On the correction for polar motion in gravimetry and in 3-D positioning

In the correction for polar motion, terrestrial gravimetry and 3-D positioning follow different conventions. The 3-D positions were first corrected to refer to the "mean pole" (IERS Conventions up to 2010) and now to the "secular pole" (IERS Conventions update since 2018). In any case, the pole reference evolves in time and describes the track of secular or low-frequency polar wander. However, since 1988 terrestrial gravimetry follows the IAGBN (International Gravity Basestation Network) Processing Standards where the gravity values are corrected to refer to the IERS Reference Pole, a fixed quantity. This may lead to discrepancies when for instance gravity change rates from absolute gravity measurements are interpreted together with vertical velocities from GNSS. I discuss the size and geographical distribution of the possible discrepancies and how to account for them in geodynamical problems. The fixed reference of the IAGBN Processing Standards has served the gravity community well, by providing a stable system for terrestrial gravity for the last 30 years during which time the pole reference in the IERS Conventions has been revised several times. In fact, the recently proposed conventions for the International Gravity Reference System (IGRS) and the International Gravity Reference Frame (IGRF) maintain the IAGBN principle. However, it appears that with the adoption of the “secular pole” the reference in 3-D positioning may have become predictable for the foreseeable future. Therefore, it could be suggested that now is the time to harmonize terrestrial gravity with 3-D, by adopting the time-dependent secular pole as a reference for it as well, especially as this is already happening with satellite gravity. I argue that at present the practical drawbacks from such a change of reference would outweigh any theoretical advantages, and the harmonization, where necessary, can be simply performed a-posteriori to published gravity trends/values.