Elevation Correction

Status: Current implemented behavior. Audience: Users and contributors. Implementation: features/activity/elevation, features/manualentry/activity/routeimport, features/manualentry/activity/ActivityEntryViewModel.kt, core/geo, features/settings. Navigation: Settings, Activities, Elevation correction. Related: Feature map, Add elevation tiles, GPX/KML/KMZ/TCX route import, FIT files import, Offline maps support.

A barometric altimeter drifts with the weather, sweat, and age; a watch or phone without one reports noisy GPS altitude. Both inflate elevation gain. Elevation correction replaces the altitude of every point in an imported or recorded route with the height from a digital elevation model (DEM) tile stored on the phone, then recomputes the gain from that profile.

For where to get tiles and how to import them, see Add elevation tiles.

What Gets Corrected

Correction runs when a file is imported: GPX, TCX, KML, KMZ, and FIT files from Settings, Data Importers and Activity Entry, and a folder of FIT files. All of these go through one importer, so they cannot differ.

It also runs when a GPS recording on the phone is finished. The route the review form shows, and the elevation gain saved with the activity, come from the tile. The figure shown while recording is still the phone's own barometer or GPS reading; see Recording of activity.

Activities already saved in Health Connect are never rewritten. Only new imports and new recordings are corrected.

Coverage And Skip Rule

A route is corrected only when every one of its points falls inside an imported tile and none of them lands on a void sample. If a single point is uncovered, the whole file keeps the altitudes it came with, and the import log says so. A profile half from the DEM and half from a drifting sensor would be worse than either.

When correction applies, the file's own ascent total is replaced too, since it came from the sensor that drifted. Distance, timing, heart rate, cadence, and speed are untouched. Elevation gain is computed from the corrected profile with the same smoothing filter a recording uses.

Tiles

Tiles are SRTM-style .hgt files, one per one-degree cell, named by the cell's south-west corner:

Grid	Samples	File size
3 arc-second (about 90 m)	1201 x 1201	2,884,802 bytes
1 arc-second (about 30 m)	3601 x 3601	25,934,402 bytes

N45E007.hgt covers 45 to 46 N and 7 to 8 E. A .zip holding one .hgt can be imported as is. A file that is neither size is refused.

Altitude at a position is interpolated from the four nearest grid samples.

Nothing Is Downloaded

The app declares no internet permission. Tiles are obtained on a computer and imported with the system file picker, the same way offline map packs are.

Local Storage

Imported tiles stay on the device as app support data, not Health Connect records. Deleting a tile removes only that file; activities imported while it was present keep their corrected values. The toggle in Settings switches correction off without deleting tiles.

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Generated from docs/features/elevation-correction.md. Edit it there; changes made here are overwritten.