Index: Ringtoets/Piping/src/Ringtoets.Piping.Data/RingtoetsPipingSurfaceLine.cs
===================================================================
diff -u -r9def31196320039cc9075ba0e1fb12ce87c6769e -r0f8ecad9de486dac8a81bb9de57e76ca6cf8a104
--- Ringtoets/Piping/src/Ringtoets.Piping.Data/RingtoetsPipingSurfaceLine.cs	(.../RingtoetsPipingSurfaceLine.cs)	(revision 9def31196320039cc9075ba0e1fb12ce87c6769e)
+++ Ringtoets/Piping/src/Ringtoets.Piping.Data/RingtoetsPipingSurfaceLine.cs	(.../RingtoetsPipingSurfaceLine.cs)	(revision 0f8ecad9de486dac8a81bb9de57e76ca6cf8a104)
@@ -37,7 +37,7 @@
     public class RingtoetsPipingSurfaceLine
     {
         private Point3D[] geometryPoints;
-        private double entryPointX;
+        private double entryPointL;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="RingtoetsPipingSurfaceLine"/> class.
@@ -95,21 +95,21 @@
         public Point3D DitchDikeSide { get; private set; }
 
         /// <summary>
-        /// Gets or sets the x-coördinate of the entree point.
+        /// Gets or sets the L-coördinate of the entry point.
         /// </summary>
-        public double EntryPointX
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="value"/> is not in range of the LZ-projected <see cref="Points"/>.</exception>
+        /// <exception cref="InvalidOperationException"><see cref="Points"/> is empty.</exception>
+        public double EntryPointL
         {
             get
             {
-                return entryPointX;
+                return entryPointL;
             }
             set
             {
-                if (value < Points.First().X || value > Points.Last().X)
-                {
-                    throw new ArgumentOutOfRangeException("value", "Cannot set entry point x-coordinate beyond the geometry.");
-                }
-                entryPointX = value;
+                ValidateHasPoints();
+                ValidateInRange(value, ProjectGeometryToLZ().ToArray(), Resources.RingtoetsPipingSurfaceLine_EntryPointL_Cannot_set_entry_point_at_L_0);
+                entryPointL = value;
             }
         }
 
@@ -223,17 +223,15 @@
         /// <returns>The height of the <see cref="RingtoetsPipingSurfaceLine"/> at L=<paramref name="l"/>.</returns>
         /// <exception cref="RingtoetsPipingSurfaceLineException">Thrown when the <see cref="RingtoetsPipingSurfaceLine"/>
         /// intersection point at <paramref name="l"/> have a significant difference in their y coordinate.</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="l"/> is not in range of the LZ-projected <see cref="Points"/>.</exception>
         /// <exception cref="InvalidOperationException"><see cref="Points"/> is empty.</exception>
         public double GetZAtL(double l)
         {
-            if (!Points.Any())
-            {
-                throw new InvalidOperationException(Resources.RingtoetsPipingSurfaceLine_SurfaceLine_has_no_Geometry);
-            }
+            ValidateHasPoints();
 
             var projectGeometryToLz = ProjectGeometryToLZ().ToArray();
 
-            ValidateInRange(l, projectGeometryToLz);
+            ValidateInRange(l, projectGeometryToLz, Resources.RingtoetsPipingSurfaceLine_GetZAtL_Cannot_determine_height);
             
             var segments = new Collection<Segment2D>();
             for (int i = 1; i < projectGeometryToLz.Length; i++)
@@ -254,23 +252,37 @@
         }
 
         /// <summary>
+        /// Checks whether the current <see cref="Points"/> collection is not empty.
+        /// </summary>
+        /// <exception cref="InvalidOperationException"><see cref="Points"/> is empty.</exception>
+        private void ValidateHasPoints()
+        {
+            if (!Points.Any())
+            {
+                throw new InvalidOperationException(Resources.RingtoetsPipingSurfaceLine_SurfaceLine_has_no_Geometry);
+            }
+        }
+
+        /// <summary>
         /// Checks whether <paramref name="l"/> is in range of the <paramref name="lzGeometry"/>.
         /// </summary>
         /// <param name="l">The value to check for.</param>
         /// <param name="lzGeometry">The 2 dimensional geometry which determines the valid range for <paramref name="l"/>, where
-        /// elements are expected to be ordered on the x-coordinate.</param>
+        ///     elements are expected to be ordered on the x-coordinate.</param>
+        /// <param name="ringtoetsPipingSurfaceLineGetZatLCannotDetermineHeightCause"></param>
         /// <exception cref="ArgumentOutOfRangeException">Throw when:
         /// <list type="bullet">
         /// <item><paramref name="l"/> &lt; {first point's x-coordinate}</item>
         /// <item><paramref name="l"/> &gt; {last point's x-coordinate}</item>
         /// </list></exception>
-        private static void ValidateInRange(double l, Point2D[] lzGeometry)
+        private static void ValidateInRange(double l, Point2D[] lzGeometry, string operationDescription)
         {
             if (lzGeometry.First().X > l || lzGeometry.Last().X < l)
             {
-                var outOfRangeMessage = string.Format(Resources.RingtoetsPipingSurfaceLine_L_needs_to_be_in_0_1_range_to_be_able_to_determine_height,
-                                                      lzGeometry.First().X,
-                                                      lzGeometry.Last().X);
+                var outOfRangeMessage = string.Format(Resources.RingtoetsPipingSurfaceLine_0_L_needs_to_be_in_1_2_range,
+                    operationDescription,                                  
+                    lzGeometry.First().X,
+                    lzGeometry.Last().X);
                 throw new ArgumentOutOfRangeException("l", outOfRangeMessage);
             }
         }