GTFS-R Train Times Differ from GPS time


#1

What are the observed train timestamps in the GTFS-R feed based on? I’m seeing evidence that the times are 30-40 seconds off GPS or UTC time.

Background

I’m trying to match an app user’s location with train trips and have an issue where phone timestamps differ from the time when a user’s train passed that section of the line.

e.g. In the attached image - I have a user point and a train point 30m apart (yellow = user, red = train) that have a time difference of 33 seconds. The phone is a recent iPhone with iOS 12 so it is unlikely the user’s GPS timestamp is incorrect. Also, the GPS accuracy for the user is good (5m).

This issue is present along the entire trip - note: the green user locations are the only user GPS points that occur within timestamps of the train points (blue and red), the yellow user points do not.


#2

Hi @minus34, we suspect this might be related to the refresh rate or update interval for real-time data. For trains it’s at least 15 seconds but can be more due to network issues or how long data takes to flow through the various systems. You can learn more about this on our troubleshooting page https://opendata.transport.nsw.gov.au/troubleshooting

Basically, if you try to track a vehicle second by second or make a comparison like you’re doing then you’re always going to get some discrepancies. This is also why most apps update their vehicle positions every 15 or 30 seconds.

In your example you will find that after the green dots the data will again match up at the next data refresh point if that makes sense.

Thanks,
Alex