MKMapView setRegion "snaps" to predefined zoom levels?

Question

Can anyone confirm that setRegion "snaps" to predefined zoom levels and whether or not this behavior is as designed (although undocumented) or a known bug? Specifically, it appears that setRegion snaps to the same zoom levels that correspond to the zoom levels used when the user double-taps the map.

I'm trying to restore a previously saved region but this behavior makes it impossible if the saved region was set via a pinch zoom and not a double-tap zoom.

A big clue to me that things are broken on the mapkit side is what occurs if I call regionThatFits on the map's current region. It should return the same region (since it obviously fits the map's frame) but it returns the region that corresponds to the next higher predefined zoom level instead.

setVisibleMapRect behaves similarly.

Any further insight or information would be appreciated.

I found these related posts but neither included a solution or definitive confirmation that this is in fact a mapkit bug:

MKMapView setRegion: odd behavior?

MKMapView show incorrectly saved region

EDIT:

Here is an example that demonstrates the problem. All values are valid for my map view's aspect ratio:

MKCoordinateRegion initialRegion;
initialRegion.center.latitude = 47.700200f;
initialRegion.center.longitude = -122.367109f;
initialRegion.span.latitudeDelta = 0.065189f;
initialRegion.span.longitudeDelta = 0.067318f;
[map setRegion:initialRegion animated:NO];
NSLog(@"DEBUG initialRegion:  %f  %f  %f  %f", initialRegion.center.latitude, initialRegion.center.longitude, initialRegion.span.latitudeDelta, initialRegion.span.longitudeDelta);
NSLog(@"DEBUG map.region:  %f  %f  %f  %f", map.region.center.latitude, map.region.center.longitude, map.region.span.latitudeDelta, map.region.span.longitudeDelta);

OUTPUT:

DEBUG initialRegion:  47.700199  -122.367111  0.065189  0.067318
DEBUG map.region:  47.700289  -122.367096  0.106287  0.109863

Note the discrepancy in the latitude/longitude delta values. The map's values are almost double what I requested. The larger values correspond to one of the zoom levels used when the user double-taps the map.

Answer 1

Yes, it snaps to discrete levels. I've done quite a bit of experimentation, and it seems to like multiples of 2.68220906e-6 degrees of longitude per pixel.

So if your map fills the whole width of the screen, the first level spans .0008583 degrees, then the next level up you can get is twice that, .001717, and then the next one is twice that, .003433, and so on. I'm not sure why they chose to normalize by longitude, it means that fixes zoom levels vary depending on what part of the world you are looking at.

I've also spent a lot of time trying to understand the significance of that number .68220906e-6 degrees. It comes out to about 30cm at the equator, which kind of makes sense since the high resolution photos used by Google Maps have a 30cm resolution, but I would have expected them to use latitude instead of longitude to establish the zoom levels. That way, at maximum zoom, you always the native resolution of the satellite images, but who knows, they probably have some smart-people reason for making it work like that.

In my application I need to display a certain range of latitude. I'm gonna work on some code to try to zoom the map as close as possible to that. If anyone is interested, contact me.

Answer 2

I found a solution.

If the received snapped zoom level, is, lets say a factor of 1.2 bigger than the desired one: use this algorithm to correct:
Asumption: you want to set the map view to exactly show "longitudinalMeters" from left to right
1) Calculate the correction scale:
Calculate the relation between longitudinal span you received, to that one you have got.

    MKCoordinateRegion region = MKCoordinateRegionMakeWithDistance(center, 0, longitudinalMeters);

    MKCoordinateRegion regionFits = [mapView regionThatFits: region];
    double correctionFactor = regionFits.span.longitudeDelta / region.span.longitudeDelta;

2) Create the transformation and apply it to the map

   CGAffineTransform mapTransform = CGAffineTransformMakeScale(correctionScale, correctionScale);       
   CGAffineTransform pinTransform = CGAffineTransformInvert(mapTransform);
   [mapView setTransform:mapTransform];

3) Apply the inverse transformation to the Map pins, to keep them at original size

 [mapView setTransform:mapTransform];
 for (id<MKAnnotation> annotation in self.mapView.annotations)
 {
     [[self.mapView viewForAnnotation:annotation] setTransform:pinTransform];
 }