在google map上画圆挺简单的，但是要想画的舒心却没找到现成代码，所以自己改了一段。效果如下，支持圆心和半径拖拽。

code部分，只整理了主要的，需要清空区域换marker样式什么自己搞就行。

var mapCircle = new google.maps.Circle({
	map : gMap,
	strokeColor : '#ff0000',
	strokeOpacity : 0.6,
	strokeWeight : 4
});

var startMarker = null;
var endMarker = null;

function createCircle() {

	gMap.setOptions({
		draggableCursor : 'crosshair'
	});

	google.maps.event.addListener(gMap, 'click', function(point) {
		if (startMarker == null) {
			startMarker = new google.maps.Marker({
				position : point.latLng,
				map : gMap
			});
			mapCircle.setCenter(point.latLng);
			gMap.setOptions({
				draggableCursor : 'pointer'
			});
		}
		mapCircle.setMap(gMap);
	});

	google.maps.event.addListener(gMap, 'mousemove', function(point) {

		if (startMarker != null && endMarker == null) {
			var distance = google.maps.geometry.spherical
					.computeDistanceBetween(startMarker.getPosition(),
							point.latLng);
			mapCircle.setRadius(distance);
		}
	});

	google.maps.event.addListener(mapCircle, 'click', function(point) {

		if (endMarker == null) {
			// startMarker is center point. now set radius
			endMarker = new google.maps.Marker({
				position : point.latLng,
				draggable : true,
				raiseOnDrag : false,
				map : gMap
			});
			google.maps.event.addListener(startMarker, 'drag', drawCircle);
			google.maps.event.addListener(endMarker, 'drag', drawCircle);
			startMarker.setDraggable(true);
			startMarker.setAnimation(null);
			drawCircle();
		}
		mapCircle.setMap(gMap);
	});

	google.maps.event.addListener(mapCircle, 'mousemove', function(point) {
		if (startMarker != null && endMarker == null) {
			var distance = google.maps.geometry.spherical
					.computeDistanceBetween(startMarker.getPosition(),
							point.latLng);
			mapCircle.setRadius(distance);
		}
	});
};

function drawCircle() {
	mapCircle.bindTo('center', startMarker, 'position');
	var distance = google.maps.geometry.spherical.computeDistanceBetween(
			startMarker.getPosition(), endMarker.getPosition());
	mapCircle.setRadius(distance);

};

画多边形也不复杂，但是在多边形编辑方面 google maps api v3之后不支持类似于v2的enableEdit，一下就变得麻烦起来，这里推荐这个多边形编辑js，用起来效果比较好。