Two-dimensional maps in (ℓ, b) space coloured by the vertex deviation, l_v. Top row: maps for the large-scale Milky Way model without a nuclear component (left) and with an axisymmetric NSD model (right). The NSD is indicated by the blue circle in the inset. Middle panels: maps for the S-NSB model with the nuclear bar at various orientations, α. Insets show the geometric configuration of the outer bar (red line) and nuclear bar (blue line). White markers in the top-left panel indicate the fields used in the analysis of Sect. 3.3. Bottom panels: same as above but for the W-NSB model.