Correlation between observed N₂H⁺(1–0) integrated intensity and H₂ column density derived from SPIRE dust-continuum data. Points within 45′′ of an embedded YSO have been excluded to avoid effects of stellar heating. The open squares in the B7/L1495 panel indicate points likely affected by V892 Tau (see text). The black lines represent the analytic expression discussed in the text.