How to use GTFS data to track transit vehicles in realtime

Ably has been developing tools to distribute open data sources such as transport systems and weather changes through the Ably Hub using their Ingester tool. They have created useful internal and external tools, but historically they haven't done much to convert the data into something more than its original structure. For transport sources defined by General Transit Feed Specification Realtime (GTFS-R), they have kept the data in the same structures. However, they are now working on a new Hub Product which would represent all transit sources in the same way and provide realtime updates for as many agencies as possible. They have developed a spatiotemporal representation of the data that can be restricted to certain geofenced regions, with the limit of a message’s data being either a max time or max region. This method was decided on as it strikes a good balance between message publish rate and granularity of information provided. They have also developed methods for predicting positions at set times and converting times and distances to positions. The result is a tool that can be scaled easily, with accurate positioning based on schedules and realtime updates, all while providing subscribers with fine-grained control over the data they're interested in.