[转] Making GTFS query more convenient

url：http://ontrakinfo.wordpress.com/2012/10/29/making-gtfs-query-more-convenient/

这简直说出了我的心声。

I have been spending a lot of time parsing the GTFS database. On the surface it is just a simple CSV files. But to extract useful information from GTFS is often unexpected difficult. For example, find the stops from a bus line in sequential order might sounds like basic thing to do. But it is actually non-trivial with GTFS.

One reason is transit service is more complex it seems. It might seems a bus service just hit all the stops in sequence. But the actual service has a lot of variables. The schedule is often different in weekend compare to weekdays. And so does the exact route that it covers. Sometimes a bus is scheduled to run a short route rather than covering the whole length. In more complex case there can be branching where there is a common main trunk and then the buses split to serve two or more alternative destination.

This is the reason why in GTFS one “route” may associate with multiple “shapes”. To find out what shapes are associate with a route, we will have to make a query like this

SELECT
 shape_id
FROM route
 JOIN trips
 JOIN shape
GROUP BY shape_id;

To find out the stops is even more complex. Here we need to join one more table the stop_times. It is also the biggest tables in the GTFS. So this is also the most computation intensive query to do.

SELECT
 shape_id, stop_id
FROM route
 JOIN trips
 JOIN stop_times
 JOIN stops
GROUP BY shape_id, stop_id;

Still most people have a clear concept of what a transit line is where it runs. It shouldn’t be such a pain to compute. A more useful structure should look like below.

    GTFS             More Useful
  Structure           Structure

    route              line
     |                   |
     |                   V
     |                 route*
     |                   | \
     |    shape          |  +-> route_shape
     |     ^             |  |
     |    /              |  +-> route_stops*
     |   /               |
     V  /                V
    trips              trips
     |                   |
     |        stops      |          stops
     |        ^          |
     |       /           |
     V      /            V
    stop_times         stop_times

Here a shift the terminology a bit. The top level entity is a line (i.e. GTFS’ route). This is service that people know of, like a numbered bus line or a metro line. Below that is routes. These are the collection of alternative routes a line may run. The routes are not explicitly represented in GTFS. You can find that by querying all unique shape_id using the first SQL. Another missing piece is the stops. If we can pre-compute all the route_stops using the second SQL once, for the most part we don’t need the giant stop_times table. For applications that do not deal with scheduled time, this is a huge saver. The is one assumption my structure makes though. It is that different lines do not shape that same route. If should be a reasonable assumption. And if there is indeed share route and shape, we should just replicated them as two separate entities.

The original GTFS structure seems to have a transit operator centric view. It allows them maximum flexibility to author and publish their service data. But for application developers, it is not structured for easy traversal. By adding the route and route_stops tables as indicated, it will greatly facilitate the query and operation of transit information.