GeoJSON (FeatureCollection) to EPSG:3857

geojson_to_epsg3857.rb

#!/usr/bin/env ruby
# frozen_string_literal: true

require 'rgeo'
require 'rgeo/geo_json'
require 'rgeo/proj4'
require 'optparse'

OptionParser.new do |opts|
  opts.banner = 'Usage: geojson_to_epsg3857.rb [options]'

  opts.on('-f', '--file FILE', 'GeoJSON file to convert') do |file|
    @file = file
  end

  opts.on('-h', '--help', 'Prints this help') do
    puts opts
    exit
  end
end.parse!

if @file.nil?
  puts 'Please specify a GeoJSON file to convert.'
  exit
end

factory_4326 = RGeo::Geos.factory(coord_sys: "EPSG:4326", srid: 4326)
factory_3857 = RGeo::Geos.factory(coord_sys: "EPSG:3857", srid: 3857)

open(@file, 'r') do |f|
  geojson = RGeo::GeoJSON.decode(f.read, json_parser: :json, geo_factory: factory_4326)
  converted_features = geojson.map do |feature|
    RGeo::GeoJSON::Feature.new(
      feature.geometry.transform(factory_3857),
      feature.feature_id,
      feature.properties
    )
  end
  new_geojson = RGeo::GeoJSON::FeatureCollection.new(converted_features)
  puts RGeo::GeoJSON.encode(new_geojson)
end

same as ogr2ogr -f GeoJSON -s_srs EPSG:4326 -t_srs EPSG:3857 /vsistdout/ foobar.geojson