[ios] Fix user location horizontal accuracy ring inaccuracy

Accuracy was off by +25%, depending on latitude.

platform/ios/CHANGELOG.md

@@ -6,6 +6,7 @@ Mapbox welcomes participation and contributions from everyone. Please read [CONT
 
 * Added an MGLStyle.localizesLabels property, off by default, that localizes any Mapbox Streets–sourced symbol layer into the user’s preferred language. ([#9582](https://github.com/mapbox/mapbox-gl-native/pull/9582))
 * Added an additional camera method to MGLMapView that accepts an edge padding parameter. ([#9651](https://github.com/mapbox/mapbox-gl-native/pull/9651))
+* Fixed an issue with the scaling of the user location annotation’s horizontal accuracy indicator. ([#9721](https://github.com/mapbox/mapbox-gl-native/pull/9721))
 
 ## 3.6.1 - July 28, 2017
 

platform/ios/src/MGLFaux3DUserLocationAnnotationView.m

@@ -460,11 +460,8 @@ const CGFloat MGLUserLocationAnnotationArrowSize = MGLUserLocationAnnotationPuck
 
 - (CGFloat)calculateAccuracyRingSize
 {
-    CGFloat latitudeRadians = MGLRadiansFromDegrees(self.userLocation.coordinate.latitude);
-    CGFloat metersPerPoint = [self.mapView metersPerPointAtLatitude:self.userLocation.coordinate.latitude];
-    CGFloat pixelRadius = self.userLocation.location.horizontalAccuracy / cos(latitudeRadians) / metersPerPoint;
-
-    return pixelRadius * 2.0;
+    // diameter in screen points
+    return self.userLocation.location.horizontalAccuracy / [self.mapView metersPerPointAtLatitude:self.userLocation.coordinate.latitude] * 2.0;
 }
 
 - (UIImage *)headingIndicatorTintedGradientImage