While developing a location-based application, I was experiencing strange problems with the GPS service accuracy. My location varied wildly all over the place and I needed a better way to visualize my location. Along with drawing a marker on the map indicating where I am, I also a wanted to draw a circle around it to see what is the accuracy threshold for it.

When GeoCoordinateWatcher sends a notification about a new position, you can retrieve the horizontal accuracy information via the GeoCoordinate.HorizontalAccuracy property. Since there is no circle or ellipse shape for map, you can approximate it with MapPolygon. I’ve declared it in the following way:

<my:MapPolygon Fill="#99FF0000" Stroke="Red" StrokeThickness="1" Opacity="0.6" x:Name="precision" />

The code is adapted from Steve Strong’s blog post. The full code for drawing a circle:

private void UpdatePrecision(GeoCoordinate location) { precision.Visibility = Visibility.Collapsed; if (location == null) return; precision.Visibility = Visibility.Visible; precision.Locations = new LocationCollection(); var earthRadius = 6371; var lat = location.Latitude * Math.PI / 180.0; //radians var lon = location.Longitude * Math.PI / 180.0; //radians var d = location.HorizontalAccuracy / 1000 / earthRadius; // d = angular distance covered on earths surface for (int x = 0; x <= 360; x++) { var brng = x * Math.PI / 180.0; //radians var latRadians = Math.Asin(Math.Sin(lat) * Math.Cos(d) + Math.Cos(lat) * Math.Sin(d) * Math.Cos(brng)); var lngRadians = lon + Math.Atan2(Math.Sin(brng) * Math.Sin(d) * Math.Cos(lat), Math.Cos(d) - Math.Sin(lat) * Math.Sin(latRadians)); var pt = new GeoCoordinate(180.0 * latRadians / Math.PI, 180.0 * lngRadians / Math.PI); precision.Locations.Add(pt); } }

On the image below you can see it in action.

Since emulator is very “precise”, I had to zoom in a bit. In regular usage the accuracy will vary.

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