Warnings that run in two hours, then stop
The optional elevation-warning feature does background work only while you are near a risky elevation, in sessions that shut themselves down. How the 2-hour sessions are designed, and what happens to the battery.
Published on 3 min read
Sea Level has exactly one notification, and it is optional, local, and short-lived. You can enable it, forget it exists, and it will tell you once when the ground under you drops below the 70-meter line. This post is about the 2-hour session that makes that promise without turning the app into a constant background watcher.
What a session is
When you turn on Elevation warnings, you are not granting the app a permanent background license. You are granting it a lease: a session, up to 2 hours, that starts and stops on its own. Plan it as "if I am about to cross a coastal basin this afternoon, warn me at the crossing", not "watch every second of my life".
The mechanics are straightforward. While the session runs, the app checks your position on the 70-meter map at intervals, and from the moment your location enters a cell that would be flooded, it fires one local notification. The notification is generated on the device; there is no push service and no server involved. When the 2 hours are up, or when you leave the risky area, the session ends. It does not renew itself.
Why the limit exists
Apple and Google both restrict what background location may do, and both expect background location to be earned, not assumed. A running app that keeps the location stream on continuously is the difference between a feature and a tax on the battery. The 2-hour session is the design that respects that division: it is short, so the battery cost is small; it is finite, so the system stops asking you to re-justify it; and it is explicit, so the permission dialog can say exactly what it means.
That last part matters more than the battery. Foreground location is scoped to "the app is open", which is easy to agree to. Background location is scoped to "the app is not open", which deserves a warning in its own right. On iOS, background sessions need the Allow Always option; on Android, Allow all the time. The app asks for those only when you enable warnings, not at first launch, and never both at once.
What it means for privacy
A 2-hour location session is location data being collected, and the policy treats it as such. The checks happen and the results are cached on the device, exactly like the foreground measurements. There is no analytics attached and nothing new leaves the phone: the same coordinate-to-grid lookups happen as when you hold the app open. You can revoke the permission, or switch the warnings off, at any moment in Settings, and the session stops immediately.
When the notification does not come
Two cases worth knowing. First, the check interval is coarse enough that a fast car crossing a narrow coastal band can slip through between checks; the feature is for walks and drives, not for evacuation duties. Second, the 70-meter map is an envelope, not a forecast, and no warning should ever be read as "water is coming now". If you need a reason not to read it that way, the map's own page states its assumptions in full.
The honest summary of the feature is its limit: two hours, on your side, one message, no tracking. That is the whole interface, and the whole cost.