When you create or import an alignment, ideally the alignments have tangential segments i.e. the alignment doesn’t have any deflection angles at the element end points. You can check for this using CTRL and Click on the COG Icon (Settings) in the alignment editor and then look for the word Tangent on all elements. If you see a D followed by an angle that is a deflection.
Alignment Based Surface Models require that the alignment has 100% tangential elements and no deflection angles. If your alignment has one deflection in it, then when we create the surface model we model it first using the Alignment Based Method and if that fails for this reason, then we model it using normal triangulation processes. If we fail the Alignment Based approach, we put a Red Flag on the Corridor Surface / Surface Model in the Project Explorer tree.
The video below shows an example where the HAL has a 179 degree deflection in it and once fixed how to rectify the surface model and remove such a flag in the Project Explorer.