MKMapViewの地図をピン（annotation）が全て表示される大きさに調整する

gistfile1.m

// 
// initでannotationクラスのcoordinateプロパティにINVALID_COORDINATEをセットしておき、
// coordinateプロパティがまだセットされていない状態であることを示す。
//
// if (CLLocationCoordinate2DIsValid(coordinate))で検証する
// 
#define INVALID_LOCATION_DEGREE -1000
#define INVALID_COORDINATE CLLocationCoordinate2DMake(INVALID_LOCATION_DEGREE, INVALID_LOCATION_DEGREE)
// 最低限の地図の大きさ
#define MINIMUM_MAP_SPAN MKCoordinateSpanMake(0.002, 0.002)

- (void)zoomToFitMapAnnotations:(MKMapView *)mapView { 
  if ([mapView.annotations count] == 0) return; 
  
  CLLocationCoordinate2D topLeftCoord; 
  topLeftCoord.latitude = -90; 
  topLeftCoord.longitude = 180; 
  
  CLLocationCoordinate2D bottomRightCoord; 
  bottomRightCoord.latitude = 90; 
  bottomRightCoord.longitude = -180; 
  
  for(id<MKAnnotation> annotation in mapView.annotations) {
    if (CLLocationCoordinate2DIsValid(annotation.coordinate)) {
      topLeftCoord.longitude = fmin(topLeftCoord.longitude, annotation.coordinate.longitude); 
      topLeftCoord.latitude = fmax(topLeftCoord.latitude, annotation.coordinate.latitude); 
      bottomRightCoord.longitude = fmax(bottomRightCoord.longitude, annotation.coordinate.longitude); 
      bottomRightCoord.latitude = fmin(bottomRightCoord.latitude, annotation.coordinate.latitude); 
    }
  }
  MKCoordinateRegion region; 
  region.center.latitude = topLeftCoord.latitude - (topLeftCoord.latitude - bottomRightCoord.latitude) * 0.5; 
  region.center.longitude = topLeftCoord.longitude + (bottomRightCoord.longitude - topLeftCoord.longitude) * 0.5;      
  
  // 端ギリギリにピンが立つのを避けるため少し広げる
  region.span.latitudeDelta = fabs(topLeftCoord.latitude - bottomRightCoord.latitude) * 1.1; 
  region.span.longitudeDelta = fabs(bottomRightCoord.longitude - topLeftCoord.longitude) * 1.1; 
  // 拡大しすぎの場合は最低限の縮尺にする
  region.span.latitudeDelta = fmax(MINIMUM_MAP_SPAN.latitudeDelta, region.span.latitudeDelta);
  region.span.longitudeDelta = fmax(MINIMUM_MAP_SPAN.longitudeDelta, region.span.longitudeDelta);
  
  region = [mapView regionThatFits:region]; 
  [mapView setRegion:region animated:YES]; 
}

参考
http://stackoverflow.com/questions/1336370/positioning-mkmapview-to-show-multiple-annotations-at-once
http://stackoverflow.com/questions/1663276/handle-empty-structs-in-objective-c-coordinate-in-custom-class
http://developer.apple.com/library/ios/#DOCUMENTATION/CoreLocation/Reference/CoreLocationFunctions/Reference/reference.html
http://developer.apple.com/library/ios/#documentation/MapKit/Reference/MapKitFunctionsReference/Reference/reference.html