Tidal Corrections in Geodetic Height Determination

Tidal forces produce a wide spectrum of phenomena, from primary luni—solar attractional effects to secondary induced effects like earth and ocean tides, and third order ocean loading effects. Since these phenomena affect precise geodetic observations and make them time-dependent it is necessary to reduce time-variable geodetic observables and derived quantities in order to correspond to a quasi-stationary, time-invariant state. Here we evaluate the tidal corrections to geodetic height determination; (geometrical) height differences observed by GPS are considered as well as physical (orthometric or normal) heights based on geodetic levelling. Special emphasis is given to the impact of the permanent M₀S₀ partial tide, in particular to the indirect effect induced by the permanent deformation. We propose to treat the corresponding tidal corrections to height determination consistently with the IAG Resolution of 1983; this procedure requires another correction term in addition to the generally applied approach. Finally we prove that the tidal correction cannot be neglected in highly precise engineering surveying.