def geturl(self, lat1, lon1, lat2, lon2, zoom, w, h):
      try:
        x1, y1 = self.xy_to_co(lat1, lon1)
        x2, y2 = self.xy_to_co(lat2, lon2)
        return self.url + "&BBOX=%f,%f,%f,%f&WIDTH=%i&HEIGHT=%i&ext=.png" % (x1, y1, x2, y2, w, h)
      except RuntimeError, e:
        return None