从特定的给定点获取高度 - mapkit

Question

我试图从一个给定的点找出高度，但我没有用我的iPad得到它。 我已经定义了我的高度变量：

 var altitude: CLLocationDistance = 0.0

我做了这个功能：

       func getAltitude(latitude: CLLocationDistance, longitude: CLLocationDistance) -> CLLocationDistance {

    //Get altitude of touched point
    locationManager.requestWhenInUseAuthorization()
    locationManager.distanceFilter = kCLDistanceFilterNone
    locationManager.desiredAccuracy = kCLLocationAccuracyBest
    let touchPointLocation = CLLocation(latitude: latitude, longitude: longitude)
    let altitude = touchPointLocation.altitude

    return altitude

}

例如，当我触摸地图时，我试图在长时间内获得这样的高度：

let touchPoint = gestureRecognizer.location(in: self.map)
let coord = map.convert(touchPoint, toCoordinateFrom: self.map)        //now this coord is working ok. Its touched point coordinates

let altitude = getAltitude(latitude: coord.latitude, longitude: coord.longitude)

print(altitude) //I get 0.0 here :(

为什么这是错的？我怎么能这样做？