We’re building a new subway/bus app at the MTA. Our system includes roughly 300 underground stations, around 150 elevated stations (i.e., above street level), and about 5 at-grade stations (i.e., at street level). We serve roughly 5 million riders a day. We’re diving deep into Core Location on iOS and have found that the altitude values returned from two fields we’re testing aren’t accurate enough for our use case: CLLocation.altitude CMAbsoluteAltitudeData.altitude We need to reliably distinguish whether a user is: At street level On an elevated platform (see attached picture) On any platform in an underground station — most have a single platform level, but some, like 59 St (see attached), have multiple platforms at different elevations. These levels typically differ by at least 15 feet, which should in theory be well within the precision range of a properly calibrated barometric pressure sensor. However, the absolute altitude values we’re seeing from these APIs are often inaccurate and inconsistent — not only compared to ground truth, but also across devices. For example, holding two phones side-by-side frequently yields altitude readings that differ by more than 15 feet. That level of variation makes the data unreliable for our needs. Please see the below photos for more context. URLs.md

We (at the NYC MTA) are building a new subway/bus app and diving deep into location tracking on iOS. We’re encountering an issue with how Core Location functions in the subway, specifically regarding how long it takes to update a passenger’s location as they travel from station to station. As an example, please see this video: https://drive.google.com/file/d/1yaddkjyPEETvTEmClPAJ2wks8b-_whqB/view?usp=sharing The red dot is set manually (via a tap gesture) and represents the ground truth of where the phone actually is at that moment. The most critical moment to observe is when the train physically arrives at a station (i.e., when I can see the platform outside my window). At this moment, I update the red dot to the center of the station on the map. Similarly, I adjust the red dot when the train departs a station, placing it just outside the station in the direction of travel. The trip shown is from Rector St to 14 St. All times are in EST. I’d like to investigate this issue further since providing a seamless underground location experience is crucial for customers. As a point of comparison, Android phones exhibit near-perfect behavior, proving that this is technically feasible. We want to ensure the iOS experience is just as smooth.