Hide MKAnnotations when user zooms out and annotations are too close to each other

Hide MKAnnotations when user zooms out and annotations are too close to
each other

I've seen this in some apps so far that when the user zooms out then the
annotations move closer to each other and if they're too close then
they're replaced by e.g. a '+5' pin or something else.
How to do that? I think it should be done in regionDidChangeAnimated and
checking the distance on mapview (not the real distance) for each Pin to
each other.
Would that be the right approach? How to get the distance on mapview
instead of distance inside it (e.g. distance between NY and SF will always
be the same, but if user zooms out, the distance between the pins on the
map shrink)