diff --git a/gprMax/vtkhdf.py b/gprMax/vtkhdf.py
index f9403de9..0a2e6278 100644
--- a/gprMax/vtkhdf.py
+++ b/gprMax/vtkhdf.py
@@ -225,6 +225,8 @@ class VtkImageData(VtkHdfFile):
     SPACING_ATTR = "Spacing"
     WHOLE_EXTENT_ATTR = "WholeExtent"
 
+    DIMENSIONS = 3
+
     @property
     def TYPE(self) -> Literal["ImageData"]:
         return "ImageData"
@@ -236,26 +238,33 @@ class VtkImageData(VtkHdfFile):
         origin: Optional[npt.NDArray[np.single]] = None,
         spacing: Optional[npt.NDArray[np.single]] = None,
         direction: Optional[npt.NDArray[np.single]] = None,
-        comm: Optional[MPI.Cartcomm] = None,
+        comm: Optional[MPI.Comm] = None,
     ) -> None:
         super().__init__(filename, comm)
 
+        if len(shape) == 0:
+            raise ValueError(f"Shape must not be empty.")
+        if len(shape) > self.DIMENSIONS:
+            raise ValueError(f"Shape must not have more than {self.DIMENSIONS} dimensions.")
+        elif len(shape) < self.DIMENSIONS:
+            shape = np.concatenate((shape, np.ones(self.DIMENSIONS - len(shape), dtype=np.intc)))
+
         self.shape = shape
-        self.points_shape = shape + 1
-        whole_extent = np.zeros(2 * len(self.shape), dtype=np.intc)
+
+        whole_extent = np.zeros(2 * self.DIMENSIONS, dtype=np.intc)
         whole_extent[1::2] = self.shape
         self._set_root_attribute(self.WHOLE_EXTENT_ATTR, whole_extent)
 
         if origin is None:
-            origin = np.zeros(len(self.shape), dtype=np.single)
+            origin = np.zeros(self.DIMENSIONS, dtype=np.single)
         self.set_origin(origin)
 
         if spacing is None:
-            spacing = np.ones(len(self.shape), dtype=np.single)
+            spacing = np.ones(self.DIMENSIONS, dtype=np.single)
         self.set_spacing(spacing)
 
         if direction is None:
-            direction = np.diag(np.ones(len(self.shape), dtype=np.single)).flatten()
+            direction = np.diag(np.ones(self.DIMENSIONS, dtype=np.single)).flatten()
         self.set_direction(direction)
 
     @property
@@ -275,30 +284,39 @@ class VtkImageData(VtkHdfFile):
         return self._get_root_attribute(self.DIRECTION_ATTR)
 
     def set_origin(self, origin: npt.NDArray[np.single]):
+        if len(origin) != self.DIMENSIONS:
+            raise ValueError(f"Origin attribute must have {self.DIMENSIONS} dimensions.")
         self._set_root_attribute(self.ORIGIN_ATTR, origin)
 
     def set_spacing(self, spacing: npt.NDArray[np.single]):
+        if len(spacing) != self.DIMENSIONS:
+            raise ValueError(f"Spacing attribute must have {self.DIMENSIONS} dimensions.")
         self._set_root_attribute(self.SPACING_ATTR, spacing)
 
     def set_direction(self, direction: npt.NDArray[np.single]):
+        if len(direction) != self.DIMENSIONS * self.DIMENSIONS:
+            raise ValueError(
+                f"Direction attribute must have {self.DIMENSIONS * self.DIMENSIONS} dimensions."
+            )
         self._set_root_attribute(self.DIRECTION_ATTR, direction)
 
     def add_point_data(
         self, name: str, data: npt.NDArray, offset: Optional[npt.NDArray[np.intc]] = None
     ):
-        if offset is None and any(data.shape != self.points_shape):  # type: ignore
+        points_shape = self.shape + 1
+        if offset is None and any(data.shape != points_shape):  # type: ignore
             raise ValueError(
-                f"If no offset is specified, data.shape {data.shape} must match the shape of the"
-                f" VtkImageData point datasets {self.points_shape}"
+                "If no offset is specified, data.shape must be one larger in each dimension than"
+                f" this vtkImageData object. {data.shape} != {points_shape}"
             )
-        return super().add_point_data(name, data, self.points_shape, offset)
+        return super().add_point_data(name, data, points_shape, offset)
 
     def add_cell_data(
         self, name: str, data: npt.NDArray, offset: Optional[npt.NDArray[np.intc]] = None
     ):
         if offset is None and any(data.shape != self.shape):  # type: ignore
             raise ValueError(
-                f"If no offset is specified, data.shape {data.shape} must match the shape of the"
-                f" VtkImageData {self.shape}"
+                "If no offset is specified, data.shape must match the dimensions of this"
+                f" vtkImageData object. {data.shape} != {self.shape}"
             )
         return super().add_cell_data(name, data, self.shape, offset)