License plate codes in Bavaria

License plates on cars in Germany start with a one, two or three letter codes corresponding (roughly) to small administrative circonscriptions. The keyword for Wikipedia is Kfz-Kennzeichen.

Warning

A more comprehensive map with coats of arms is constructed in the corresponding section.

Data acquisition

Download all district (Kreisen) borders for Bavaria (Bayern in German)

from cartes.osm import Overpass

bayern = Overpass.request(
    # There are other ways to select Bavaria, esp. with name.
    # ISO codes are reliable though
    area={"ISO3166-2": "DE-BY", "admin_level": 4, },
    rel={ "boundary": "administrative", "admin_level": 6}
)

Data preprocessing

Simplify the borders for all districts and assign colours so that no two neighbouring districts get the same colour.

# 1st step: simplify the borders
# 2nd step: assign a color using graph coloring
bayern = bayern.simplify(5e3).coloring()

Data visualisation

import altair as alt

bayern_chart = alt.Chart(bayern)

alt.layer(
    bayern_chart.mark_geoshape()
    .encode(alt.Color("coloring:N", legend=None), alt.Tooltip("name:N"))
    .properties(width=500, height=600),
    bayern_chart.mark_text(font="Ubuntu", fontSize=12)
    .encode(
        alt.Latitude("latitude:Q"), alt.Longitude("longitude:Q"),
        alt.Text("license_plate_code:N"), alt.Tooltip("name:N"),
    )
    # do not display empty data
    .transform_filter("datum.license_plate_code != null")
)

Coats of arms

The first map shows one main issue: only Landkreis display a license plate code, not cities outside a Kreis (Kreisfreie Stadt).

Since the data returned by OpenStreetMap contains a Wikidata identifier, we use it to fill the missing information. Since the whole process involves many small downloads which can be run in parallel, we use in the following example the asynchronous library aiohttp rather than the popular “blocking” requests library.

Wikidata returns comprehensive information in JSON format, with popular fields encoded with identifiers. We focus here on:

• P394: licence plate code;

• P94: coat of arms (just because it’s beautiful).

The following code gets all the necessary complementary information:

import aiohttp
import bs4  # beautifulsoup4

async def fetch(wikidata, session):
    result = dict()

    async with session.get(
        f"https://www.wikidata.org/wiki/Special:EntityData/{wikidata}.json"
    ) as resp:
        json = await resp.json()

    # The licence plate code
    p395 = [
        elt["mainsnak"]["datavalue"]["value"]
        for elt in json["entities"][wikidata]["claims"]["P395"]
    ]
    # Some districts have several official licence plate codes
    # Then the second looks more natural if we have to pick one
    result["P395"] = p395[0] if len(p395) == 1 else p395[1]

    # The name of the SVG file for the coat of arms
    p94 = next(
        elt["mainsnak"]["datavalue"]["value"]
        for elt in json["entities"][wikidata]["claims"]["P94"]
    )

    # The full path to the SVG file is to be found on that page
    async with session.get(f"https://commons.wikimedia.org/wiki/File:{p94}") as resp:
        page = bs4.BeautifulSoup(await resp.text())
        svg_link = page.find("a", href=re.compile("https://.*\.svg$"))
        if svg_link is not None:
            result["P94"] = svg_link.attrs["href"]

    return result


async def wikidata():
    async with aiohttp.ClientSession() as session:
        futures = list(
            fetch(elt.wikidata, session) for _, elt in bayern.data.iterrows()
        )
        return list(result for result in await asyncio.gather(*futures))


records = await wikidata()  # only valid in notebooks, otherwise asyncio.run(main())

bayern_complete = bayern.data.merge(
    pd.DataFrame.from_records(records, index=bayern.data.id_),
    left_on="id_",
    right_index=True,
)

Then we can build the full map:

bayern_chart = alt.Chart(bayern_complete)
selector = alt.selection_single(on="mouseover", nearest=True, empty="none")

alt.layer(
    # Same background map
    bayern_chart.mark_geoshape()
    .encode(alt.Color("coloring:N", legend=None), alt.Tooltip("name:N"))
    .properties(width=500, height=600),
    # The text is now taken from P395 when not available
    bayern_chart.mark_text(font="Ubuntu", fontSize=12)
    .encode(
        alt.Text("display:N"), alt.Tooltip("name:N"),
        alt.Latitude("latitude:Q"), alt.Longitude("longitude:Q"),
    )
    .transform_calculate(
        # The switch happens here
        display=(
            "if(isValid(datum.license_plate_code), "
            "datum.license_plate_code, datum.P395)"
        )
    )
    .add_selection(selector),
    # We place a map on the top right corner, according to the selected text
    bayern_chart.mark_image(width=100, height=150, align="right", baseline="line-top")
    .encode(alt.Url("P94:N"))
    .transform_filter(selector),
)