OpenLayers图层 - HelloWorld开发者社区

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    <title>理解基础层和非基础层</title>
    <link rel="stylesheet" href="./OpenLayers-2.12/theme/default/style.css" type="text/css" />
    <script src="./OpenLayers-2.12/lib/OpenLayers.js"></script>
    <script type="text/javascript">
        function init(){
        //初始化左侧地图
        //应用特定DOM元素创建地图
        var map_a = new OpenLayers.Map("base_nonbase_map_a");
        //添加WMS层
        var wms = new OpenLayers.Layer.WMS("OpenLayers WMS Basic", "http://vmap0.tiles.osgeo.org/wms/vmap0",
        {
            layers: 'basic'
        },
        {
            isBaseLayer: true
        });
        map_a.addLayer(wms);
        // 添加WMS层
        var topo = new OpenLayers.Layer.WMS("USA Topo Maps", "http://terraservice.net/ogcmap.ashx",
        {
            layers: "DRG"
        },
        {
            opacity: 0.5,
            isBaseLayer: false
        });
        map_a.addLayer(topo);
        //添加层选择控件
        map_a.addControl(new OpenLayers.Control.LayerSwitcher());         
        //将地图视图范围设置为全图视窗
        //注意：如果没有一个基础层，将创建失败
        map_a.setCenter(new OpenLayers.LonLat(-100, 40), 5);   
        //初始化右侧地图
        //应用特定DOM元素创建地图
        var map_b = new OpenLayers.Map("base_nonbase_map_b");
        // 添加WMS层
        var wms = new OpenLayers.Layer.WMS("OpenLayers WMS Basic", "http://vmap0.tiles.osgeo.org/wms/vmap0",
        {
            layers: 'basic'
        });
        map_b.addLayer(wms);
        // 添加WMS层
        var topo = new OpenLayers.Layer.WMS("USA Topo Maps", "http://terraservice.net/ogcmap.ashx",
        {
            layers: "DRG"
        });
        map_b.addLayer(topo);
        //添加层选择控件
        map_b.addControl(new OpenLayers.Control.LayerSwitcher());        
        //将地图视图范围设置为全图视窗
        //注意：如果没有一个基础层，将创建失败
        map_b.setCenter(new OpenLayers.LonLat(-100, 40), 5);
    }
    </script>
</head>
<body onload="init()">
    <table style="width: 100%; height: 95%;">
        <tr>
            <td>
                <p>Map with one non base layer:</p>
                <div id="base_nonbase_map_a" style="width: 100%; height: 500px;"></div>
            </td>
            <td>
                <p>Map with two base layers</p>
                <div id="base_nonbase_map_b" style="width: 100%; height: 500px;"></div>
            </td>
        </tr>
    </table>
    <div style="width:100%; height:100%" id="map"></div>
</body>
</html>
理解基础层和非基础层
Map with one non base layer:
Map with two base layers