come straight out of the sky. GPS can tell the antenna its three-dimensional position, but the vertical part of GPS is the least accurate part of it, and a smartwatch on a wrist is not an antenna. So the app plays three sensors off one another. This post is the whole trick, in the order the app trusts them.
The order of trust
Sea Level ranks its sources by how steady they are, and mixes them with a simple rule: each source wins only where it is reliable, and the barometer, when there is one, is the secret weapon.
- Barometer, when the phone has one (most modern phones do). A barometer measures air pressure, which falls by about one hectopascal per 8.5 meters of altitude. That makes it the most stable elevation sensor a phone carries: short-term relative changes are accurate to a meter or two. What it is not, alone, is an absolute answer, because pressure also changes with the weather. At sea level today it might read 1013 hPa, tomorrow 990.
- GPS gives the absolute anchor the barometer lacks, with an ugly vertical error of maybe 5 to 15 meters in open sky, far worse under trees between buildings. GPS is used to pin the baseline.
- Public elevation data is the tiebreaker: the app sends your coordinates to the open elevation service, which samples a global 30-meter elevation grid and returns the ground height at that exact point. Ground elevation, not where the device is held: The app combines the two so that "your elevation" means the height of the ground under you, matching the 70-meter map it compares it to.
The ranking exists because the strengths line up with the weaknesses. The barometer is precise but relative. GPS is absolute but noisy. The grid is exact but static and coarser than reality. No single source is trustworthy, and the blend is.
Why the number still drifts
Even blended, your elevation will wander, and it is worth knowing which direction the noise comes from. Barometer calibration depends on the last known reference, so a phone moved from a coastal city to a mountain town can carry a stale baseline for a ride. Cloudless pressure swings of 30 hPa in a storm front translate into 250 meters of apparent climb, which is why the app keeps computing, and why it rarely quotes you a single frozen number.
The honest phrasing is that the app shows you your elevation to the precision its sensors allow today, usually within a few meters when the sky is open, and it draws the 70-meter comparison the same way. Two meters of sensor noise is nothing against a seventy-meter rise, and everything against a flood map drawn to the meter. The compare chart keeps both truths at once.
The one request that leaves the phone
Everything above runs on the device except the grid lookup. The coordinates leave the phone once, without a name, an email or a device identifier, to the open elevation service, and the result comes back. Nothing else crosses the boundary, which is the entire subject of the privacy policy.
Meters or feet
The default is metric, because the scenario pools under discussion are computed in meters and because most of the world reads them in meters. There is a switch, and it changes the app, not the calculation: the underlying number never leaves meters.