在mapview上将跨度值转换为米

Question

每当用户放大或缩小地图时,我都需要知道地图上当前表示了多少米(宽度或高度).

我需要的是MKCoordinateRegionMakeWithDistance的反函数来计算当前地图范围所代表的距离.

我尝试了以下代码但是我得到了错误的结果:

- (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);
}

如果我将地图区域设置为1500米,那么我得到1800这样的结果.

谢谢你的帮助,文森特

Answer 1

实际上这是一个非常愚蠢的错误,如果我沿X轴进行相同的操作,那么我得到正确的值:

- (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);
}

Answer 2

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

根据文档http://developer.apple.com/library/ios/#documentation/MapKit/Reference/MapKitDataTypesReference/Reference/reference.html,

latitudeDelta

要在地图上显示的南北距离(以度为单位)的数量.与纵向距离不同,纵向距离根据纬度而变化,一个纬度总是大约111公里(69英里).