# Map data

`japan_prefectures.geojson` derives from **Natural Earth**
(<https://www.naturalearthdata.com/>), *Admin 1 – States, Provinces*, 1:10m
cultural vectors, 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.

`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.