Time lags and POS speeds from synthetic t − d maps. Top panel: time lags (coloured symbols) obtained from the synthetic t − d maps for each wavelength as a function of the projected distance along the loop. Vertical bars (comparable to the size of the symbols) on these values indicate the associated errors from the quadratic fit performed to obtain the time lags. The solid lines denote spline fits to the data, which are almost indistinguishable. Bottom panel: propagation speeds obtained from the spline fits to time lag values along with the associated errors.