水準測量における天文潮汐と海洋潮汐荷重の影響について

―日本の一等水準測量観測の一部における試算―

抄録

We quantitatively estimate the corrections for astronomical tides and ocean tidal loading to first-order leveling survey in Japan along its typical routes. Tidal force of the astronomical bodies such as the Moon and the Sun makes the geopotential surfaces and the deformable Earth’s surface tilt differently in both permanent and periodical manners. The force also redistributes the oceanic masses as ocean tides, producing differential tilts between those surfaces through their direct gravitational effects and load-induced elastic bending of the crust. Those differential tilts may affect the readings of spirit leveling only slightly, but may yield significant changes in orthometric heights on a national/ continental scale. The results show that temporally changing parts of the cumulative effects of astronomic tides along the routes are comparable to or even larger than, but not linearly related to their permanent parts, that those total cumulative effects possibly enlarge closure errors in loops, and that the cumulative effects of ocean tidal loading are generally minor but may become significant towards the ocean in high tide areas. The inclusion of astronomical tidal corrections in the analysis of nationwide precise leveling will be worthwhile in future for improvement in comparison of geoidal undulations among GPS/leveling data and gravimetric geoid models and for accuracy enhancement of the national height datum.