diff --git a/gprMax/subgrids/precursor_nodes.py b/gprMax/subgrids/precursor_nodes.py
index 3b96b07f..e9eeac4b 100644
--- a/gprMax/subgrids/precursor_nodes.py
+++ b/gprMax/subgrids/precursor_nodes.py
@@ -636,14 +636,9 @@ class PrecursorNodes(PrecusorNodes2dBase):
 
     def interpolate_to_sub_grid(self, field, coords):
         x, z, x_sg, z_sg = coords
-        ex_t = np.transpose(field)
-        f = interpolate.interp2d(x, z, ex_t, kind=self.interpolation)
-        #f = interpolate.RectBivariateSpline(x, z, field)
-        ex_inter_t = f(x_sg, z_sg)
-        ex_inter = np.transpose(ex_inter_t)
-        #ex_inter = ex_inter_t
-
-        return ex_inter
+        interp_f = interpolate.RectBivariateSpline(x, z, field, kx=self.interpolation, ky=self.interpolation)
+        f_i = interp_f(x_sg, z_sg)
+        return f_i
 
 
 class PlaneError(Exception):
diff --git a/gprMax/subgrids/precursor_nodes_filtered.py b/gprMax/subgrids/precursor_nodes_filtered.py
index c8a0d41d..eadfa680 100644
--- a/gprMax/subgrids/precursor_nodes_filtered.py
+++ b/gprMax/subgrids/precursor_nodes_filtered.py
@@ -741,14 +741,9 @@ class PrecursorNodes(PrecusorNodes2dBase):
 
     def interpolate_to_sub_grid(self, field, coords):
         x, z, x_sg, z_sg = coords
-        ex_t = np.transpose(field)
-        f = interpolate.interp2d(x, z, ex_t, kind=self.interpolation)
-        #f = interpolate.RectBivariateSpline(x, z, field)
-        ex_inter_t = f(x_sg, z_sg)
-        ex_inter = np.transpose(ex_inter_t)
-        #ex_inter = ex_inter_t
-
-        return ex_inter
+        interp_f = interpolate.RectBivariateSpline(x, z, field, kx=self.interpolation, ky=self.interpolation)
+        f_i = interp_f(x_sg, z_sg)
+        return f_i
 
 
 class PlaneError(Exception):
diff --git a/gprMax/subgrids/user_objects.py b/gprMax/subgrids/user_objects.py
index d481a5d5..40ecf0bf 100644
--- a/gprMax/subgrids/user_objects.py
+++ b/gprMax/subgrids/user_objects.py
@@ -177,7 +177,7 @@ class SubGridHSG(SubGridBase):
                  is_os_sep=3,
                  pml_separation=4,
                  subgrid_pml_thickness=6,
-                 interpolation='linear',
+                 interpolation=1,
                  filter=True,
                  **kwargs):
         """Constructor."""