diff --git a/wellpathpy/rad_curv.py b/wellpathpy/rad_curv.py
index c3e76dd..792c1f1 100644
--- a/wellpathpy/rad_curv.py
+++ b/wellpathpy/rad_curv.py
@@ -28,9 +28,9 @@ def radius_curvature(md, inc, azi):
     .. math::
         northing = \sum (md_l - md_u)
                    \cdot cos(inc_u) - cos(inc_l)
-                   \cdot \\frac{sin(azi_l) - sin(azi_u)}{(inc_l - inc_u)}
-                   \cdot (azi_l - azi_u)
+                   \cdot \\frac{sin(azi_l) - sin(azi_u)}{(inc_l - inc_u) \cdot (azi_l - azi_u)}
 
+    northing = np.cumsum((md_lower - md_upper) * (np.cos(incl_upper) - np.cos(incl_lower)) * (np.sin(azi_lower) - np.sin(azi_upper)) / (delta_inc * delta_azi))
 
     :math:`easting = sum((md_l - md_u) * cos(inc_u) - cos(inc_l) * cos(azi_u) - cos(azi_l) / (inc_l - inc_u) * (azi_l - azi_u))`
     :math:`tvd = sum((md_l - md_u) * sin(inc_l) - sin(inc_u) / (inc_l - inc_u))`