Top: measurements of log N_CaII as a function of stellar distance collected by Welsh et al. (2010). Red pluses indicate upper limits. Bottom: log N_CaII–E(B−V) correlations obtained by converting first E(B−V) into log N_HI, using the relation given by Savage & Mathis (1979, SM79; black), Diplas & Savage (1994, DS94; red), Güver & Özel (2009, GO09; blue), Predehl & Schmitt (1995, PS95; purple), and Gudennavar et al. (2012, G12; yellow), and then log N_HI into log N_CaII, using either the log N_HI–log N_CaII relation given by Wakker & Mathis (2000, WM00; solid lines) or by using the Ca ii ISM ionisation fraction measured by Frisch & Slavin (2003, FS03) and assuming a solar Ca abundance (Asplund et al. 2009, A09; dashed lines). The “X” in the legend stands for any of SM79, DS94, GO09, PS95, or G12. The vertical dotted lines indicate the average distance at which a given E(B−V) value can be expected following the ISM extinction maps of Amôres & Lépine (2005). This figure justifies the upper boundary of log N_CaII = 14 used for the analysis carried out in Sects. 4 and 5.