Ask your phone where you are and it’s accurate to a few metres. Ask it how high you are and it can miss by 10–20 m — vertical error is GPS’s weakest axis, and it gets worse in steep terrain, where the satellites you’d need for a clean fix are hidden behind the mountain itself.
For a ski tracker this matters everywhere: vertical drop is the number skiers brag about, run detection works by watching altitude trends, and airtime detection needs a clean altitude signal. If the raw GPS altitude wobbles ±15 m, a parked phone “descends” and “climbs” all afternoon.
Modern iPhones carry a barometric altimeter alongside GPS. The barometer is the opposite of GPS: beautifully smooth — it resolves altitude changes under a metre — but relative. It doesn’t know your elevation, only how it’s changing, and its baseline drifts as weather systems move through (a passing front can shift it tens of metres over hours).
So you have one instrument that’s accurate but noisy, and one that’s precise but unanchored.
The fix is to fuse them: take the barometer’s smooth shape and pin it to GPS’s absolute level with a slowly-learned offset. The offset has to adapt slowly — over minutes, not seconds — so it absorbs weather drift without re-importing GPS wobble.
This is what Alpy does while you track. Every recorded fix rides the fused altitude: in our tests the fusion cuts GPS altitude noise by more than 65%, which means cleaner run detection, honest vertical totals, and an airtime detector that isn’t fooled by satellite jitter. On phones without a barometer the app falls back to GPS alone — and it tells you which it used, because an instrument that doesn’t disclose its method isn’t an instrument.
Alpy is a free ski app for iPhone — no account, no ads, and your GPS never leaves the phone. What it does.