<div id="ch1_avoid_baselayer" style="width: 100%; 
height: 100%;"></div>

// Create the map using the specified DOM element 
var map = new OpenLayers.Map("ch1_avoid_baselayer", { 
    allOverlays: true 
});

// Add a WMS layer 
var wms = new OpenLayers.Layer.WMS("OpenLayers WMS Basic", "http://vmap0.tiles.osgeo.org/wms/vmap0", 
{ 
    layers: 'basic' 
}); 
map.addLayer(wms); 
// Add a WMS layer 
var topo = new OpenLayers.Layer.WMS("USA Topo Maps", "http://terraservice.net/ogcmap.ashx", 
{ 
    layers: "DRG" 
},
{ 
    opacity: 0.5 
}); 
map.addLayer(topo); 
// Add LayerSwitcher control 
map.addControl(new OpenLayers.Control.LayerSwitcher()); 

// Set view to zoom maximum map extent 
// NOTE: This will fail if there is no base layer defined 
map.setCenter(new OpenLayers.LonLat(-100, 40), 5);