Using the "Swept Surface along Curves" tool to Construct a centerline element between two curves/lines, for example to determine the axes of a road, can sometimes misfire and create a result as shown below.
A surface like this can be caused by the different number of segments/nodes for eache curve or linestring.In this example case 257 and 273
To resolve this, both line strings need to have an equal account of segments.This can be fixed as follows:
Under Surface Modeling select Rebuild Curve (short key W21)
Set, or leave, the Method to 'Number of Points' and change the value of 'Control Points' to a number close to an average of the current segments.As I like round numbers, I have chosen for 250 in this example. Leave the 'Order' as it is (4).
Select both curves and Accept.The curves, originally for this example Line Strings, have been converted to B-spline curves with 250 Control Points
Next, follow the steps as described in the wiki article Construct centerline element between two curves/lines to create your centerline
Construct centerline element between two curves/lines