Using Python to plot the city graph of France

Author: Tran Thu Le & ChatGPT
Date: 14/02/2023

Here’s a tutorial on how to plot the city graph of France using Python:

To begin, we’ll load the city data in the geojson format by importing the required libraries and accessing the dataset available for download at France-geojson - Departements. The downloaded file should be saved as france.json.

import numpy as np
import matplotlib.pyplot as plt
import networkx as nx
import geopandas as gpd
from scipy.spatial import Delaunay

cities = gpd.read_file('france.geojson')

Next, we’ll define a function get_data that extracts the necessary data for the graph, including the centroids of each city, the simplices (triangles) of the Delaunay triangulation, and the edges of the graph:

def get_data(cities):
    cen = cities.centroid
    x = np.array(cen.x)
    y = np.array(cen.y)
    centroids = np.vstack([x, y]).T  # shape (n, 2)
    tri = Delaunay(centroids)

    edges = set()
    for simplex in tri.simplices:
        edges.add((simplex[0], simplex[1]))
        edges.add((simplex[1], simplex[2]))
        edges.add((simplex[2], simplex[0]))

    edges = [list(e) for e in edges]
    edge_lengths = []
    for e in edges:
        length = np.linalg.norm(centroids[e[0]] - centroids[e[1]])
        e.append({"len": length})
    return centroids, tri.simplices, edges

Now, we’ll call get_data to obtain the necessary data for the graph and define a dictionary pos that maps each node index to its corresponding centroid coordinates. Now create a networkx.Graph object and add the edges to it. We’ll also remove edges that are too long.

centroids, simplices, edges = get_data(cities)
pos = dict(enumerate(centroids))
G = nx.Graph()
G.add_edges_from(edges)
LONG_DISTANCE = 1.5
for e in G.edges:
    if G.edges[e]["len"] > LONG_DISTANCE:
        G.remove_edge(*e)

Finally, we’ll plot the cities and the graph using geopandas and networkx, respectively:

ax = cities.plot(figsize=(10, 10))
nx.draw(G, pos, ax=ax)
nx.draw_networkx_nodes(G, pos, ax=ax, node_size=20, node_color="red", )
plt.show()

This will create a plot of the cities of France with the corresponding graph overlayed on top. The nodes of the graph will be displayed as small red circles as you see at the begining of this post.