Get meters per pixel at latitude

android/src/main/java/com/mapbox/mapboxgl/MapboxMapController.java

@@ -513,6 +513,13 @@ public void onMethodCall(MethodCall call, MethodChannel.Result result) {
         result.success(reply);
         break;
       }
+      case "map#getMetersPerPixelAtLatitude": {
+        Map<String, Object> reply = new HashMap<>();
+        Double retVal = mapboxMap.getProjection().getMetersPerPixelAtLatitude((Double)call.argument("latitude"));
+        reply.put("metersperpixel", retVal);
+        result.success(reply);
+        break;
+      }
       case "camera#move": {
         final CameraUpdate cameraUpdate = Convert.toCameraUpdate(call.argument("cameraUpdate"), mapboxMap, density);
         if (cameraUpdate != null) {

example/lib/map_ui.dart

@@ -309,6 +309,10 @@ class MapUiBodyState extends State<MapUiBody> {
         Point convertedPoint = await mapController.toScreenLocation(latLng);
         LatLng convertedLatLng = await mapController.toLatLng(point);
         print("Map long press converted: ${convertedPoint.x},${convertedPoint.y}   ${convertedLatLng.latitude}/${convertedLatLng.longitude}");
+        double metersPerPixel = await mapController.getMetersPerPixelAtLatitude(latLng.latitude);
+
+        print ("Map long press The distance measured in meters at latitude ${latLng.latitude} is $metersPerPixel m");
+
 
         List features = await mapController.queryRenderedFeatures(point, [], null);
         if (features.length>0) {

ios/Classes/MapboxMapController.swift

@@ -176,6 +176,13 @@ class MapboxMapController: NSObject, FlutterPlatformView, MGLMapViewDelegate, Ma
             reply["x"] = returnVal.x as NSObject
             reply["y"] = returnVal.y as NSObject
             result(reply)
+        case "map#getMetersPerPixelAtLatitude":
+             guard let arguments = methodCall.arguments as? [String: Any] else { return }
+             var reply = [String: NSObject]()
+             guard let latitude = arguments["latitude"] as? Double else { return }
+             let returnVal = mapView.metersPerPoint(atLatitude:latitude)
+             reply["metersperpixel"] = returnVal as NSObject
+             result(reply)
         case "map#toLatLng":
             guard let arguments = methodCall.arguments as? [String: Any] else { return }
             guard let x = arguments["x"] as? Double else { return }

lib/src/controller.dart

@@ -790,4 +790,10 @@ class MapboxMapController extends ChangeNotifier {
     return MapboxGlPlatform.getInstance(_id).toLatLng(screenLocation);
   }
 
+  /// Returns the distance spanned by one pixel at the specified [latitude] and current zoom level.
+  /// The distance between pixels decreases as the latitude approaches the poles. This relationship parallels the relationship between longitudinal coordinates at different latitudes.
+  Future<double> getMetersPerPixelAtLatitude(double latitude) async{
+    return MapboxGlPlatform.getInstance(_id).getMetersPerPixelAtLatitude(latitude);
+  }
+
 }

mapbox_gl_platform_interface/lib/src/mapbox_gl_platform_interface.dart

@@ -254,4 +254,9 @@ abstract class MapboxGlPlatform {
       throw UnimplementedError(
         'toLatLng() has not been implemented.');
   }
+
+  Future<double> getMetersPerPixelAtLatitude(double latitude) async{
+    throw UnimplementedError(
+        'getMetersPerPixelAtLatitude() has not been implemented.');
+  }
 }

mapbox_gl_platform_interface/lib/src/method_channel_mapbox_gl.dart

@@ -575,4 +575,18 @@ class MethodChannelMapboxGl extends MapboxGlPlatform {
       return new Future.error(e);
     }
   }
+
+  @override
+  Future<double> getMetersPerPixelAtLatitude(double latitude) async{
+    try {
+      var latLngMap = await _channel
+          .invokeMethod('map#getMetersPerPixelAtLatitude', <String, dynamic>{
+        'latitude': latitude,
+      });
+      return latLngMap['metersperpixel'];
+    } on PlatformException catch (e) {
+      return new Future.error(e);
+    }
+  }
+
 }

mapbox_gl_web/lib/mapbox_gl_web.dart

@@ -5,6 +5,7 @@ import 'dart:async';
 // ignore_for_file: avoid_web_libraries_in_flutter
 import 'dart:html';
 import 'dart:js';
+import 'dart:math';
 import 'dart:typed_data';
 import 'dart:ui' as ui;
 

mapbox_gl_web/lib/src/mapbox_map_controller.dart

@@ -628,4 +628,12 @@ class MapboxMapController extends MapboxGlPlatform
     var lngLat = _map.unproject(mapbox.Point(screenLocation.x, screenLocation.y));
     return LatLng(lngLat.lat, lngLat.lng);
   }
+
+  @override
+  Future<double> getMetersPerPixelAtLatitude(double latitude) async{
+    //https://wiki.openstreetmap.org/wiki/Zoom_levels
+    var circumference = 40075017.686;
+    var zoom = _map.getZoom();
+    return circumference * cos(latitude * (pi/180)) / pow(2, zoom + 9);
+  }
 }

pubspec.lock

@@ -15,6 +15,13 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "1.6.0"
+  characters:
+    dependency: transitive
+    description:
+      name: characters
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.0.0"
   charcode:
     dependency: transitive
     description:
@@ -28,7 +35,7 @@ packages:
       name: collection
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.14.12"
+    version: "1.14.13"
   convert:
     dependency: transitive
     description:
@@ -59,14 +66,14 @@ packages:
       name: image
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.13"
+    version: "2.1.14"
   js:
     dependency: transitive
     description:
       name: js
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.6.1+1"
+    version: "0.6.2"
   mapbox_gl_dart:
     dependency: transitive
     description:
@@ -78,20 +85,16 @@ packages:
     dependency: "direct main"
     description:
       path: mapbox_gl_platform_interface
-      ref: HEAD
-      resolved-ref: fa9dac81b6d0f3a5f01688a6695ee938b71874e5
-      url: "https://github.com/tobrun/flutter-mapbox-gl.git"
-    source: git
-    version: "0.7.0"
+      relative: true
+    source: path
+    version: "0.8.0"
   mapbox_gl_web:
     dependency: "direct main"
     description:
       path: mapbox_gl_web
-      ref: HEAD
-      resolved-ref: fa9dac81b6d0f3a5f01688a6695ee938b71874e5
-      url: "https://github.com/tobrun/flutter-mapbox-gl.git"
-    source: git
-    version: "0.7.0"
+      relative: true
+    source: path
+    version: "0.8.0"
   meta:
     dependency: transitive
     description:
@@ -124,7 +127,7 @@ packages:
       name: typed_data
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.6"
+    version: "1.2.0"
   vector_math:
     dependency: transitive
     description:
@@ -138,7 +141,7 @@ packages:
       name: xml
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "3.7.0"
+    version: "4.5.1"
 sdks:
-  dart: ">=2.7.0 <3.0.0"
+  dart: ">=2.9.0-14.0.dev <3.0.0"
   flutter: ">=1.12.13+hotfix.4 <2.0.0"