How to find the radius of visible MKMapView visible screen area?

Question

I want to know the radius of visible area in iphone screen, as I will zoomout and zoom in the visible area will change, so I want to know the radius of that particular area, how can I do it?

Answer 1

Its not radius what is required.

You need to use the region parameter from mapView.

Check out apple docs, it is pretty much clear from those.

Go thru this tutorial. It will help you a lot

icode blog mapkit demo

specifically you need to set something like this..

MKCoordinateSpan span = [self coordinateSpanWithMapView:self centerCoordinate:centerCoordinate andZoomLevel:zoomLevel];
MKCoordinateRegion region = MKCoordinateRegionMake(centerCoordinate, span);
[self setRegion:region animated:animated];

where span can be calculated as

- (MKCoordinateSpan)coordinateSpanWithMapView:(MKMapView *)mapView
                         centerCoordinate:(CLLocationCoordinate2D)centerCoordinate
                             andZoomLevel:(NSUInteger)zoomLevel
{
// convert center coordiate to pixel space
double centerPixelX = [self longitudeToPixelSpaceX:centerCoordinate.longitude];
double centerPixelY = [self latitudeToPixelSpaceY:centerCoordinate.latitude];

// determine the scale value from the zoom level
NSInteger zoomExponent = 20 - zoomLevel;
double zoomScale = pow(2, zoomExponent);

// scale the map’s size in pixel space
CGSize mapSizeInPixels = mapView.bounds.size;
double scaledMapWidth = mapSizeInPixels.width * zoomScale;
double scaledMapHeight = mapSizeInPixels.height * zoomScale;

// figure out the position of the top-left pixel
double topLeftPixelX = centerPixelX - (scaledMapWidth / 2);
double topLeftPixelY = centerPixelY - (scaledMapHeight / 2);

// find delta between left and right longitudes
CLLocationDegrees minLng = [self pixelSpaceXToLongitude:topLeftPixelX];
CLLocationDegrees maxLng = [self pixelSpaceXToLongitude:topLeftPixelX + scaledMapWidth];
CLLocationDegrees longitudeDelta = maxLng - minLng;

// find delta between top and bottom latitudes
CLLocationDegrees minLat = [self pixelSpaceYToLatitude:topLeftPixelY];
CLLocationDegrees maxLat = [self pixelSpaceYToLatitude:topLeftPixelY + scaledMapHeight];
CLLocationDegrees latitudeDelta = -1 * (maxLat - minLat);

// create and return the lat/lng span
MKCoordinateSpan span = MKCoordinateSpanMake(latitudeDelta, longitudeDelta);
return span;
}

Cheers :)

Answer 2

I might be misunderstanding the question, but isn't it as simple as:

- (void)mapView:(MKMapView *)mapView regionDidChangeAnimated:(BOOL)animated {
    CGFloat latD = mapView.region.span.latitudeDelta;
    CGFloat lngD = mapView.region.span.longitudeDelta;
    NSLog(@"This is the latitude delta of the visible map: %f", latD);
    NSLog(@"This is the longitude delta of the visible map: %f", lngD);
}