计算Google地图中可见地图用户的半径

Question

What I want to do is calculate a circle's radius by which the user is viewing the map.
I've written the solution so far as follows (which is true):

mMap.setOnCameraChangeListener(new GoogleMap.OnCameraChangeListener() {
        @Override
        public void onCameraChange(CameraPosition cameraPosition) {
            String TAG = AppController.TAG;
            LatLngBounds bounds = mMap.getProjection().getVisibleRegion().latLngBounds;
            LatLng target = cameraPosition.target;
            LatLng northEast = bounds.northeast;
            LatLng southEast = bounds.southwest;
            float[] results1 = new float[1];
            float[] results2 = new float[1];
            Location.distanceBetween(target.latitude, target.longitude, northEast.latitude, northEast.longitude, results1);
            Location.distanceBetween(target.latitude, target.longitude, southEast.latitude, southEast.longitude, results2);
            double distance = results1[0] > results2[0] ? results1[0] : results2[0];
            Log.d(TAG, "onCameraChange:" + results1[0] + "  " + results2[0]);
        }
    });

I'm facing two questions here:
1- First of all why the distane between the center and north east isn't equal to south west?
2- Is there any built in method to achieve the same result?

Answer 1

as you can see from the map above, the horizontal line (latitude) gets shorter the further away you get from the equator.

on the screen, the map is 'distorted' to flatten the curved surface of the earth, with +/- the same amount of longitude and latitude from the center of the view.

so unless you are taking measurements directly at the equator, you will have slight differences in the radius. for small distances, this is negligible.

if accuracy is not important, you can try using the smaller radius (if you want to draw a circle on the view that will not be partially hidden), or use the bigger radius (if you want to do calculations for places within that cicle).