GTFS, subway and GPS (I)

platform-gpsThe subway is a very complex mean of transportation. It cannot be reduced to point. While The dimensions of the stations are on average between 80 and 150 meters, sometimes even up to 200 m. The present case is about a station with a central platform, but it can be easily adapted for a station with separate platforms.  The problem here is the position of the station on a map, especially from the GTFS point of view.

The subway stations and GTFS

Let’s say we have the timetables of the subway, along with the lines, interconnections and so on.  The position of the station on a map will be given by its GPS coordinates. But, wait, what are the GPS coordinates of a subway station ? Today, it is customary to give the position of point X, i.e. the geometric center of the platform.  This is not very useful. First, in case of a long platform, it does not separate the two ways or semi-platform (see figure at the beginning of the article). platform-sep-gpsThen, for stations with separate platforms, the point is an imaginary place, right on the tracks. In other words, a place hard to reach. Not very realistic. What can we do about that ?  There are several solutions. We are analyzing one such solution.

The proposal

Instead of using one central point marked with X, we use four different points, marking  the two ends of each of the semi-platforms. This can work in case of stations with more than two tracks, too.

The idea is to mark the two ends of the semi-platform. Even for the stations with a curved shape, the limitations on the radius of the tracks allow a precise positioning. The real connection between two subway lines can be expressed with much greater precision and transfer times care closer to the reality.

For the GTFS, the GPS position of the station becomes the GPS  position of the head of the train, for example point A. For the other way, we can use a different GPS position, for example D. Also, for mobile applications, the position of the passenger is expressed in a better way. A train is said to be in the station if both head and tail are between points A and B (for example).

Proposed change in GTFS

Today, stations have a single GPS position. For a big square, with N individual bus stops, we have N different GPS positions. For the subway, we can have a different positioning scheme. Each semi-platforms has two ends, which could be named head and tail. Why not use a supplementary file with with all the GPS coordinates (points A to D)?

We will talk about indoor positioning in another article.

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