我正在使用OpenLayers开展一个 Map 项目，我想在其上显示街道数据，这些数据位于Google图像底图的顶部 . ESRI通过他们的ArcGIS REST服务器提供了良好的街道覆盖，但是我已经尝试过遵循文档和this example，并且't figure out what I' m可以做的不同's making it not work. It will put a new layer in the layers panel along with the Google Satellite basemap, but nothing loads in that layer--it'空白 . 这是我的代码：

function addOpenLayers() {
    var mapcenter = new OpenLayers.LonLat(-90.8, 46.45)
    var base = new OpenLayers.Layer.Google("Google Satellite", {type: google.maps.MapTypeId.SATELLITE, numZoomLevels:22});

    var olOptions = {
        controls: [new OpenLayers.Control.PanZoomBar, new OpenLayers.Control.LayerSwitcher, new OpenLayers.Control.Navigation],
    }
    var olMap = new OpenLayers.Map("map", olOptions);

    olMap.addLayer(base);

    olMap.setCenter(mapcenter.transform(
        new OpenLayers.Projection("EPSG:4326"),
        olMap.getProjectionObject()
    ), 10);

    //This is the part that doesn't work:
    var esriRoadsUrl = "http://server.arcgisonline.com/ArcGIS/rest/services/Reference/World_Transportation/MapServer";
    var esriRoads = new OpenLayers.Layer.ArcGIS93Rest("Roads", esriRoadsUrl, {layers: "show:0"});
    esriRoads.isBaseLayer = false;

    olMap.addLayer(esriRoads);
}
openlayers esri