@transitapp just launched underground subway directions. Yes, even in areas without GPS! In deep subways and metros, GPS accuracy is bad. And while some cities have WiFi and 5G in their tunnels, many don't. So Transit's engineers got to work and found a way to triangulate your location using just your phone’s vibrations. First they distinguished “train moving” vibrations from other ones, like “walking" or "going up the escalator". Their engineers collected vibration data in Montreal, then went to NYC to collect more to make their model generalizable to different types of trains, tracks, and stop patterns. They found that train vibrations are usually around 5 Hz, while walking ones around 2 Hz — but the frequency bands are noisy. So they used Fourier transforms to match frequencies to motion types, then trained a motion classifier to get the prediction to 90% accuracy. Now, whenever you use GO in Transit app, the app will detect you're on a moving train. It will use that insight, look at your last-known GPS coordinates and the train schedule, and then update your location and ETA so you know when to get off. Best of all? The feature works entirely offline — no need for clouds in the tunnel ;) Now subway riders in can get step-by-step navigation even in GPS deadzones. No more trying to figure out where you are by reading the little map in the train car.