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Convert span value into meters on a mapview

Whenever the user zoom in or out the map i need to know how many meters are currently represented on the map (width or height).

What i need is the inverse function of MKCoordinateRegionMakeWithDistance to calculate the distance represented by the current map span.

I tried the following code but i get wrong results :

- (void)mapView:(MKMapView *)mapView regionDidChangeAnimated:(BOOL)animated {
    MKMapRect mRect = self.map.visibleMapRect;
    MKMapPoint northMapPoint = MKMapPointMake(MKMapRectGetMidX(mRect), MKMapRectGetMinY(mRect));
    MKMapPoint southMapPoint = MKMapPointMake(MKMapRectGetMidX(mRect), MKMapRectGetMaxY(mRect));

    self.currentDist = MKMetersBetweenMapPoints(northMapPoint, southMapPoint);
}

If i set the map region to 1500 meters i get something like 1800 as a result..

Thanks for your help, Vincent

like image 716
vdaubry Avatar asked Mar 11 '11 08:03

vdaubry


4 Answers

Actually it was a really stupid mistake, if i do the same operation along the X axis then i get the correct value :

- (void)mapView:(MKMapView *)mapView regionDidChangeAnimated:(BOOL)animated {
MKMapRect mRect = self.map.visibleMapRect;
MKMapPoint eastMapPoint = MKMapPointMake(MKMapRectGetMinX(mRect), MKMapRectGetMidY(mRect));
MKMapPoint westMapPoint = MKMapPointMake(MKMapRectGetMaxX(mRect), MKMapRectGetMidY(mRect));

self.currentDist = MKMetersBetweenMapPoints(eastMapPoint, westMapPoint);
}
like image 72
vdaubry Avatar answered Nov 01 '22 01:11

vdaubry


- (void)mapView:(MKMapView *)map regionDidChangeAnimated:(BOOL)animated {
  MKCoordinateSpan span = mapView.region.span;
  NSLog(@" 1 = ~111 km -> %f = ~ %f km ",span.latitudeDelta,span.latitudeDelta*111);
}

According to the docs http://developer.apple.com/library/ios/#documentation/MapKit/Reference/MapKitDataTypesReference/Reference/reference.html,

latitudeDelta

The amount of north-to-south distance (measured in degrees) to display on the map. Unlike longitudinal distances, which vary based on the latitude, one degree of latitude is always approximately 111 kilometers (69 miles).

like image 43
johndpope Avatar answered Nov 01 '22 03:11

johndpope


Thanks for the post all. I have an app that required a mile radius to figure out how many location records to fetch so this came in handy. Here is the swift equivalent for anyone who might come across this in the future.

let mRect: MKMapRect = self.mapView.visibleMapRect
        let eastMapPoint = MKMapPointMake(MKMapRectGetMinX(mRect), MKMapRectGetMidY(mRect))
        let westMapPoint = MKMapPointMake(MKMapRectGetMaxX(mRect), MKMapRectGetMidY(mRect))
        let currentDistWideInMeters = MKMetersBetweenMapPoints(eastMapPoint, westMapPoint)
        let milesWide = currentDistWideInMeters / 1609.34  // number of meters in a mile
        println(milesWide)
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Christopher Wade Cantley Avatar answered Nov 01 '22 01:11

Christopher Wade Cantley


Swift 4.2

    func mapView(_ mapView: MKMapView, regionDidChangeAnimated animated: Bool) {

        let mapRect = mapView.visibleMapRect
        let westMapPoint = MKMapPoint(x: mapRect.minX, y: mapRect.midY)
        let eastMapPoint = MKMapPoint(x: mapRect.maxX, y: mapRect.midY)
        let visibleDistance = westMapPoint.distance(to: eastMapPoint)
    }
like image 2
rbaldwin Avatar answered Nov 01 '22 01:11

rbaldwin