openlayers2加载小块区域地图，限制拖动范围值怎么设置?

var map = new OpenLayers.Map({
div: "map_id",
projection: "EPSG:3857",
maxExtent: [-18924313.432222, -15538711.094146, 18924313.432222, 15538711.094146],
restrictedExtent: [-13358338.893333, -9608371.5085962, 13358338.893333, 9608371.5085962],
center: [-12356463.476333, 5621521.4854095]
});
想知道 限制地图拖动范围边界 restrictedExtent的值怎么算的。。

http://blog.csdn.net/longshengguoji/article/details/49000785