More robust seabed slope support

- Seabed slope for each Line is now calculated based
  on the depth beneath each endpoint (assuming a
  uniformly sloped seabed underneath the line) rather
  than looking at the panel slope under end A.
- This should be more robust by avoiding inconsistencies
  when lines cross panels, though could reduce accuracy
  for finer grids with Lines that only slightly touch.

moorpy/line.py

@@ -791,6 +791,11 @@ def staticSolve(self, reset=False, tol=0.0001, profiles=0):
             w_total = self.w
 
 
+        # horizontal and vertical dimensions of line profile (end A to B)
+        LH = np.linalg.norm(dr[:2])
+        LV = dr[2]
+
+
         # apply a rotation about Z' to align the line profile with the X'-Z' plane
         theta_z = -np.arctan2(dr[1], dr[0])
         R_z = rotationMatrix(0, 0, theta_z)
@@ -802,8 +807,10 @@ def staticSolve(self, reset=False, tol=0.0001, profiles=0):
         if self.rA[2] <= -depthA or self.rB[2] <= -depthB:
             nvecA_prime = np.matmul(R, nvecA)
 
-            dz_dx = -nvecA_prime[0]*(1.0/nvecA_prime[2])  # seabed slope components
-            dz_dy = -nvecA_prime[1]*(1.0/nvecA_prime[2])  # seabed slope components
+            #dz_dx = -nvecA_prime[0]*(1.0/nvecA_prime[2])  # seabed slope components
+            #dz_dy = -nvecA_prime[1]*(1.0/nvecA_prime[2])  # seabed slope components
+            # Seabed slope along line direction (based on end A/B depth)
+            dz_dx = (-depthB + depthA)/LH  
             # we only care about dz_dx since the line is in the X-Z plane in this rotated situation
             alpha = np.degrees(np.arctan(dz_dx))
             cb = self.cb
@@ -815,10 +822,6 @@ def staticSolve(self, reset=False, tol=0.0001, profiles=0):
                 alpha = 0
                 cb = self.cb
 
-        # horizontal and vertical dimensions of line profile (end A to B)
-        LH = np.linalg.norm(dr[:2])
-        LV = dr[2]
-
 
         # ----- call catenary function or alternative and save results -----