function split_markers_by_zone(arr) {
  /*
    Build a dictionary which holds the objects based on a property
  */
  var sorted = {};
  for( var i = 0, max = arr.length; i < max ; i++ ){
      var val = arr[i].getAttribute("data-zone-uid");
      if( sorted[val] == undefined ){
        sorted[val] = [];
      }
      sorted[val].push(arr[i]);
  }
  return sorted;
}