Maximum amplitudes of the northern warp overlaid with the vertical heights of HMSFR masers. Similar to X. Chen et al. (2019), all maser samples are presented merely to illustrate a simple warping trend. The gray dots indicate the HMSFR masers collected from the literature, while the cyan triangle marks G040.96+02.48. The curves show maximum warp amplitudes derived from different studies: DS 01, dust (R. Drimmel & D. N. Spergel 2001); LC02, red clump stars (M. López-Corredoira et al. 2002); Y04, pulsars (I. Yusifov 2004); L06, H I (E. S. Levine et al. 2006); C19, Cepheids (X. Chen et al. 2019); P25a (young giants); and P25b (Cepheids; E. Poggio et al. 2025).