Index: Core/Common/src/Core.Common.Geometry/AdvancedMath2D.cs
===================================================================
diff -u -r67284323e2785c651633d9c52049ba12a9c70e6a -r58bcdff0e0effb884294342067cc723e06961379
--- Core/Common/src/Core.Common.Geometry/AdvancedMath2D.cs	(.../AdvancedMath2D.cs)	(revision 67284323e2785c651633d9c52049ba12a9c70e6a)
+++ Core/Common/src/Core.Common.Geometry/AdvancedMath2D.cs	(.../AdvancedMath2D.cs)	(revision 58bcdff0e0effb884294342067cc723e06961379)
@@ -59,6 +59,38 @@
         }
 
         /// <summary>
+        /// Completes a line shape so that it becomes a polygon by adding two bottom points to the shape.
+        /// The location of the bottom points are determined by the <paramref name="line"/>'s first and 
+        /// last points' x-coordinate and by <paramref name="completingPointsLevel"/> for the y-coordinate.
+        /// </summary>
+        /// <param name="line">The line to complete.</param>
+        /// <param name="completingPointsLevel">The level at which to place the points completing the polygon.</param>
+        /// <returns>A new collection of <see cref="Point2D"/>, with the line's point and
+        /// the two new bottom points.</returns>
+        /// <exception cref="ArgumentNullException">Thrown when <paramref name="line"/> is <c>null</c>.
+        /// </exception>
+        /// <exception cref="ArgumentException">Thrown when <paramref name="line"/> contains
+        /// less than 2 points.</exception>
+        public static IEnumerable<Point2D> CompleteLineToPolygon(IEnumerable<Point2D> line, double completingPointsLevel)
+        {
+            if (line == null)
+            {
+                throw new ArgumentNullException(nameof(line));
+            }
+            if (line.Count() < 2)
+            {
+                throw new ArgumentException(@"The line needs to have at least two points to be able to create a complete polygon.", nameof(line));
+            }
+
+            foreach (Point2D point in line)
+            {
+                yield return point;
+            }
+            yield return new Point2D(line.Last().X, completingPointsLevel);
+            yield return new Point2D(line.First().X, completingPointsLevel);
+        }
+
+        /// <summary>
         /// Transforms X coordinates in a 2D X, Y plane using:
         /// - A reference point as starting point of the line.
         /// - An offset at which the reference coincides with the X axis.
Index: Core/Common/test/Core.Common.Geometry.Test/AdvancedMath2DTest.cs
===================================================================
diff -u -r67284323e2785c651633d9c52049ba12a9c70e6a -r58bcdff0e0effb884294342067cc723e06961379
--- Core/Common/test/Core.Common.Geometry.Test/AdvancedMath2DTest.cs	(.../AdvancedMath2DTest.cs)	(revision 67284323e2785c651633d9c52049ba12a9c70e6a)
+++ Core/Common/test/Core.Common.Geometry.Test/AdvancedMath2DTest.cs	(.../AdvancedMath2DTest.cs)	(revision 58bcdff0e0effb884294342067cc723e06961379)
@@ -376,6 +376,59 @@
             }, points);
         }
 
+        [Test]
+        public void CompleteLineToPolygon_WithoutLine_ThrowsArgumentNullException()
+        {
+            // Call
+            TestDelegate test = () => AdvancedMath2D.CompleteLineToPolygon(null, double.NaN).ToArray();
+
+            // Assert
+            var exception = Assert.Throws<ArgumentNullException>(test);
+            Assert.AreEqual("line", exception.ParamName);
+        }
+
+        [Test]
+        public void CompleteLineToPolygon_LineWithLessThanTwoPoints_ThrowsArgumentNullException([Range(0,1)] int pointCount)
+        {
+            // Setup
+            IEnumerable<Point2D> points = Enumerable.Repeat(new Point2D(3, 2), pointCount);
+
+            // Call
+            TestDelegate test = () => AdvancedMath2D.CompleteLineToPolygon(points, double.NaN).ToArray();
+
+            // Assert
+            const string message = "The line needs to have at least two points to be able to create a complete polygon.";
+            var exception = TestHelper.AssertThrowsArgumentExceptionAndTestMessage<ArgumentException>(test, message);
+            Assert.AreEqual("line", exception.ParamName);
+        }
+
+
+        [Test]
+        [TestCase(3)]
+        [TestCase(0)]
+        [TestCase(-9)]
+        [TestCase(double.NaN)]
+        public void CompleteLineToPolygon_LineWithTwoPoints_TwoPointsAtBottomLevelAdded(double completingPointsLevel)
+        {
+            // Setup
+            var random = new Random(21);
+            int firstPointX = random.Next();
+            int lastPointX = random.Next();
+            var points = new []
+            {
+                new Point2D(firstPointX, random.Next()), 
+                new Point2D(random.Next(), random.Next()), 
+                new Point2D(lastPointX, random.Next()), 
+            };
+            
+            // Call
+            Point2D[] pointsOfPolygon = AdvancedMath2D.CompleteLineToPolygon(points, completingPointsLevel).ToArray();
+
+            // Assert
+            Assert.AreEqual(new Point2D(lastPointX, completingPointsLevel), pointsOfPolygon.ElementAt(3));
+            Assert.AreEqual(new Point2D(firstPointX, completingPointsLevel), pointsOfPolygon.ElementAt(4));
+        }
+
         private static double[] ThreeRandomXCoordinates()
         {
             var random = new Random(21);
Index: Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Data/SoilProfileUnderSurfaceLineFactory.cs
===================================================================
diff -u -r677ec9937ae7eff73a09bf937804ad22e0dc5a4b -r58bcdff0e0effb884294342067cc723e06961379
--- Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Data/SoilProfileUnderSurfaceLineFactory.cs	(.../SoilProfileUnderSurfaceLineFactory.cs)	(revision 677ec9937ae7eff73a09bf937804ad22e0dc5a4b)
+++ Ringtoets/MacroStabilityInwards/src/Ringtoets.MacroStabilityInwards.Data/SoilProfileUnderSurfaceLineFactory.cs	(.../SoilProfileUnderSurfaceLineFactory.cs)	(revision 58bcdff0e0effb884294342067cc723e06961379)
@@ -54,10 +54,11 @@
             }
             Point2D[] localizedSurfaceLine = surfaceLine.LocalGeometry.ToArray();
 
-            IEnumerable<Point2D> surfaceLineGeometry = CreateSurfaceLineAreaToDepth(localizedSurfaceLine, soilProfile.Bottom);
+            double geometryBottom = Math.Min(soilProfile.Bottom, localizedSurfaceLine.Min(p => p.Y)) - 1;
+            IEnumerable<Point2D> surfaceLineGeometry = AdvancedMath2D.CompleteLineToPolygon(localizedSurfaceLine, geometryBottom);
             IEnumerable<TempSoilLayerGeometry> layerGeometries = soilProfile.Layers.Select(
                 layer => As2DGeometry(
-                    layer, 
+                    layer,
                     soilProfile,
                     localizedSurfaceLine.First().X,
                     localizedSurfaceLine.Last().X));
@@ -81,8 +82,8 @@
 
             IEnumerable<SoilLayerUnderSurfaceLine> layersUnderSurfaceLine = soilProfile.Layers.Select(
                 layer => new SoilLayerUnderSurfaceLine(
-                    RingToPoints(layer.OuterRing), 
-                    layer.Holes.Select(RingToPoints), 
+                    RingToPoints(layer.OuterRing),
+                    layer.Holes.Select(RingToPoints),
                     layer.Properties));
 
             return new SoilProfileUnderSurfaceLine(layersUnderSurfaceLine);
@@ -101,7 +102,7 @@
 
             foreach (TempSoilLayerGeometry layer in layerGeometries)
             {
-                foreach (Point2D[] soilLayerArea in CreateSoilLayerAreas(surfaceLineGeometryArray, layer.OuterLoop))
+                foreach (Point2D[] soilLayerArea in GetSoilLayerWithSurfaceLineIntersection(surfaceLineGeometryArray, layer.OuterLoop))
                 {
                     collection.Add(new SoilLayerUnderSurfaceLine(soilLayerArea, layer.Properties));
                 }
@@ -124,22 +125,6 @@
             }, layer.Properties);
         }
 
-        private static IEnumerable<Point2D> CreateSurfaceLineAreaToDepth(Point2D[] localizedSurfaceLine, double soilProfileBottom)
-        {
-            foreach (Point2D point in localizedSurfaceLine)
-            {
-                yield return point;
-            }
-            double geometryBottom = Math.Min(soilProfileBottom, localizedSurfaceLine.Min(p => p.Y)) - 1;
-            yield return new Point2D(localizedSurfaceLine.Last().X, geometryBottom);
-            yield return new Point2D(localizedSurfaceLine.First().X, geometryBottom);
-        }
-
-        private static IEnumerable<Point2D[]> CreateSoilLayerAreas(IEnumerable<Point2D> surfaceLineGeometry, IEnumerable<Point2D> soilLayerGeometry)
-        {
-            return GetSoilLayerWithSurfaceLineIntersection(surfaceLineGeometry, soilLayerGeometry);
-        }
-
         private static IEnumerable<Point2D[]> GetSoilLayerWithSurfaceLineIntersection(IEnumerable<Point2D> surfaceLineGeometry, IEnumerable<Point2D> soilLayerGeometry)
         {
             return AdvancedMath2D.PolygonIntersectionWithPolygon(surfaceLineGeometry, soilLayerGeometry).Where(arr => arr.Length > 2);
Index: Ringtoets/Piping/src/Ringtoets.Piping.Forms/Factories/PipingChartDataPointsFactory.cs
===================================================================
diff -u -rd6fe8399e8398224cf1bda9259052232d85b85a4 -r58bcdff0e0effb884294342067cc723e06961379
--- Ringtoets/Piping/src/Ringtoets.Piping.Forms/Factories/PipingChartDataPointsFactory.cs	(.../PipingChartDataPointsFactory.cs)	(revision d6fe8399e8398224cf1bda9259052232d85b85a4)
+++ Ringtoets/Piping/src/Ringtoets.Piping.Forms/Factories/PipingChartDataPointsFactory.cs	(.../PipingChartDataPointsFactory.cs)	(revision 58bcdff0e0effb884294342067cc723e06961379)
@@ -277,14 +277,10 @@
 
             double topLevel = soilLayer.Top;
             double bottomLevel = topLevel - soilProfile.GetLayerThickness(soilLayer);
-            double surfaceLineLowestPointY = surfaceLineAsPolygon.Select(p => p.Y).Min();
 
-            double closingSurfaceLineToPolygonBottomLevel = Math.Min(surfaceLineLowestPointY, bottomLevel) - 1;
+            double closingSurfaceLineToPolygonBottomLevel = Math.Min(surfaceLineAsPolygon.Select(p => p.Y).Min(), bottomLevel) - 1;
 
-            surfaceLineAsPolygon.Add(new Point2D(surfaceLineAsPolygon.Last().X, closingSurfaceLineToPolygonBottomLevel));
-            surfaceLineAsPolygon.Add(new Point2D(surfaceLineAsPolygon.First().X, closingSurfaceLineToPolygonBottomLevel));
-
-            return surfaceLineAsPolygon.ToArray();
+            return AdvancedMath2D.CompleteLineToPolygon(surfaceLineLocalGeometry, closingSurfaceLineToPolygonBottomLevel).ToArray();
         }
 
         private static Point2D[] CreateSurfaceLineWideSoilLayer(Point2D[] surfaceLineLocalGeometry, PipingSoilLayer soilLayer, PipingSoilProfile soilProfile)