diff --git a/Assets/Scripts/AnnotationPass/CityFieldsPass.cs b/Assets/Scripts/AnnotationPass/CityFieldsPass.cs
index 1e40aff..68e562e 100644
--- a/Assets/Scripts/AnnotationPass/CityFieldsPass.cs
+++ b/Assets/Scripts/AnnotationPass/CityFieldsPass.cs
@@ -132,11 +132,7 @@ namespace Assets.AnnotationPass
             }
 
             // remove outside field
-            city.Fields.RemoveAll(field =>
-            {
-                var points = field.Boundary.Take(3).ToArray();
-                return !PointUtils.IsClockwise(points[0], points[1], points[2]);
-            });
+            city.Fields.RemoveAll(field => !PointUtils.IsClockwise(field.Boundary));
         }
 
         public void Annotate(Map map)
diff --git a/Assets/Scripts/Common/PointUtils.cs b/Assets/Scripts/Common/PointUtils.cs
index 1a15d2e..bf3c2a4 100644
--- a/Assets/Scripts/Common/PointUtils.cs
+++ b/Assets/Scripts/Common/PointUtils.cs
@@ -60,6 +60,11 @@ namespace Assets.Common
             return xa * yb - xb * ya < 0;
         }
 
+        public static bool IsClockwise(IEnumerable<Point> points)
+        {
+            return points.Zip(points.RotateRight(), (a, b) => (b.x - a.x) * (b.y + a.y)).Aggregate((a, b) => a + b) < 0;
+        }
+
         public static bool IsPointInside(Point point, Point[] polygon)
         {
             var j = polygon.Length - 1;