Q1. How is the absolute gravity value at LOGG determined as the national reference?
A： An absolute gravimeter is used at a permanent pier for at least 12 hours of measurements to obtain the absolute gravity value at 100 cm above the pier. The raw observations are then reduced to standard conditions (standard atmosphere, no polar motion, average groundwater level, average soil moisture, etc.) so that the value is time-invariant; tidal effects of the solid Earth and ocean loading are also removed. A relative gravimeter is then used to measure the local gravity gradient, transferring the value from the 100 cm height to the pier surface, which is provided as the official reference for further surveys.