MKMapView+ZoomLevel.h

//
//  MKMapView+ZoomLevel.h
//
//  Created by Wouter De Schuyter on 30/11/14.
//  Copyright (c) 2014 Wouter De Schuyter. All rights reserved.
//

#import <MapKit/MapKit.h>

@interface MKMapView (ZoomLevel)

- (double)longitudeToPixelSpaceX:(double)longitude;
- (double)latitudeToPixelSpaceY:(double)latitude;
- (double)pixelSpaceXToLongitude:(double)pixelX;
- (double)pixelSpaceYToLatitude:(double)pixelY;
- (void)setCenterCoordinate:(CLLocationCoordinate2D)centerCoordinate zoomLevel:(NSUInteger)zoomLevel animated:(BOOL)animated;
- (MKCoordinateSpan)coordinateSpanWithMapView:(MKMapView *)mapView centerCoordinate:(CLLocationCoordinate2D)centerCoordinate andZoomLevel:(NSUInteger)zoomLevel;

@end

MKMapView+ZoomLevel.m

//
//  MKMapView+ZoomLevel.m
//
//  Created by Wouter De Schuyter on 30/11/14.
//  Copyright (c) 2014 Wouter De Schuyter. All rights reserved.
//

#import "MKMapView+ZoomLevel.h"

#define MERCATOR_OFFSET 268435456
#define MERCATOR_RADIUS 85445659.44705395

@implementation MKMapView (ZoomLevel)

- (double)longitudeToPixelSpaceX:(double)longitude
{
    return round(MERCATOR_OFFSET + MERCATOR_RADIUS * longitude * M_PI / 180.0);
}

- (double)latitudeToPixelSpaceY:(double)latitude
{
    return round(MERCATOR_OFFSET - MERCATOR_RADIUS * logf((1 + sinf(latitude * M_PI / 180.0)) / (1 - sinf(latitude * M_PI / 180.0))) / 2.0);
}

- (double)pixelSpaceXToLongitude:(double)pixelX
{
    return ((round(pixelX) - MERCATOR_OFFSET) / MERCATOR_RADIUS) * 180.0 / M_PI;
}

- (double)pixelSpaceYToLatitude:(double)pixelY
{
    return (M_PI / 2.0 - 2.0 * atan(exp((round(pixelY) - MERCATOR_OFFSET) / MERCATOR_RADIUS))) * 180.0 / M_PI;
}

- (void)setCenterCoordinate:(CLLocationCoordinate2D)centerCoordinate zoomLevel:(NSUInteger)zoomLevel animated:(BOOL)animated
{
    zoomLevel = MIN(zoomLevel, 28);
    
    MKCoordinateSpan span = [self coordinateSpanWithMapView:self centerCoordinate:centerCoordinate andZoomLevel:zoomLevel];
    MKCoordinateRegion region = MKCoordinateRegionMake(centerCoordinate, span);
    
    [self setRegion:region animated:animated];
}

- (MKCoordinateSpan)coordinateSpanWithMapView:(MKMapView *)mapView centerCoordinate:(CLLocationCoordinate2D)centerCoordinate andZoomLevel:(NSUInteger)zoomLevel
{
    double centerPixelX = [self longitudeToPixelSpaceX:centerCoordinate.longitude];
    double centerPixelY = [self latitudeToPixelSpaceY:centerCoordinate.latitude];
    
    NSInteger zoomExponent = 20 - zoomLevel;
    double zoomScale = pow(2, zoomExponent);
    
    CGSize mapSizeInPixels = mapView.bounds.size;
    double scaledMapWidth = mapSizeInPixels.width * zoomScale;
    double scaledMapHeight = mapSizeInPixels.height * zoomScale;
    
    double topLeftPixelX = centerPixelX - (scaledMapWidth / 2);
    double topLeftPixelY = centerPixelY - (scaledMapHeight / 2);
    
    CLLocationDegrees minLng = [self pixelSpaceXToLongitude:topLeftPixelX];
    CLLocationDegrees maxLng = [self pixelSpaceXToLongitude:topLeftPixelX + scaledMapWidth];
    CLLocationDegrees longitudeDelta = maxLng - minLng;
    
    CLLocationDegrees minLat = [self pixelSpaceYToLatitude:topLeftPixelY];
    CLLocationDegrees maxLat = [self pixelSpaceYToLatitude:topLeftPixelY + scaledMapHeight];
    CLLocationDegrees latitudeDelta = -1 * (maxLat - minLat);
    
    MKCoordinateSpan span = MKCoordinateSpanMake(latitudeDelta, longitudeDelta);
    
    return span;
}

@end