* way point has been used to modify the area.
*/
private LatLon calcAreaForWayPoint(WayPoint p, LatLon previous) {
tick();
LatLon c = p.getCoor();
if (previous == null || c.greatCircleDistance(previous) > buffer_dist) {
// we add a buffer around the point.
r.setRect(c.lon() - buffer_x, c.lat() - buffer_y, 2 * buffer_x, 2 * buffer_y);
a.add(new Area(r));
return c;
}