见这个网址：http://api.map.baidu.com/library/GeoUtils/1.2/examples/simple.html
查看其源代码即可。
下面的代码可以判断某个点是否属于一个多边形
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>GeoUtils示例</title>
<script type="text/javascript" src="http://api.map.baidu.com/api?v=1.2"></script>
<script type="text/javascript" src="http://api.map.baidu.com/library/GeoUtils/1.2/src/GeoUtils_min.js"></script>
<style type="text/css">
table{
font-size:14px;
}
</style>
</head>
<body>
<div style="float:left;width:600px;height:500px;border:1px solid gray" id="container"></div>
</div>
</body>
</html>
<script type="text/javascript">
var map = new BMap.Map("container");
var pt = new BMap.Point(116.404, 39.915);
map.centerAndZoom(pt, 16);
map.enableScrollWheelZoom();//开启滚动缩放
map.enableContinuousZoom();//开启缩放平滑
var pts = [];
var pt1 = new BMap.Point(116.395, 39.910);
var pt2 = new BMap.Point(116.394, 39.914);
var pt3 = new BMap.Point(116.403, 39.920);
var pt4 = new BMap.Point(116.402, 39.914);
var pt5 = new BMap.Point(116.410, 39.913);
pts.push(pt1);
pts.push(pt2);
pts.push(pt3);
pts.push(pt4);
pts.push(pt5);
var ply = new BMap.Polygon(pts);
var pt =new BMap.Point(116.400, 39.914);
var result = BMapLib.GeoUtils.isPointInPolygon(pt, ply);
if(result == true){
alert("in polygon");
} else {
alert("点在多边形外")
}
//演示：将面添加到地图上
map.clearOverlays();
var mkr = new BMap.Marker(pt);
map.addOverlay(mkr);
map.addOverlay(ply);
</script>