Best-fitted models of polynomial-logarithmic (PL) spirals (Hou & Han 2014). Black points represent the GSs. Panel a: GSs located at |b | <= 2°. Panel b: GSslocated at 2° < |b| < 5°. Panel c: GSslocated at |b| >= 5°. The Sun is represented by a blue asterisk and the Galactic centre by a yellow dot. The concentric circles around the Galactic centre are separated by 10 kpc and indicate the distance to the Galactic centre.