Every ski tracker ends the day with a top speed, and every group chat has someone claiming 70 km/h off it. Here’s the uncomfortable bit: GPS speed is an estimate with a known error, and at the moment your app recorded that peak, the error might have been ±2 km/h — or ±10 km/h, if you were between trees or mid-turn with a poor satellite view.
Phones don’t compute speed by dividing distance by time between two
fixes (that would be noisy beyond use). They use the Doppler shift of
the satellite signals themselves, which is far better — and still
imperfect. iOS actually reports its own confidence alongside every
speed reading: a speedAccuracy value in metres per second. Most
apps throw it away.
Alpy keeps the accuracy value from the exact moment your day’s top speed was set, and prints the headline number with it: 54 ±3 km/h. Two things follow:
No other ski app we’ve found publishes its measurement error. We think that’s backwards: the error bar is what makes the number mean something. It’s the same reason Alpy labels turn counts and airtime as estimates — measured numbers and estimated numbers both have a place, as long as you can tell which is which.
Alpy is a free ski app for iPhone that labels every number by how it was measured. More.