title: calculate walking distance between gps points
body: <p>I have pairs of gps coordinates (longitude latitude) and I would like to calculate the walking distance between them. i.e. using road data (from google maps or another open source) calculate the km of the shortest route between the two gps points. I could do it using google maps, but I have thousands of pairs so I would like to find a more automated way. </p>
  
  <p>Does somebody know how to do it?</p>