title: How to put multiple circle overlays on a WP7 Bing Map control?
body: <p>I want to put a number of (transparent) circles on a Bing Map Control as overlays. Think about a "blast radius" or "sphere of influence" kind of thing. I'll need at least two dozen of these circles on the map, together with some other polygons, so making sure performance does not suffer and the maps continues to be easily manipulative seems key here.</p>
  
  <p>So far I can think of three options (of which I tried two):</p>
  
  <ul>
  <li><p>Add n-sided <code>MapPolygon</code> instances to the map for each circle. Tried this, and works fine in principle. However, the number of vertices per polygon is a bit of pickle. Too little, and when you zoom in it looks hideous. Too many, and performance will start to suffer.</p></li>
  <li><p>Add <code>Pushpin</code> instances for each circle, and style/template them as circle that are centered around the pushpin location. Seems to work, except that the size of the cicle is then fixed in screen size, not map size. So when you zoom, the circles stay the same size on screen, while they should zoom with the map. <em>Can you bind the size of <code>Ellipse</code> control (in the template in the style I use) to the <code>Zoom</code> of the parent map control in some way..?</em></p></li>
  <li><p>Create a custom subclass of <code>MapShapeBase</code> to represent each circle. This implementation could dynamically change the number of vertices used to draw the polygon depending on whether the shape is visible on the map or not (=inside current view port) and what the zoom level is. That is, if map shapes have access to this information. I haven't tried this option yet.</p></li>
  </ul>
  
  <p>What have you used? What would you use? How would you work around the problems mentioned above, <em>especially binding a property of a control in a template used in a style to a parent control property using XAML</em>? Or do you have any other options I could try to get this to work?</p>