gpx_parser.rb

require 'date'
require 'nokogiri'
require 'gnuplot'

class Point
  attr_accessor :lon, :lat, :speed, :time

  def initialize(lon, lat, speed, time)
    @lon = lon
    @lat = lat
    @speed = speed
    @time = time
  end
end

doc = Nokogiri::XML(open(ARGV.first))

raw_points = doc.xpath('//xmlns:trkpt').map{|trk|
  Point.new(
    trk['lon'].to_s.to_f,
    trk['lat'].to_s.to_f,
    trk.at_xpath('.//mytracks:speed').content.to_f * 0.54,
    DateTime.parse(trk.at_css('time').content)
  )
}

points = raw_points.select{|p| p.speed < 50 && p.speed > 0 }

speeds = points.map(&:speed)
times = points.map(&:time).map(&:to_s)

Gnuplot.open do |gp|
  Gnuplot::Plot.new(gp) do |plot|
    plot.terminal "png"
    plot.output "./speeds.png"
    plot.ylabel "knots"
    plot.yrange "[0:10]"
    plot.xrange "[0:4600]"
    plot.data << Gnuplot::DataSet.new([speeds]) do |ds|
      ds.with = "lines"
      ds.smooth = "bezier"
    end
  end
end