Silverlight中在BingMap上画一个圆

不解释，直接上代码：

private double rad(double d)
{
        return d * Math.PI / 180;
}
//radius : km
private void DrawCircle(Location center, double radius)
{
        double EARTH_RADIUS = 6371;

        double lon = rad(center.Longitude);
        double lat = rad(center.Latitude);

        double d = radius / EARTH_RADIUS;
        polygon = new MapPolygon()
        {
                Fill = new SolidColorBrush(Colors.Red),
                Opacity = 0.6,
        };
        polygon.Locations = new LocationCollection();

        for (int i = 0; i < 360; i++)
        {
                Location location = new Location();
                double bearing = rad(i);
                location.Latitude = Math.Asin(Math.Sin(lat) * Math.Cos(d) + Math.Cos(lat) * Math.Sin(d) * Math.Cos(bearing));
                location.Longitude = ((lon + Math.Atan2(Math.Sin(bearing) * Math.Sin(d) * Math.Cos(lat), Math.Cos(d) - Math.Sin(lat) * Math.Sin(location.Latitude))) * 180) / Math.PI;
                location.Latitude = location.Latitude * 180 / Math.PI;
                polygon.Locations.Add(location);
        }
        MyMap.Children.Insert(0, polygon);
}