snippet.js

// In closure: initial_sw
  google.maps.event.addListener(map, "click", function(event) {
    if (!initial_sw) {
      initial_sw = map.get_bounds().getSouthWest();
    }
    var sw = map.get_bounds().getSouthWest(),
      pos = new google.maps.LatLng(
        event.latLng.lat() - (initial_sw.lat()-sw.lat()), 
        event.latLng.lng() - (initial_sw.lng()-sw.lng()));