T_eff predictions obtained by the GP model based on the BT-Settl grid of models for the four libraries of ground-based spectra listed in Sect. 4 for G = 15 (top row) and G = 20 (bottom row). The colour code is the same as used in Fig. 8. The x-axis shows the spectral types gathered from the literature cited in the spectral compilations. The grey line shows the T_eff-spectral type calibration by Stephens et al. (2009) from optical and infrared spectra (± 250 K, grey dashed lines). The right-hand panels show the residuals with respect to the calibration as in Fig. 8.