extract-rendered-geojson.js

'use strict';

const mapnik = require('mapnik');
const VectorTile = require('vector-tile').VectorTile;
const Protobuf = require('pbf');
const fs = require('fs');

mapnik.register_default_input_plugins();

const geojson = fs.readFileSync('original-russian-triangle.geojson', 'utf8');

const LAYER = 'triangles';
function getTiledFeatureGeoJson(z, x, y) {
  const mapnikTile = new mapnik.VectorTile(z, x, y);
  mapnikTile.addGeoJSON(geojson, LAYER);
  const tilePbf = mapnikTile.getDataSync();
  const tile = new VectorTile(new Protobuf(tilePbf));
  const layer = tile.layers[LAYER];
  return layer.feature(0).toGeoJSON(x, y, z);
}

const featureA = getTiledFeatureGeoJson(6, 0, 15);
const featureB = getTiledFeatureGeoJson(6, 63, 15);

const result = {
  type: 'FeatureCollection',
  features: [featureA, featureB],
};

fs.writeFileSync('tiled-russian-triangle.geojson', JSON.stringify(result, null, 2));

original-russian-triangle.geojson

tiled-russian-triangle.geojson