GTFS-route-shapes

A simple script to generate a single geoJSON shape for each transit route in a GTFS archive.

Why you might need this

In developing web and mobile applications with public transit data, we often want to display the route of a particular line on a map — let's take the MBTA Red Line in Boston as an example. The way the GTFS standard organizes data on the map shape of transit lines, however, makes that a little tricky. There are two problems:

1. Rather than providing the map shape of a route, GTFS provides the map shape of trips, of which one route may have many (see this very helpful sketch of the GTFS schema if that sounds confusing to you). This means a route may have many lines associated with it, usually redundant and overlapping. For example, the MBTA Red Line has two branches in the south, which look like this:

   

   We can overplot both lines, but then we're dealing with redundant points between the northern terminus (Alewife station) and the branch point. Not a huge deal if there's just two shapes, but for many GTFS archives, a route has many such shapes — the whole shapes.txt file for the MBTA GTFS archive is >17 MB. We really want to store the minimal collection of non-overlapping lines that describe the shape of the whole route:

   

2. The lines stored in the shapes.txt table of GTFS archives are often of unnecessarily high resolution for many applications (e.g. overview maps of transit routes), which leads to those large file sizes. We often want to store a downsampled set of points that still adequately describes the shape of the route.

What this script does

This script addresses both of these problems, by:

1. Removing any overlap in control points from shapes (lines) describing the same route, and

2. Downsampling those to a specified tolerance (I've set it to about 30m, or the approx. width of Mass. Ave., a main thoroughfare in Boston).

Input

An expanded GTFS archive. Run the script from within the directory containing the csv files of the archive.

Output

A geoJSON file named route_shapes.json, containing a FeatureCollection object with the same number of Features as there are unique route_ids in the supplied routes.txt file. Each Feature contains a MultiLineString or LineString, which traces the shape of the route, and a route_id property.

For example:

    {
      "type": "FeatureCollection",
      "features": [
        {
          "geometry": {
            "type": "LineString",
            "coordinates": [
              [
                -70.991534,
                42.413641
              ],
              [
                -70.992021,
                42.4127
              ],
              ...
            ]
          },
          "type": "Feature",
          "properties": {
            "route_id": "Blue"
          }
        },
        {
          "geometry": {
            "type": "MultiLineString",
            "coordinates": 
            ...
          },
          "type": "Feature",
          "properties": {
            "route_id": "Green-D"
          }
        },
        ...
      ]
    }