Get & set the zoom level of a MKMapView

MKMapView+Extensions

@interface MKMapView (Extensions)

-(double)as_zoomLevel;
-(void)as_setCenterCoordinate:(CLLocationCoordinate2D)centerCoordinate zoomLevel:(NSUInteger)zoomLevel animated:(BOOL)animated;

@end

static double mercadorRadius = 85445659.44705395;
static double mercadorOffset = 268435456;

@implementation MKMapView (Extensions)

-(double)as_zoomLevel {
    
    // Derived from http://stackoverflow.com/questions/7594827/how-to-find-current-zoom-level-of-mkmapview
    
    static double maxGoogleLevels = -1.0;
    
    if (maxGoogleLevels < 0.0)
        maxGoogleLevels = log2(MKMapSizeWorld.width / 256.0);
    
    CLLocationDegrees longitudeDelta = self.region.span.longitudeDelta;
    CGFloat mapWidthInPixels = self.bounds.size.width;
    double zoomScale = longitudeDelta * mercadorRadius * M_PI / (180.0 * mapWidthInPixels);
    double zoomer = maxGoogleLevels - log2( zoomScale );
    if ( zoomer < 0 ) zoomer = 0;

    return zoomer;
}


// http://troybrant.net/blog/2010/01/set-the-zoom-level-of-an-mkmapview/

#pragma mark -
#pragma mark Map conversion methods

-(double)as_longitudeToPixelSpaceX:(double)longitude {
    return round(mercadorOffset + mercadorRadius * longitude * M_PI / 180.0);
}

-(double)as_latitudeToPixelSpaceY:(double)latitude {
    return round(mercadorOffset - mercadorRadius * logf((1 + sinf(latitude * M_PI / 180.0)) / (1 - sinf(latitude * M_PI / 180.0))) / 2.0);
}

-(double)as_pixelSpaceXToLongitude:(double)pixelX {
    return ((round(pixelX) - mercadorOffset) / mercadorRadius) * 180.0 / M_PI;
}

-(double)as_pixelSpaceYToLatitude:(double)pixelY {
    return (M_PI / 2.0 - 2.0 * atan(exp((round(pixelY) - mercadorOffset) / mercadorRadius))) * 180.0 / M_PI;
}

#pragma mark -
#pragma mark Helper methods

-(MKCoordinateSpan)as_coordinateSpanWithMapView:(MKMapView *)mapView centerCoordinate:(CLLocationCoordinate2D)centerCoordinate andZoomLevel:(NSUInteger)zoomLevel {
    
    // convert center coordiate to pixel space
    double centerPixelX = [self as_longitudeToPixelSpaceX:centerCoordinate.longitude];
    double centerPixelY = [self as_latitudeToPixelSpaceY:centerCoordinate.latitude];
    
    // determine the scale value from the zoom level
    NSInteger zoomExponent = 20 - zoomLevel;
    double zoomScale = pow(2, zoomExponent);
    
    // scale the map’s size in pixel space
    CGSize mapSizeInPixels = mapView.bounds.size;
    double scaledMapWidth = mapSizeInPixels.width * zoomScale;
    double scaledMapHeight = mapSizeInPixels.height * zoomScale;
    
    // figure out the position of the top-left pixel
    double topLeftPixelX = centerPixelX - (scaledMapWidth / 2);
    double topLeftPixelY = centerPixelY - (scaledMapHeight / 2);
    
    // find delta between left and right longitudes
    CLLocationDegrees minLng = [self as_pixelSpaceXToLongitude:topLeftPixelX];
    CLLocationDegrees maxLng = [self as_pixelSpaceXToLongitude:topLeftPixelX + scaledMapWidth];
    CLLocationDegrees longitudeDelta = maxLng - minLng;
    
    // find delta between top and bottom latitudes
    CLLocationDegrees minLat = [self as_pixelSpaceYToLatitude:topLeftPixelY];
    CLLocationDegrees maxLat = [self as_pixelSpaceYToLatitude:topLeftPixelY + scaledMapHeight];
    CLLocationDegrees latitudeDelta = -1 * (maxLat - minLat);
    
    // create and return the lat/lng span
    MKCoordinateSpan span = MKCoordinateSpanMake(latitudeDelta, longitudeDelta);
    return span;
}

-(void)as_setCenterCoordinate:(CLLocationCoordinate2D)centerCoordinate zoomLevel:(NSUInteger)zoomLevel animated:(BOOL)animated {
    
    // clamp large numbers to 28
    zoomLevel = MIN(zoomLevel, 28);
    
    // use the zoom level to compute the region
    MKCoordinateSpan span = [self as_coordinateSpanWithMapView:self centerCoordinate:centerCoordinate andZoomLevel:zoomLevel];
    MKCoordinateRegion region = MKCoordinateRegionMake(centerCoordinate, span);
    
    // set the region like normal
    [self setRegion:region animated:animated];}

@end