# Map data

Both maps derive from **Natural Earth** (<https://www.naturalearthdata.com/>),
release v5.1.2. Natural Earth's map data is in the public domain:

> All versions of Natural Earth raster and vector map data found on this
> website are in the public domain. You may use the maps in any manner,
> including modifying the content and design, electronic dissemination, and
> offset printing.

## Japan's prefectures

`japan_prefectures.geojson` derives from *Admin 1 – States, Provinces*, 1:10m
cultural vectors. `apps/learn/tool/maps/japan_prefectures.py` builds the file: it keeps
Japan's 47 prefectures, simplifies their outlines (Douglas–Peucker, 0.006°),
drops islands too small to see (each prefecture keeps its largest), and rounds
coordinates to 4 decimals. The outlines are an approximation for display, not
survey-accurate data. Regions are named by their ISO 3166-2 codes (`JP-01` …
`JP-47`), with Natural Earth's English and Japanese names.

## Countries of the world

`world_countries.geojson` derives from *Admin 0 – Countries*, 1:50m cultural
vectors (Gibraltar and the US Outlying Islands, which it leaves out, from the
1:10m layer). `apps/learn/tool/maps/world_countries.py` builds the file: one region per
ISO 3166-1 code (239), areas without a code of their own joined to the country
ISO counts them under, outlines kept as Natural Earth generalised them (islands
under 0.001 square degrees dropped), coordinates rounded to 3 decimals. Borders
are drawn as Natural Earth draws them, as they are on the ground.

The countries' names come from the **Unicode Common Locale Data Repository**
(CLDR, <https://cldr.unicode.org/>), release 48.2.3, in every language a
studyset can be in. CLDR is under the Unicode License v3, whose notice is in
`LICENSE-CLDR.txt` and in the app's licence list.