  We investigate the cross-calibration of the Hinode/SOT-SP and SDO/HMI
instrument meta-data, specifically the correspondence of the scaling and
pointing information. Accurate calibration of these datasets gives the
correspondence needed by inter-instrument studies and learning-based
magnetogram systems, and is required for physically-meaningful photospheric
magnetic field vectors. We approach the problem by robustly fitting geometric
models on correspondences between images from each instrument's pipeline. This
technique is common in computer vision, but several critical details are
required when using scanning slit spectrograph data like Hinode/SOT-SP. We
apply this technique to data spanning a decade of the Hinode mission. Our
results suggest corrections to the published Level 2 Hinode/SOT-SP data. First,
an analysis on approximately 2,700 scans suggests that the reported pixel size
in Hinode/SOT-SP Level 2 data is incorrect by around 1%. Second, analysis of
over 12,000 scans show that the pointing information is often incorrect by
dozens of arcseconds with a strong bias. Regression of these corrections
indicates that thermal effects have caused secular and cyclic drift in
Hinode/SOT-SP pointing data over its mission. We offer two solutions. First,
direct co-alignment with SDO/HMI data via our procedure can improve alignments
for many Hinode/SOT-SP scans. Second, since the pointing errors are
predictable, simple post-hoc corrections can substantially improve the
pointing. We conclude by illustrating the impact of this updated calibration on
derived physical data products needed for research and interpretation. Among
other things, our results suggest that the pointing errors induce a hemispheric
bias in estimates of radial current density.
