Left: scatter plot of δτ_SN from HMI (y-axis) vs. MDI (x-axis) for all available latitudes and travel distances ranging from 10.2° to 45°. The center-to-limb variation and MDI P-angle error are not corrected. Right: same as in the left panel but with corrections to the center-to-limb variation and the MDI P-angle. The red lines refer to linear functions that fit the data points assuming equal errors in both coordinates (Press et al. 1992, Sect. 15.3). The Pearson correlation coefficients, r_xy, between the two data sets and the fitted linear functions are labeled in each panel.