diff --git a/tests/test_grid3d_hc_thickness4.py b/tests/test_grid3d_hc_thickness4.py
index 6b08939..5fdd552 100644
--- a/tests/test_grid3d_hc_thickness4.py
+++ b/tests/test_grid3d_hc_thickness4.py
@@ -43,4 +43,4 @@ def test_hc_thickness4a():
     mapfile = os.path.join(td, 'all--hc4a_rockthickness.gri')
     mymap = RegularSurface(mapfile)
 
-    assert_almostequal(mymap.values.mean(), 0.76590, 0.0001)
+    assert_almostequal(mymap.values.mean(), 0.76590, 0.001)