diff --git a/Billing.Shared/Billing.Shared.projitems b/Billing.Shared/Billing.Shared.projitems
index 3d02ec0..4490d62 100644
--- a/Billing.Shared/Billing.Shared.projitems
+++ b/Billing.Shared/Billing.Shared.projitems
@@ -22,6 +22,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Helper.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Languages\PlatformCulture.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Languages\Resource.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)LocationExtension.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)MainShell.xaml.cs">
       <DependentUpon>MainShell.xaml</DependentUpon>
       <SubType>Code</SubType>
diff --git a/Billing.Shared/Helper.cs b/Billing.Shared/Helper.cs
index 897162c..69223a3 100644
--- a/Billing.Shared/Helper.cs
+++ b/Billing.Shared/Helper.cs
@@ -147,39 +147,6 @@ namespace Billing
             }
             return status;
         }
-
-        private static double TransformLatitude(double x, double y)
-        {
-            var ret = -100.0 + 2.0 * x + 3.0 * y + 0.2 * y * y + 0.1 * x * y + 0.2 * Math.Sqrt(Math.Abs(x));
-            ret += (20.0 * Math.Sin(6.0 * x * Math.PI) + 20.0 * Math.Sin(2.0 * x * Math.PI)) * 2.0 / 3.0;
-            ret += (20.0 * Math.Sin(y * Math.PI) + 40.0 * Math.Sin(y / 3.0 * Math.PI)) * 2.0 / 3.0;
-            ret += (160.0 * Math.Sin(y / 12.0 * Math.PI) + 320 * Math.Sin(y * Math.PI / 30.0)) * 2.0 / 3.0;
-            return ret;
-        }
-
-        private static double TransformLongitude(double x, double y)
-        {
-            var ret = 300.0 + x + 2.0 * y + 0.1 * x * x + 0.1 * x * y + 0.1 * Math.Sqrt(Math.Abs(x));
-            ret += (20.0 * Math.Sin(6.0 * x * Math.PI) + 20.0 * Math.Sin(2.0 * x * Math.PI)) * 2.0 / 3.0;
-            ret += (20.0 * Math.Sin(x * Math.PI) + 40.0 * Math.Sin(x / 3.0 * Math.PI)) * 2.0 / 3.0;
-            ret += (150.0 * Math.Sin(x / 12.0 * Math.PI) + 300.0 * Math.Sin(x / 30.0 * Math.PI)) * 2.0 / 3.0;
-            return ret;
-        }
-
-        public static (double longitude, double latitude) Wgs84ToGcj02(double longitude, double latitude)
-        {
-            var a = 6378245.0;
-            var ee = 0.00669342162296594323;
-            var offsetLatitude = TransformLatitude(longitude - 105.0, latitude - 35.0);
-            var offsetLongitude = TransformLongitude(longitude - 105.0, latitude - 35.0);
-            var radiusLatitude = latitude / 180.0 * Math.PI;
-            var magic = Math.Sin(radiusLatitude);
-            magic = 1 - ee * magic * magic;
-            var sqrtMagic = Math.Sqrt(magic);
-            offsetLatitude = offsetLatitude * 180.0 / (a * (1 - ee) / (magic * sqrtMagic) * Math.PI);
-            offsetLongitude = offsetLongitude * 180.0 / (a / sqrtMagic * Math.Cos(radiusLatitude) * Math.PI);
-            return (longitude + offsetLongitude, latitude + offsetLatitude);
-        }
     }
 
     public class AsyncLazy<T>
diff --git a/Billing.Shared/LocationExtension.cs b/Billing.Shared/LocationExtension.cs
new file mode 100644
index 0000000..01e2a6a
--- /dev/null
+++ b/Billing.Shared/LocationExtension.cs
@@ -0,0 +1,54 @@
+using System;
+
+namespace Billing
+{
+    public static class LocationExtension
+    {
+        private const double PI = 3.1415926535897931;
+
+        public static (double longitude, double latitude) Wgs84ToGcj02(this (double lon, double lat) position)
+        {
+            if (IsOutOfChina(position.lon, position.lat))
+            {
+                return position;
+            }
+
+            var a = 6378245d;
+            var ee = 0.00669342162296594323;
+
+            var offsetLatitude = TransformLatitude(position.lon - 105d, position.lat - 35d);
+            var offsetLongitude = TransformLongitude(position.lon - 105d, position.lat - 35d);
+            var radiusLatitude = position.lat / 180d * PI;
+            var magic = 1d - ee * Math.Pow(Math.Sin(radiusLatitude), 2);
+            var sqrtMagic = Math.Sqrt(magic);
+            offsetLatitude = offsetLatitude * 180d / (a * (1d - ee) / (magic * sqrtMagic) * PI);
+            offsetLongitude = offsetLongitude * 180d / (a / sqrtMagic * Math.Cos(radiusLatitude) * PI);
+            return (position.lon + offsetLongitude, position.lat + offsetLatitude);
+        }
+
+        private static bool IsOutOfChina(double lon, double lat)
+        {
+            return lon < 72.004 || lon > 137.8347
+                || lat < 0.8293 || lat > 55.8171;
+        }
+
+        private static double TransformLatitude(double x, double y)
+        {
+            var ret = -100d + 2d * x + 3d * y + 0.2 * y * y + 0.1 * x * y + 0.2 * Math.Sqrt(Math.Abs(x));
+            ret += (20d * Math.Sin(6d * x * PI) + 20d * Math.Sin(2d * x * PI)) * 2d / 3d;
+            ret += (20d * Math.Sin(y * PI) + 40d * Math.Sin(y / 3d * PI)) * 2d / 3d;
+            ret += (160d * Math.Sin(y / 12d * PI) + 320d * Math.Sin(y * PI / 30d)) * 2d / 3d;
+            return ret;
+        }
+
+        private static double TransformLongitude(double x, double y)
+        {
+            var ret = 300d + x + 2d * y + 0.1 * x * x + 0.1 * x * y + 0.1 * Math.Sqrt(Math.Abs(x));
+            ret += (20d * Math.Sin(6d * x * PI) + 20d * Math.Sin(2d * x * PI)) * 2d / 3d;
+            ret += (20d * Math.Sin(x * PI) + 40d * Math.Sin(x / 3d * PI)) * 2d / 3d;
+            ret += (150d * Math.Sin(x / 12d * PI) + 300d * Math.Sin(x / 30d * PI)) * 2d / 3d;
+            return ret;
+        }
+
+    }
+}
diff --git a/Billing.Shared/Views/ViewLocationPage.cs b/Billing.Shared/Views/ViewLocationPage.cs
index f1ef086..2fb6c55 100644
--- a/Billing.Shared/Views/ViewLocationPage.cs
+++ b/Billing.Shared/Views/ViewLocationPage.cs
@@ -14,7 +14,7 @@ namespace Billing.Views
 
             if (bill.Latitude != null && bill.Longitude != null)
             {
-                var (longitude, latitude) = Helper.Wgs84ToGcj02(bill.Longitude.Value, bill.Latitude.Value);
+                var (longitude, latitude) = (bill.Longitude.Value, bill.Latitude.Value).Wgs84ToGcj02();
                 var position = new Position(latitude, longitude);
                 var mapSpan = new MapSpan(position, 0.01, 0.01);
                 Content = new Map(mapSpan)