Using GeoJson

Loading data

Let us load a GeoJSON file representing the US states.

import requests

geo_json_data = requests.get(
    "https://raw.githubusercontent.com/python-visualization/folium-example-data/main/us_states.json"
).json()

It is a classical GeoJSON FeatureCollection (see https://en.wikipedia.org/wiki/GeoJSON) of the form :

{
    "type": "FeatureCollection",
    "features": [
        {
            "properties": {"name": "Alabama"},
            "id": "AL",
            "type": "Feature",
            "geometry": {
                "type": "Polygon",
                "coordinates": [[[-87.359296, 35.00118], ...]]
                }
            },
        {
            "properties": {"name": "Alaska"},
            "id": "AK",
            "type": "Feature",
            "geometry": {
                "type": "MultiPolygon",
                "coordinates": [[[[-131.602021, 55.117982], ... ]]]
                }
            },
        ...
        ]
    }

A first way of drawing it on a map, is simply to use folium.GeoJson :

m = folium.Map([43, -100], zoom_start=4)

folium.GeoJson(geo_json_data).add_to(m)

m

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Note that you can avoid loading the file on yourself, by providing a (local) file path or a url.

m = folium.Map([43, -100], zoom_start=4)

url = "https://raw.githubusercontent.com/python-visualization/folium-example-data/main/us_states.json"

folium.GeoJson(url).add_to(m)

m

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You can pass a geopandas object.

import geopandas

gdf = geopandas.read_file(url)

m = folium.Map([43, -100], zoom_start=4)

folium.GeoJson(
    gdf,
).add_to(m)

m

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Click on zoom

You can enable an option that if you click on a part of the geometry the map will zoom in to that.

Try it on the map below:

m = folium.Map([43, -100], zoom_start=4)

folium.GeoJson(geo_json_data, zoom_on_click=True).add_to(m)

m

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Styling

Now this is cool and simple, but we may be willing to choose the style of the data.

You can provide a function of the form lambda feature: {} that sets the style of each feature.

For possible options, see:

• For Point and MultiPoint, see https://leafletjs.com/reference.html#marker

• For other features, see https://leafletjs.com/reference.html#path and https://leafletjs.com/reference.html#polyline

m = folium.Map([43, -100], zoom_start=4)

folium.GeoJson(
    geo_json_data,
    style_function=lambda feature: {
        "fillColor": "#ffff00",
        "color": "black",
        "weight": 2,
        "dashArray": "5, 5",
    },
).add_to(m)

m

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What’s cool in providing a function, is that you can specify a style depending on the feature. For example, if you want to visualize in green all states whose name contains the letter ‘E’, just do:

m = folium.Map([43, -100], zoom_start=4)

folium.GeoJson(
    geo_json_data,
    style_function=lambda feature: {
        "fillColor": "green"
        if "e" in feature["properties"]["name"].lower()
        else "#ffff00",
        "color": "black",
        "weight": 2,
        "dashArray": "5, 5",
    },
).add_to(m)

m

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Wow, this looks almost like a choropleth. To do one, we just need to compute a color for each state.

Let’s imagine we want to draw a choropleth of unemployment in the US.

First, we may load the data:

import pandas

unemployment = pandas.read_csv(
    "https://raw.githubusercontent.com/python-visualization/folium-example-data/main/us_unemployment_oct_2012.csv"
)

unemployment.head(5)

	State	Unemployment
0	AL	7.1
1	AK	6.8
2	AZ	8.1
3	AR	7.2
4	CA	10.1

Now we need to create a function that maps one value to a RGB color (of the form #RRGGBB). For this, we’ll use colormap tools from folium.colormap.

from branca.colormap import linear

colormap = linear.YlGn_09.scale(
    unemployment.Unemployment.min(), unemployment.Unemployment.max()
)

print(colormap(5.0))

colormap

#d8f0a3ff

We need also to convert the table into a dictionary, in order to map a feature to it’s unemployment value.

unemployment_dict = unemployment.set_index("State")["Unemployment"]

unemployment_dict["AL"]

np.float64(7.1)

Now we can do the choropleth.

m = folium.Map([43, -100], zoom_start=4)

folium.GeoJson(
    geo_json_data,
    name="unemployment",
    style_function=lambda feature: {
        "fillColor": colormap(unemployment_dict[feature["id"]]),
        "color": "black",
        "weight": 1,
        "dashArray": "5, 5",
        "fillOpacity": 0.9,
    },
).add_to(m)

folium.LayerControl().add_to(m)

m

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Of course, if you can create and/or use a dictionary providing directly the good color. Thus, the finishing seems faster:

color_dict = {key: colormap(unemployment_dict[key]) for key in unemployment_dict.keys()}

m = folium.Map([43, -100], zoom_start=4)

folium.GeoJson(
    geo_json_data,
    style_function=lambda feature: {
        "fillColor": color_dict[feature["id"]],
        "color": "black",
        "weight": 1,
        "dashArray": "5, 5",
        "fillOpacity": 0.9,
    },
).add_to(m)

<folium.features.GeoJson at 0x108c57200>

Note that adding a color legend may be a good idea.

colormap.caption = "Unemployment color scale"
colormap.add_to(m)

m

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Caveat

When using style_function in a loop you may encounter Python’s ‘Late Binding Closure’ gotcha! See https://docs.python-guide.org/writing/gotchas/#late-binding-closures for more info. There are a few ways around it from using a GeoPandas object instead, to “hacking” your style_function to force early closure, like:

for geom, my_style in zip(geoms, my_styles):
    style = my_style
    style_function = lambda x, style=style: style
    folium.GeoJson(
        data=geom,
        style_function=style_function,
    ).add_to(m)

Highlight function

The GeoJson class provides a highlight_function argument, which works similarly to style_function, but applies on mouse events. In the following example the fill color will change when you hover your mouse over a feature.

m = folium.Map([43, -100], zoom_start=4)

folium.GeoJson(
    geo_json_data,
    highlight_function=lambda feature: {
        "fillColor": (
            "green" if "e" in feature["properties"]["name"].lower() else "#ffff00"
        ),
    },
).add_to(m)

m

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Keep highlighted while popup is open

The GeoJson class provides a popup_keep_highlighted boolean argument. Whenever a GeoJson layer is associated with a popup and a highlight function is defined, this argument allows you to decide if the highlighting should remain active while the popup is open.

m = folium.Map([43, -100], zoom_start=4)

popup = folium.GeoJsonPopup(fields=["name"])

folium.GeoJson(
    geo_json_data,
    highlight_function=lambda feature: {
        "fillColor": (
            "green" if "e" in feature["properties"]["name"].lower() else "#ffff00"
        ),
    },
    popup=popup,
    popup_keep_highlighted=True,
).add_to(m)

m

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