30blay | 1 year ago | on: No GPS required: our app can now locate underground trains
30blay's comments
30blay | 1 year ago | on: No GPS required: our app can now locate underground trains
Great question! I'm one of the developers behind this project and your guesses are good.
Other difficult cases include trains stopping between two stations (doesn't happen everywhere, but it's frequent in NYC), or a user walking fast onboard a moving train, which can be mistaken for the user having gotten off the train.
Taking a train in the opposite directions will break the assumptions we make and we won't know until the next GPS location
30blay | 1 year ago | on: No GPS required: our app can now locate underground trains
I'm one of the developers behind this project, so I want to jump in because we did explore some of these solutions.
You are correct in saying the low frequency acceleration from starts, stops and turns can be distinguished from the higher frequency noise.
One big challenge was with orientation. Acceleration can look the same as deceleration and turns from the sensor's perspective, if you turn the phone around. Taking the integral of the gyro reading, the error would grow quadratically, and we found magnetometer readings unreliable depending on the vehicles.
Your point about the hunting oscillation is interesting and I agree, estimating the speed would be a great improvement.
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As others have stated, not all phones have pressure sensors, and the quality of the readings also varies a lot between different models. For example, we had one device where the readings would spike when squeezing the phone.
Transit also doesn't have permission to read the pressure sensor, and our use case wouldn't justify asking for it.