Question

I’m having an issue with both a custom MKOverlayView and standard MKPolygonView being clipped at certain zoom levels when there are multiple overlays added to a map.

The overlay of Algeria at two double tap zoom level.

The overlay of Algeria at three double tap zoom level. Note the clipping.

A few observations:

• This occurs regardless of whether or not I use a custom MKOverlayView or return an MKPolygonView with the same polygons.
• If I only draw one overlay, this problem does not occur.
• This does not occur for all overlays – only some.

As far as code goes: this adds the overlay to an NSMutableArray (borderOverlays), which is then accessed elsewhere to load the overlay for a specific country ID. minX/minY/maxX/maxY are latitude/longitude values; polygon is a path constructed from an ESRI shapefile.

CLLocationCoordinate2D mbrMin = CLLocationCoordinate2DMake(minY, minX);
CLLocationCoordinate2D mbrMax = CLLocationCoordinate2DMake(maxY, maxX);
MKMapPoint minPoint = MKMapPointForCoordinate(mbrMin);
MKMapPoint maxPoint = MKMapPointForCoordinate(mbrMax);
MKMapSize size = MKMapSizeMake(maxPoint.x - minPoint.x, maxPoint.y - minPoint.y);
MKMapRect rect = MKMapRectMake(minPoint.x, minPoint.y, size.width, size.height);

if ( spans180 ) {
    rect = MKMapRectMake(minPoint.x, minPoint.y, MKMapSizeWorld.width * 2, size.height);
}

CustomMKOverlay* overlay = [[CustomMKOverlay alloc] initWithPolygon:polygon withBoundingMapRect:rect];
[borderOverlays addObject:overlay];

The overlay is added to the map via:

[mapView addOverlay:overlay];

viewForOverlay:

- (MKOverlayView *)mapView:(MKMapView*)aMapView viewForOverlay:(id<MKOverlay>)overlay
{
    if ( [overlay isKindOfClass:[CustomMKOverlay class]] ) {
        /* Note: the behaviour if this chunk is not commented is the exact same as below. 
        CustomMKOverlayView* overlayView = [[[CustomMKOverlayView alloc] initWithOverlay:overlay withMapView:aMapView] autorelease];
        [borderViews addObject:overlayView];
        return overlayView; */

        MKPolygonView* view = [[[MKPolygonView alloc] initWithPolygon:((CustomMKOverlay*)overlay).polygon] autorelease];
        view.fillColor = [((CustomMKOverlay*)overlay).colour colorWithAlphaComponent:0.5f];
        view.lineWidth = 5.0f;
        view.strokeColor = [UIColor blackColor];
        [borderViews addObject:view];
        return view;
    }
}

When MKPolygonView is used, there is no drawing code (the example shown). For completion’s sake, though, here’s my custom drawing code, and the same issue occurs. The outlines normally draw – this is actually debugging drawing, which draws a rect around the boundingMapRect of the overlay and fills it without mucking around with the outlines.

- (void)drawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale inContext:(CGContextRef)context
{    
    CustomMKOverlay* overlay = (CustomMKOverlay*)self.overlay;

    CGRect clipRect = [self rectForMapRect:overlay.boundingMapRect];
    CGContextAddRect(context, clipRect);
    CGContextClip(context);

    UIColor* colour = [UIColor redColor];
    colour = [colour colorWithAlphaComponent:0.5f];
    CGContextSetFillColorWithColor(context, [colour CGColor]);
    CGRect fillRect = [self rectForMapRect:overlay.boundingMapRect];
    CGContextFillRect(context, fillRect);
}

Suffice to say, I’m a bit stumped at this point – it’s almost as if the zoomed tiled that’s being loaded draws over the overlay. I’ve poured over various examples regarding TileMap and HazardMap, but as I am not loading my own map tiles, they’re not very helpful.

I’m probably missing something painfully obvious. Any help would be appreciated. I’m happy to provide more code/context if necessary.

Answer 1

It would appear that the culprit is:

CLLocationCoordinate2D mbrMin = CLLocationCoordinate2DMake(minY, minX);
CLLocationCoordinate2D mbrMax = CLLocationCoordinate2DMake(maxY, maxX);

Bounding rectangles for MKOverlays apparently need to be based on the northwest/southeast coordinates of the bounding region, and not southwest/northeast (which is the format the ESRI shapefile stores its bounding coordinates in). Changing the offending code to:

CLLocationCoordinate2D mbrMin = CLLocationCoordinate2DMake(maxY, minX);
CLLocationCoordinate2D mbrMax = CLLocationCoordinate2DMake(minY, maxX);

Appears to resolve all issues with zooming and strange outline anomalies. I hope this helps anyone who comes across this problem in the future (and I’d like to hear about it if it doesn’t, since this solution works a treat for me).

Also: if anyone can point to any documentation that states this, I’d like to see it.