Outdoor walk/run (GPS)

Outdoors there’s no treadmill to report speed and incline, so OmniBandBridge (OBB) reconstructs them from the phone’s GPS plus, where useful, elevation maps (DEM) and the barometer — while the Mi Band / footpod still provides cadence and the biomechanical metrics. GPS and the band are complementary: each covers what the other can’t.

Indoor equivalent

This is the outdoor counterpart of the Treadmill (FTMS) page. On a treadmill, belt speed and incline come from the machine (FTMS or manual). Outdoors, speed comes from GPS and incline/grade — which plays exactly the same role in the power model — comes from a DEM elevation map or the barometer instead of FTMS.

GPS and the band are complementary

Metric	Mi Band / footpod	Phone GPS
Cadence, stride, ground contact, biomechanics	✅ direct	❌
Instantaneous speed	✅ (foot accel)	✅ (Doppler)
Total distance	⚠️ step integration (±5%)	✅ better on long tracks
Route track (lat/lon)	❌	✅
Elevation / grade	❌	⚠️ noisy (DEM/barometer better)

Cadence always comes from the band, never from GPS. Running cadence is ~160–200 spm (≈ 2.7–3.3 Hz); detecting it from a 1 Hz GPS signal is impossible (Nyquist), and even 10 Hz sport chips can’t do it reliably in a consumer app. So the band stays the only trustworthy source for cadence, stride and biomechanics — outdoors too.

Speed is cross-validated between the two: GPS speed is more accurate over long distances, while the band’s speed is a good fallback when GPS drops out (urban canyon, tunnel), and vice-versa.

Elevation & grade — the outdoor replacement for FTMS incline

Grade matters because running uphill costs far more energy than the same speed on the flat — it’s a key input to OBB’s metabolic power. On a treadmill the incline is handed to us by FTMS; outdoors OBB has to derive it, and it does not use raw GPS altitude for it.

Why not GPS altitude

GPS horizontal error (lat/lon) is small, but the vertical error is ±3–5 m — far too noisy to compute a reliable grade from the difference between two fixes. So OBB uses the accurate lat/lon to look up elevation another way.

DEM elevation maps (primary source)

OBB reads ground elevation from a DEM (Digital Elevation Model) at the fix’s lat/lon — the same technique Strava and Garmin use. Because lat/lon is accurate, the DEM elevation is close to the truth and free of GPS vertical noise.

• Offline & real-time. Grade is needed live for power, but online DEM APIs have rate limits, latency and network dependence. So at workout start (first GPS fix) OBB pre-downloads DEM tiles for a bounding box around the start (generous radius) and caches them locally; every lookup during the run is then local — no network, no rate limit, privacy preserved.
• Tile sources. Global baseline from SRTM / AWS Terrarium tiles (free, keyless, elevation encoded in RGB), with an optional finer 10 m layer (e.g. TINITALY) where available for a cleaner grade. A repository tries the fine source first and falls back to the global one.
• Bilinear interpolation + smoothing. GPS fixes rarely land on a DEM grid node and consecutive 1 Hz fixes (~3 m apart) often fall in the same cell, so OBB interpolates between the 4 surrounding nodes and smooths grade over a ~20–30 m distance window. The result is grade averaged over tens of metres — exactly what the power model wants (power responds to sustained grade, not to a single step).
• Post-workout too. The DEM also rewrites elevation and total ascent at export time, populating <AltitudeMeters> in the TCX file.

DEM caveats

The DEM is the terrain, so bridges/overpasses/tunnels return ground elevation (usually fine on roads). Never mix GPS altitude (WGS84 ellipsoid) with DEM altitude (geoid) — OBB uses the DEM consistently. And sending a full track to an online DEM API would be sharing location data, which is why OBB pre-downloads tiles locally instead.

Barometer (complementary, if present)

A barometer gives high-frequency relative altitude change with low latency and no GPS vertical noise, so it’s the natural complement to the DEM: the DEM provides the absolute anchor (and corrects barometric drift from weather), while the barometer captures fast relative changes. OBB reads it via Android’s Sensor.TYPE_PRESSURE when available — but not every phone has a barometer (many mid-range models don’t), so the DEM is the primary source and the barometer is fused in only when present.

Barometer is outdoor-only

A barometer measures air pressure, so it can’t sense a treadmill incline (you’re not actually climbing — the pressure doesn’t change). Indoors, incline is an FTMS/manual value; see Treadmill (FTMS).

From speed & grade to metabolic power

Just like on the treadmill, OBB feeds speed (GPS) and grade (DEM/barometer) into its personalized metabolic power model — grade here is the outdoor equivalent of the treadmill incline. The estimate carries a bit more uncertainty than indoors (GPS speed vs. an exact belt readout, terrain grade vs. an exact machine value, plus wind is no longer eliminated), but the DEM-based grade keeps it solid.

Where the model is explained

This page only covers where the numbers come from. The metabolic power model itself — how OBB turns speed + grade + your profile into watts — is covered in Personalized metabolic power.

Good to know

• Optional “wait for GPS” gate before starting, so the first segments aren’t recorded with a poor fix.
• Distance: on long outdoor tracks OBB prefers GPS distance over step integration; the route (lat/lon) is saved and exported.
• Cadence/biomechanics still require the band — GPS can’t provide them (see above).