Analyzing the long-term changes in China’s National Height Datum

China’s National Height Datum was determined by the mean sea level at the Qingdao tide gauge station. Research was conducted to study the long-term changes in China’s National Height Datum. The first step was to analyze the long-term changes in the Qingdao Huanghai mean sea level relative to the crust by analyzing the tide data from 1952 to 2011. The frequencies and amplitudes of main tides, including 180 short-period tides and 6 long-period tides, were obtained by fast Fourier transform. Then, the changes in the mean sea level at the Qingdao tide gauge from 1952 to 2011 were studied using harmonic analysis based on least-squares estimation and the 18.61-year shifting average of the tidal heights. The results of the first step showed that the mean sea level at the Qingdao tide gauge was at the rate of 1.07 mm/yr from 1952 to 1980 and increased to 1.62 mm/yr from 1980 to 2011. The second step was to ascertain the long-term changes in the crustal subsidence by analyzing the daily data from the Qingdao global navigation satellite system (GNSS) from 2005 to 2015. Initially, the daily relax solutions for coordinates of this station were computed in a combined adjustment involving a global GNSS network of about 50 International GNSS Service stations and a regional network of 43 Crustal Monitoring Observation network of China stations. Then, a time series spanning 10 years for station coordinates aligned to the international terrestrial reference frame (ITRF) 2014 was obtained. The time series of height components was analyzed to detect entities such as outliers, offsets, frequencies, and seasonal periodic parts. Through statistical detection, we found that the vertical velocity of the crust in the Qingdao GNSS station was zero. Based on the combined analysis of tide data and Qingdao GNSS data, it can be concluded that the mean sea level at epoch 2019.0 is 247.446 cm, which is 4.556 cm higher than the 1985 National Height Datum.