Extracting town road networks from OpenStreetMap

A while ago I worked on  a project that analysed the road networks in a number of towns and cities in England. Initially, we used the Ordnance Survey’s Integrated Transport Network (ITN). The process to extract an ITN network for a specific town was as follows:

  1. Using Digimap, extract the ITN network for a bounding box around the specific town.
  2. Extract the boundary polygon for the town and then trim the network to that polygon
  3. Tidy the road network – removing nodes (junctions) without edges (links), edges without a node at either end and disconnected islands. Fortunately, these tasks can be carried out within QGIS.

As you can imagine, this process is time-consuming and it limited the number of towns that we could analyse.

Since then OSMNX has been released and it promises:

  1. the automated downloading of political boundaries
  2. downloading and constructing of street network data from OpenStreetMap
  3. the algorithmic correction of network topology
  4. the ability to save street networks to disk as shapefiles
  5. the ability to analyze street networks, including calculating routes, projecting and visualizing networks, and calculating metric and topological measures (Boeing, 2017)

Implementing OSMNX

The first iteration of the code used OSMNX’s graph_from_place method, which takes, as a parameter, the name of the place of interest, in the format:

Blackpool,United Kingdom
Bournemouth,United Kingdom
Bristol,United Kingdom

This raises two problems. Firstly, the call to the OSM Nominatem does not always yield a polygon, which graph_from_place requires, but a point. This can be seen by typing Blackpool, United Kingdom into OSM’s search box:

OSM nominatim example
OSM nominatim example

Clicking the first result returns a point, and the second result a polygon. This can be solved by trapping the error and using the which_result=2 parameter value for graph_from_place if the first attempt fails.

The second issue is less easily soluble; many of the OSM boundaries do not match the town’s limits. Consider the neighbouring towns of Harrogate and York:

Harrogate and York
Harrogate (left) and York boundaries (right) from OSM with their respective OSM road networks.

Here OSM has used the local authority’s boundaries for each of the two towns rather than the urban extent. This was a common occurance amongst many of the towns investigated. Clearly a different set of town boundaries is required.

One solution is to to use the ONS Major Towns and Cities (December 2015) Boundaries. This dataset provides a better match to the built-up areas; for example, with Harrogate:

harrogate ONS boundary
Harrogate ONS boundary

… and York:

York ONS boundary
York ONS boundary

Now that we have a usable set of boundaries we can extract the OSM networks for the towns and cities.

We can supply these boundaries to the OSMNX method graph_from_polygon method but the polygons need to be in lat/lng co-ordinates (specifically EPSG 4326). The output of the script is a dictionary containing the basic statistics for each town’s road network, which is pickled and can be unpickled for examination.

The Python script:

  • Reads the town ONS boundaries from a Shapefile
  • Convert the boundaries to a EPSG 4326 reference system
  • For each town boundary
    • get the geometry for the boundary
    • extract the network for that geometry (graph_from_polygon)
    • save the network(graph) as graphML format
    • save the network’s nodes and edges as Shapefiles in OSGB (EPSG 27700) reference system (note that this is not particularly straightforward – see the script for more details).
    • add the network’s basic statistics to a dictionary
  • Pickle the statistics dictionary to a file

Some issues still remain; consider Barnsley below, where the town is split by a river, and the OSM network gets disconnected.

Barnsley ONS boundary and network
Barnsley ONS boundary and OSM network

To process all 112 towns in the ONS dataset, using my laptop, took approximately 100 minutes.

The script is on GitHub, as is a small script that unpickles the network statistics.

Boeing, G., 2017. OSMnx: New methods for acquiring, constructing, analyzing, and visualizing complex street networks. Computers, Environment and Urban Systems, 65, pp.126-139.

 

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