Choropleths with folium

Visualizing Geospatial Data in Python

Mary van Valkenburg

Data Science Program Manager, Nashville Software School

folium.Map choropleth

# Construct a map object for Nashville
nashville = [36.1636,-86.7823]
m = folium.Map(location=nashville, zoom_start=10)

# Create a choropleth using the folium Choropleth class
folium.Choropleth(...)

Arguments of the folium choropleth

geo_data - the source data for the polygons (geojson file or a GeoDataFrame)
name - the name of the geometry column (or geojson property) for the polygons
data- the source DataFrame or Series for the normalized data
columns- a list of columns: one that corresponds to the polygons and one that has the value to plot

Additional arguments of the folium choropleth

key_on - a GeoJSON variable to bind the data to (always starts with feature)
fill_color - polygon fill color (defaults to blue)
fill_opacity - range between 0 (transparent) and 1 (completely opaque)
line_color - color of polygon border lines (defaults to black)
line_opacity - range between 0 (transparent) and 1 (completely opaque)
legend_name - creates a title for the legend

# Center point and map for Nashville
nashville = [36.1636,-86.7823]
m = folium.Map(location=nashville, zoom_start=10)

# Define a choropleth layer for the map
folium.Choropleth(
    geo_data=districts_with_counts,
    name='geometry',
    data=districts_with_counts,
    columns=['district', 'school_density'],
    key_on='feature.properties.district',
    fill_color='YlGn',
    fill_opacity=0.75,
    line_opacity=0.5,
    legend_name='Schools per km squared by School District'
   ).add_to(m)

folium.LayerControl().add_to_map()

display(m)

Folium choropleth of school density

folium map of school density with choropleth layer

Let's Practice!

Visualizing Geospatial Data in Python