Question

I want to be able to update the region displayed on a MKMapView by allowing the user to type in an address or location in a UIAlertView. I currently have:

        if (geocoder.geocoding)
            [geocoder cancelGeocode];

        [geocoder geocodeAddressString:[[alertView textFieldAtIndex:0] text] completionHandler:^(NSArray *placemarks, NSError *error) {
            if (!error) {
                NSLog(@"Found a location");
            } else {
                NSLog(@"Error in geocoding");
            }

            NSLog(@"Num found: %d", [placemarks count]);

            CLPlacemark *placemark = [placemarks objectAtIndex:0];
            MKCoordinateRegion region;
            region.center.latitude = placemark.region.center.latitude;
            region.center.longitude = placemark.region.center.longitude;
            MKCoordinateSpan span;
            double radius = placemark.region.radius / 1000;

            NSLog(@"Radius is %f", radius);
            span.latitudeDelta = radius / 112.0;
            //span.longitudeDelta = ??? 

            region.span = span;

            NSLog(@"Region is %f %f %f", region.center.latitude, region.center.longitude, span.latitudeDelta);

            [mapView setRegion:region animated:YES];
        }];

My problem is I am unsure how to calculate the longitude delta.

Answer 1

You could just set it equal to latitudeDelta and the map view will adjust as needed.

But you don’t need to calculate the span yourself in the first place. You can use:

region = MKCoordinateRegionMakeWithDistance(
             placemark.region.center, 
             placemark.region.radius, 
             placemark.region.radius);

Not sure about the second part of your question.

---

In iOS 7 and higher, the region returned by the CLPlacemark is actually a CLCircularRegion (see Deprecated CLRegion methods – how to get radius?).

Although the original code will still work as-is, you may get a compiler warning that radius and center are deprecated.

To avoid the warning, cast the region as a CLCircularRegion:

CLCircularRegion *pmCircularRegion = (CLCircularRegion *)placemark.region;

region = MKCoordinateRegionMakeWithDistance(
         pmCircularRegion.center,
         pmCircularRegion.radius,
         pmCircularRegion.radius);