Overlay momentarily disappears - ios

Question

Alright, the problem I'm having is that I have an overlay that is driven by the compass heading for the iphone. Whenever the compass heading changes, the line should move across the map with the heading. There are two overlays, one for the compass and another one. When only the compass overlay is being displayed, the line will flicker as it changes, but when the other line is displayed, the flicker goes away and the line moves smoothly.
- (MKOverlayView *)mapView:(MKMapView *)map viewForOverlay:(id <MKOverlay>)overlay
{
SatFinderAppDelegate *appdel = (SatFinderAppDelegate *)[[UIApplication sharedApplication]delegate];
float s = [appdel.location floatValue];
if (overlay == self.headingline)
{
headinglineView = [[MKPolylineView alloc] initWithPolyline:self.headingline];
headinglineView.fillColor = [UIColor greenColor];
headinglineView.strokeColor = [UIColor greenColor];
headinglineView.lineWidth = 5;
return self.headinglineView;
}
if (overlay == self.routeLine)
{
if (s == 0) {
return routeLineView = nil;
}
routeLineView = [[MKPolylineView alloc] initWithPolyline:self.routeLine];
routeLineView.fillColor = [UIColor cyanColor];
routeLineView.strokeColor = [UIColor cyanColor];
routeLineView.lineWidth = 5;
if (ele < 0) {
routeLineView.lineDashPattern = [NSArray arrayWithObjects:[NSNumber numberWithFloat:12],
[NSNumber numberWithFloat:8],nil];
routeLineView.strokeColor = [UIColor redColor];
}
return routeLineView;
}
return nil;
}
...
- (void)locationManager:(CLLocationManager *)manager didUpdateHeading:(CLHeading *)newHeading {
//line for heading
[self headinglineupdate];
}
....
-(void)headinglineupdate
{
[map removeOverlay:self.headingline];
self.headingline = nil;
self.headingline = [MKPolyline polylineWithCoordinates:locations count:2];
[map addOverlay:headingline];
[headinglineView setNeedsDisplay];
[map setNeedsDisplay];
}