map.Tags.add(function(tile, zoom) {
  var bound = longdo.Util.boundOfTile(map.projection(), tile);
  mockAjaxFromServer(bound, function(locationList) {
    for (var i = 0; i < locationList.length; ++i) {
      map.Overlays.add(new longdo.Marker(locationList[i], { visibleRange: { min: zoom, max: zoom } }));
    }
  });
});

function mockAjaxFromServer(bound, callback) {
  var locationList = [];
  var count = Math.random() * 5;
  for (var i = 0; i < 3; ++i) {
    locationList.push({ lon: bound.minLon + (Math.random() * (bound.maxLon - bound.minLon)),
      lat: bound.minLat + (Math.random() * (bound.maxLat - bound.minLat)) });
  }
  callback(locationList);
}