MKMapView setRegion not exact - ios6

Question

Starting Situation
iOS6, Apple Maps. I have two annotations on a MKMapView. The coordinates of the two annotations have the same values for the latitude component, but different longitudes.
What I want
I now want to zoom the map so that one annotation is exactly on the left edge of the mapView's frame, and the other annotation on the right edge of the frame. For that I tried MKMapView's setRegion and setVisibleMapRect methods.
The Problem
The problem is that those methods seem to snap to certain zoom levels and therefore not setting the region as exactly as I need it. I saw a lot of questions on Stack Overflow that point out, that this behavior is normal in iOS5 and below. But since Apple Maps are using vector graphics, the view is not bound to certain zoom levels to display imagery in proper resolution.
Tested in...
I tested on iPhone 4, 4S, 5, iPad 3 and iPad Mini (all with iOS6.1), and in the Simulator on iOS6.1.
My question
So why is setRegion and setVisibleMapRect snapping to certain zoom levels and does not adjust the exact region to the values I pass it?
Sample Code
in view did appear I define 4 different Locations as iVars, and set up the map view:
// define coords
_coord1 = CLLocationCoordinate2DMake(46.0, 13.0);
_coord2 = CLLocationCoordinate2DMake(46.0, 13.1);
_coord3 = CLLocationCoordinate2DMake(46.0, 13.2);
_coord4 = CLLocationCoordinate2DMake(46.0, 13.3);
// define frame for map in landscape mode
CGRect mainScreen = [[UIScreen mainScreen] bounds];
CGRect newSRect = mainScreen;
newSRect.size.width = mainScreen.size.height;
newSRect.size.height = mainScreen.size.width;
// setup map view
customMapView = [[MKMapView alloc] initWithFrame:newSRect];
[customMapView setDelegate:self];
[customMapView setShowsUserLocation:NO];
[self.view addSubview:customMapView];
Then I add 3 buttons. Those trigger all the same method addAnnotationsWithCoord1:coord2. The first button passes _coord1 and _coord2, the second button passes _coord1 and _coord3 and the third button passes _coord1 and _coord4. The method looks like so (TapAnnotation is my subclass of MKAnnotation):
-(void)addAnnotationsWithCoord1:(CLLocationCoordinate2D)coord1 coord2:(CLLocationCoordinate2D)coord2{
// Make 2 new annotations with the passed coordinates
TapAnnotation *annot1 = [[TapAnnotation alloc] initWithNumber:0 coordinate:coord1];
TapAnnotation *annot2 = [[TapAnnotation alloc] initWithNumber:0 coordinate:coord2];
// Remove all existing annotations
for(id<MKAnnotation> annotation in customMapView.annotations){
[customMapView removeAnnotation:annotation];
}
// Add annotations to map
[customMapView addAnnotation:annot1];
[customMapView addAnnotation:annot2];
}
After that I determine the SouthWest and NorthEast points that will determine the rect which is containing my 2 annotations.
// get northEast and southWest
CLLocationCoordinate2D northEast;
CLLocationCoordinate2D southWest;
northEast.latitude = MAX(coord1.latitude, coord2.latitude);
northEast.longitude = MAX(coord1.longitude, coord2.longitude);
southWest.latitude = MIN(coord1.latitude, coord2.latitude);
southWest.longitude = MIN(coord1.longitude, coord2.longitude);
Then I calculate the center point between the two coordinates and set the center coordinate of the map to it (remember, since all coordinates have same latitudes the following calculation should be correct):
// determine center coordinate and set to map
double centerLon = ((coord1.longitude + coord2.longitude) / 2.0f);
CLLocationCoordinate2D center = CLLocationCoordinate2DMake(southWest.latitude, centerLon);
[customMapView setCenterCoordinate:center animated:NO];
Now I try to set the region of the map so that it fits like I want:
CLLocation *loc1 = [[CLLocation alloc] initWithLatitude:southWest.latitude longitude:southWest.longitude];
CLLocation *loc2 = [[CLLocation alloc] initWithLatitude:northEast.latitude longitude:northEast.longitude];
CLLocationDistance meterSpanLong = [loc1 distanceFromLocation:loc2];
CLLocationDistance meterSpanLat = 0.1;
MKCoordinateRegion region = MKCoordinateRegionMakeWithDistance(center, meterSpanLat, meterSpanLong);
[customMapView setRegion:region animated:NO];
This does not behave as expected, so I try this:
MKCoordinateSpan span = MKCoordinateSpanMake(0.01, northEast.longitude-southWest.longitude);
MKCoordinateRegion region = MKCoordinateRegionMake(center, span);
[customMapView setRegion:region animated:NO];
This still not behaves as expected, so I try it with setVisibleMapRect:
MKMapPoint westPoint = MKMapPointForCoordinate(southWest);
MKMapPoint eastPoint = MKMapPointForCoordinate(northEast);
MKMapRect mapRect = MKMapRectMake(westPoint.x, westPoint.y,eastPoint.x-westPoint.x,1);
[customMapView setVisibleMapRect:mapRect animated:NO];
And still, it does not behave like I want. As a verification, I calculate the point distance from the left annotation to the left edge of the mapView's frame:
// log the distance from the soutwest point to the left edge of the mapFrame
CGPoint tappedWestPoint = [customMapView convertCoordinate:southWest toPointToView:customMapView];
NSLog(#"distance: %f", tappedWestPoint.x);
For _coord1 and _coord2 it shows: 138
For _coord1 and _coord3 it shows: 138
For _coord1 and _coord4 it shows: 65
So why do I get these values? If anything works as expected, these should all be 0.
Thanks for any help, struggling with this problem for a week now.

Answer 1

Read the docs on setRegion:
When setting this property, the map may adjust the new region value so that it fits the visible area of the map precisely. This is normal and is done to ensure that the value in this property always reflects the visible portion of the map. However, it does mean that if you get the value of this property right after setting it, the returned value may not match the value you set. (You can use the regionThatFits: method to determine the region that will actually be set by the map.)
You will have to figure out the math yourself if you want a precise zoom.