How do can I add 2 miles to viewRegion.span.latitudeDelta?

How do can I add 2 miles to viewRegion.span.latitudeDelta ? How do can I calculate kTwoMiles?

MKCoordinateRegion viewRegio开发者_开发技巧n;
viewRegion.center = coordinateCentre;
viewRegion.span.latitudeDelta = latitudeMax - latitudeMin + kTwoMiles;  
viewRegion.span.longitudeDelta = longitudeMax - longitudeMin + kTwoMiles;

MKCoordinateRegion adjustedRegion = [mapView regionThatFits:viewRegion];
[mapView setRegion:adjustedRegion animated:YES];

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Basically this is relatively complicated because every degree has a different delta.
At Earth's equator you will have 111 km per degree and at the poles 0 km. (http://en.wikipedia.org/wiki/Longitude#Degree_length)

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I decided to add 2 miles only in the latitudeDelta. Probably regionThatFits will resize longitudeDelta.

double kTwoMiles = (double)2/69; 
MKCoordinateRegion viewRegion;
viewRegion.center = coordinateCentre;
viewRegion.span.latitudeDelta = latitudeMax - latitudeMin + kTwoMiles;  
viewRegion.span.longitudeDelta = longitudeMax - longitudeMin;

MKCoordinateRegion adjustedRegion = [mapView regionThatFits:viewRegion];
[mapView setRegion:adjustedRegion animated:YES];`enter code here`