Virtual Earth Silverlight Control Part 2: Locate Me

It was a post by Nikhil Kothari that inspired me to create a method to report my current location on the Virtual Earth Silverlight control.

http://www.nikhilk.net/Silverlight-Locate-Me.aspx

While I was traveling around the country (see http://www.wiredwalkabout.com/), I would run Nikhil's application to see the result. For the most part, it always reported my location about 20 miles from where I really was, which I think is a problem with the web service used and not the code calling it.

http://api.hostip.info/get_html.php?position=true




Nikhil's current live sample has been updated, but it is still not at good as a GPS. The application source code is worth looking at in detail. He uses techniques that are more advanced than calling WebClient, and as a programmer it is interesting to know how to do same thing many different ways (viewing this link requires you have Silverlight 3 installed):

http://www.nikhilk.net/Content/Posts/SilverlightLocateMe/LocateMe.htm

This post is about integrating this functionality into the mapping application I am currently developing. I have decided to create a separate class of objects to mark my current location. I did this mainly because I wanted to use a different symbol to mark myself on the map -- like a "You Are Here" marker. I have extended this class to collect a time stamp, and this -- in combination with persistence -- can be used to track my movements.

    public class MyLocation : PointOfInterest



    {



        public DateTime TimeStamp { get; set; }



        public MyLocation()



            : base()



        {



            TimeStamp = DateTime.Now;



        }







        #region Serializer Methods



        public override XElement PersistTo(ModelSerializer oSerializer, XElement oSelf)



        {



            XElement oXElement = base.PersistTo(oSerializer, oSelf);







            oXElement.Add(new XAttribute("TimeStamp", TimeStamp.ToString()));







            return oXElement;



        }



        public override IMapModel RecoverFrom(ModelSerializer oSerializer, XElement oElement)



        {



            var oResult = base.RecoverFrom(oSerializer, oElement);







            string sTimeStamp = oElement.Attribute("TimeStamp").Value;



            TimeStamp = DateTime.Parse(sTimeStamp);



            return oResult;



        }



        #endregion



    }

I needed to modify the rendering code in order to display a different picture for this class. With this quick change, objects of type MyLocation now show a pushpin instead of a red rectangle.

        void RenderPoint(PointOfInterest oPOI)



        {







            if (oPOI is MyLocation)



            {



                Uri oSource = new Uri(@"Pushpin.png", UriKind.Relative);



                var oBitmap = new BitmapImage(oSource);



                var oImage = new Image() { Source = oBitmap, Width=23, Height=27 };



                m_oLayer.AddChild(oImage, oPOI.Loc, PositionMethod.BottomCenter);



            }



            else



            {



                // Create a Rectangle Shape to Show as the "Pushpin"



                // Set it's Size to 10x10



                var oRect = new System.Windows.Shapes.Rectangle() { Fill = new SolidColorBrush(Colors.Red), Width = 10, Height = 10 };







                // Add Rectangle to MapLayer



                m_oLayer.AddChild(oRect, oPOI.Loc, PositionMethod.Center);



            }



        }

The code to query the web service using WebClient follows. This is an asynchronous call that returns a string delimited by line feeds, so I used Nikhil's trick of reading from a stream to parse out and convert the Lat/Long information.

        void WhereAmI()



        {



            WebClient oClient = new WebClient();



            var oAddress = new Uri(@"http://api.hostip.info/get_html.php?position=true");



            oClient.DownloadStringCompleted += new DownloadStringCompletedEventHandler(oClient_DownloadStringCompleted);



            oClient.DownloadStringAsync(oAddress);



        }







        void oClient_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)



        {



            string sResult = e.Result;







            using (StreamReader sr = new StreamReader(new MemoryStream(Encoding.UTF8.GetBytes(sResult))))



            {



                try



                {



                    sr.ReadLine();



                    sr.ReadLine();







                    double lat = 0;



                    TryParse<Double>(sr.ReadLine(), out lat);







                    double lng = 0;



                    TryParse<Double>(sr.ReadLine(), out lng);







                    var oPOI = new MyLocation() { Loc = new Location(lat, lng) };



                    m_oList.Add(oPOI);







                    RenderPoint(oPOI);







                }



                catch { }



            }



            MessageBox.Show(sResult);



        }