GeoJSON of Japanese prefectures

Reason

When you look online for GeoJSON of Japanese prefectures, most results are approximate. The best resource out there is probably this gist but the files are already split by prefectures.

This module provides a single high definition GeoJSON file and some utility methods to interact with it

Source

Source data is from the National Land Information Division which is a high resolution GeoJSON file containing all administrative areas. All features are merged by prefectures and the resulting file is simplified with a Visvalingam simplification keeping lines within 5 meters of the original location.

Usage

You can use the GeoJSON file directly or the following utilities

npm install open-data-jp-prefectures-geojson

getPrefectures

const { getPrefectures } = require('open-data-jp-prefectures-geojson');

const shinjuku = [35.6877109,139.6919347];
const yoyogi = [35.6877109,139.6919347];
const point = { type: 'Feature', geometry: { type: 'Point', coordinates: shinjuku } };
const line = { type: 'Feature', geometry: { type: 'LineString', coordinates: [shinjuku, yoyogi] } };

console.log(await getPrefectures(point)); // [13]
console.log(await getPrefectures(line)); // [13]

Note that getPrefectures returns an array of Japanese prefectures.

Single points fully contained inside a prefecture will only have one value. Single points right at the border or lines crossing multiple prefectures will have multiple values.

If you're sure your LineStrings are short and don't overlap multiple prefectures, you can pass the option skipIntersect to speed up resolution.

await getPrefectures(line, { skipIntersect: true });

The default behaviour (skipIntersect: false) checks that the line crosses any prefecture at any point, so for instance if you're checking a straight line from Hokkaido to Tokyo, you'll get all the prefectures inbetween, but if you know your lines are small (for instance local streets, or railway station platforms) you can pass skipIntersect: false to only check the prefectures of the edges inside the line (for a straight line it'll be start and end, for more complex lines it's where the lines bend).

Development

1. Clone the repo
2. Download the source zip file into input/
3. Run ./setup.sh to generate the output geojson file

If you want to generate a smaller GeoJSON file at lower resolution, call ./setup.sh 50 (50 is going to be the simplification interval, defaults to 5)