Matt Rajca

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Rendering Google Sky Tiles with MapKit

October 08, 2013

As of iOS 7, MapKit includes a MKTileOverlay class that can be used to efficiently replace Apple Maps data with custom tiles. What’s more, MKTileOverlay’s initializer accepts URL templates with x, y, and z parameters, which are implemented by many mapping services that use Mercator projection. This makes it trivial for third-party mapping software to leverage MapKit as a renderer for custom tile data.

That’s great, but custom tile overlays aren’t just useful for terrestrial imagery. Google Sky tiles can be accessed at URLs of format:

	
	http://mw1.google.com/mw-planetary/sky/skytiles_v1/{x}_{y}_{z}.jpg
	

Sure enough, we can use MapKit to render Google Sky imagery in just 12 lines of code:

	
	- (void)awakeFromNib {
		MKMapView *mv = [[MKMapView alloc] initWithFrame:[UIScreen mainScreen].bounds];
		mv.delegate = self;
	
		MKTileOverlay *overlay = [[MKTileOverlay alloc] initWithURLTemplate:
								  @"http://mw1.google.com/mw-planetary/sky/skytiles_v1/{x}_{y}_{z}.jpg"];
		overlay.canReplaceMapContent = YES;
		[mv addOverlay:overlay level:MKOverlayLevelAboveLabels];
	
		[self.view addSubview:mv];
	}
	
	- (MKOverlayRenderer *)mapView:(MKMapView *)mapView rendererForOverlay:(id<MKOverlay>)overlay {
		return [[MKTileOverlayRenderer alloc] initWithTileOverlay:overlay];
	}
	

Under the hood, MapKit will now load Google Sky tiles on demand as the user pans and zooms.

The complete project can be found on GitHub.

Disclaimer: This project was created for experimental purposes. Google Sky data should not be used in shipping code.