How to do an intersection of geojson with rgeo?

Question

I am new to ruby and experimenting with rgeo.

I have 2 geojson files: points.geojson contains a number of points here is a gist of points.geojson

outline.geojson contains a single polygon here is a gist of my outline.geojson:

I want the intersection (points that are contained within outline)

Here is what I am trying...

require 'rgeo'
require 'rgeo/geo_json'

points_str = File.read("points.geojson")
points = RGeo::GeoJSON.decode(points_str, json_parser: :json)
puts points

outline_str = File.read("outline.geojson")
outline = RGeo::GeoJSON.decode(outline_str, json_parser: :json)
puts outline

puts points.intersection outline

The error I get: intersection.rb:12:in ': undefined method intersection' for #RGeo::GeoJSON::FeatureCollection:0x294ac44 (NoMethodError)

Answer 1

The issue with your code is that RGeo::GeoJSON.decode doesn't return geometries, but an instance of RGeo::GeoJSON::FeatureCollection.

You first have to extract the relevant geometries, i.e. the array of points and the polygon, respectively.

This snippet does what you want:

require 'rgeo'
require 'rgeo/geo_json'

points = RGeo::GeoJSON
  .decode(File.open('points.geojson'))
  .map(&:geometry)
puts points

polygon = RGeo::GeoJSON
  .decode(File.open("outline.geojson"))
  .first
  .geometry
puts polygon

contained_points = points.select { |p| p.within?(polygon) }
puts contained_points