diff --git a/LoopStructural/modelling/core/_model_exporter.py b/LoopStructural/modelling/core/_model_exporter.py
index 8442f6b51..c63305642 100644
--- a/LoopStructural/modelling/core/_model_exporter.py
+++ b/LoopStructural/modelling/core/_model_exporter.py
@@ -46,7 +46,7 @@ def get_stratigraphic_surfaces(
         units = []
         if model.stratigraphic_column is None:
             return []
-        units = model.stratigraphic_column.get_isovalues()
+        units = model.stratigraphic_column.get_isovalues(where='bottom' if bottoms else 'top')
         units_for_group = {}
         for name, u in units.items():
             if u['group'] not in model:
diff --git a/LoopStructural/modelling/core/stratigraphic_column.py b/LoopStructural/modelling/core/stratigraphic_column.py
index 943c68ba4..a2d1971f1 100644
--- a/LoopStructural/modelling/core/stratigraphic_column.py
+++ b/LoopStructural/modelling/core/stratigraphic_column.py
@@ -677,16 +677,29 @@ def from_dict(cls, data):
             column.add_element(element)
         return column
 
-    def get_isovalues(self) -> dict[str, float]:
+    def get_isovalues(self, where: str = 'bottom') -> dict[str, float]:
         """
         Returns a dictionary of isovalues for the stratigraphic units in the column.
+
+        Parameters
+        ----------
+        where : str, optional
+            'bottom' (default) returns the value at the base of each unit.
+            'top' returns the value at the top of each unit.
         """
+        if where not in ('top', 'bottom'):
+            raise ValueError("Invalid 'where' argument. Use 'top' or 'bottom'.")
         surface_values = {}
         for g in reversed(self.get_groups()):
             v = 0
             for u in reversed(g.units):
-                surface_values[u.name] = {'value': v, 'group': g.name, 'colour': u.colour}
+                base = v
                 v += u.thickness
+                surface_values[u.name] = {
+                    'value': v if where == 'top' else base,
+                    'group': g.name,
+                    'colour': u.colour,
+                }
         return surface_values
 
     def plot(self, *, ax=None, **kwargs):
diff --git a/tests/unit/modelling/test_stratigraphic_column.py b/tests/unit/modelling/test_stratigraphic_column.py
index 1f7fc2849..db27518b8 100644
--- a/tests/unit/modelling/test_stratigraphic_column.py
+++ b/tests/unit/modelling/test_stratigraphic_column.py
@@ -444,6 +444,25 @@ def test_get_isovalues_multi_unit_group(self):
         assert isovalues["B"]["value"] == 10
         assert isovalues["C"]["value"] == 15
 
+    def test_get_isovalues_where_top_returns_unit_top(self):
+        # where='top' must give each unit's top, i.e. base + thickness --
+        # the same value that where='bottom' (default) gives the next
+        # (younger) unit as its base.
+        column = StratigraphicColumn()
+        column.clear(basement=False)
+        column.add_unit("A", thickness=10, id=0)
+        column.add_unit("B", thickness=5, id=1)
+        column.add_unit("C", thickness=3, id=2)
+        isovalues = column.get_isovalues(where='top')
+        assert isovalues["A"]["value"] == 10
+        assert isovalues["B"]["value"] == 15
+        assert isovalues["C"]["value"] == 18
+
+    def test_get_isovalues_invalid_where_raises(self):
+        column = self._build_two_group_column()
+        with pytest.raises(ValueError):
+            column.get_isovalues(where="middle")
+
 
 class TestOrderingAndUpdates:
     def test_update_order_reorders_elements(self):