haversine.rb

# I cannot credit the original author
class Haversine
  #EARTH_RADIUS = 3963.19 # miles
  EARTH_RADIUS = 6371 # kilometers
  RADIAN_PER_DEGREE = Math::PI / 180.0

  def self.distance(lat1, lng1, lat2, lng2)
     lat1_radians = lat1 * RADIAN_PER_DEGREE
     lat2_radians = lat2 * RADIAN_PER_DEGREE

     distance_lat = (lat2-lat1) * RADIAN_PER_DEGREE
     distance_lng = (lng2-lng1) * RADIAN_PER_DEGREE

     a = Math.sin(distance_lat/2)**2 + Math.cos(lat1_radians) * Math.cos(lat2_radians) * Math.sin(distance_lng/2) ** 2
     c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a))
     EARTH_RADIUS * c
  end
end

mongo_better_nearby.rb

require 'haversine'
      
module LoGeek
  module NearbyFinder
    # achieve a nearby lookup then sort via the more accurate haversine distance
    def self.nearby_search(database,collection_name,lat,lon,num=10,query={})
      cmd = BSON::OrderedHash.new
      cmd['geoNear'] = collection_name
      cmd['near'] = [lat,lon]
      cmd['num'] = num
      cmd['query'] = query
      database.command(cmd)['results'].sort_by do |r|
        r['distance'] = Haversine.distance(lat,lon,r['obj']['loc'][0],r['obj']['loc'][1])
      end
    end
  end
end