Fix cross track point perturbation

ensembleperturbation/perturbation/atcf.py

@@ -802,14 +802,14 @@ def perturb(
             next_offset = get_offset(current_point, next_point, cross_track_error)
 
             # get the perpendicular offset based on the average of the forward and backward piecewise track lines adjusted so that the distance matches the actual cross_error
-            normal_offset = numpy.mean([previous_offset, next_offset])
+            normal_offset = numpy.mean([previous_offset, next_offset], axis=0)
             alpha = abs(cross_track_error) / numpy.sqrt(numpy.sum(normal_offset ** 2))
 
             if numpy.isinf(alpha):
                 alpha = 0
 
             # compute the next point and retrieve back the lat-lon geographic coordinate
-            new_point = current_point - alpha * normal_offset
+            new_point = current_point - abs(alpha) * normal_offset
             new_coordinates.append(
                 transformer.transform(
                     new_point[0].magnitude,
@@ -1796,9 +1796,9 @@ def get_offset(
         #   z**2 / (1 + pslope**2) = x**2
         #   z / (1 + pslope**2)**0.5 = x
 
-        points = numpy.concatenate([point_1, point_2], axis=0)
+        points = numpy.vstack([point_1, point_2])
 
-        difference = numpy.diff(points, axis=0)
+        difference = numpy.diff(points, axis=0).ravel()
 
         # tangential slope approximation
         slope = difference[1] / difference[0]