gpx_distance.rb

#!/usr/bin/env ruby
# -*- coding: utf-8 -*-

require 'nokogiri'
require 'pp'

module GpxDistance
  def calc_gpx_distance(gpx_file)
    distance = calc_distance(get_latlons(gpx_file))
    distance / 1000 # kilometers
  end

  private
  def get_latlons(gpx_file)
    gpx = Nokogiri::XML(open(gpx_file))
    gpx.css("gpx trk trkpt").map{|pt|
      [pt.attribute("lat").value.to_f, pt.attribute("lon").value.to_f]
    }
  end

  def calc_distance(latlons)
    distance =
      latlons.map{|latlon|
        deg_to_rad(latlon)
      }.each_cons(2).inject(0){|dist, pt|
        dist + hubeny(*pt)
      }
    distance # meters
  end

  def deg_to_rad(latlng)
    latlng.map { |deg| deg * Math::PI / 180 }
  end

  # http://yamadarake.jp/trdi/report000001.html
  # WGS84 (GPS)
  A = 6378137.000
  B = 6356752.314245

  def hubeny(latlng_1, latlng_2)
    pow_e = 1 - B ** 2 / A ** 2

    ave = (latlng_1[0] + latlng_2[0]) / 2
    dp = latlng_1[0] - latlng_2[0]
    dr = latlng_1[1] - latlng_2[1]
    sin_ave = Math.sin(ave)
    cos_ave = Math.cos(ave)
    w = Math.sqrt(1 - pow_e * sin_ave ** 2)
    m = A * (1 - pow_e) / w ** 3
    n = A / w

    distance = Math.sqrt((m * dp) ** 2 + (n * cos_ave * dr) ** 2)
    distance #meters
  end

  extend self
end

if __FILE__ == $0
  exit 1 if ARGV.length < 1
  gpx_file = ARGV[0]
  distance = GpxDistance::calc_gpx_distance(gpx_file)
  puts sprintf("%.3f km", distance)
end