example #5 for http://factormystic.net/blog/using-reactive-extensions-to-smooth-compass-data-in-windows-phone

averaged_compass_headings.cs

Observable.FromEvent<SensorReadingEventArgs<CompassReading>>(compass, "CurrentValueChanged")
          .BufferWithTime(TimeSpan.FromSeconds(2))
          .Select(headings => headings.Select(e => e.EventArgs.SensorReading.TrueHeading).Sum() / headings.Count())
          .Subscribe(heading => Debug.WriteLine(string.Format("Averaged: {0}", heading)));