Supplementary data: Absolute vertical motion of NAP

The backbone of the Amsterdam Ordnance Datum (NAP) is a network of about 400 primary subsurface markers. Relative movements between the primary subsurface markers are measured with spirit levelling once in 10-20 years. We investigated absolute movements in this primary network through a time series analysis of more than 20 years of gravity measurements at 4 stations in the Netherlands. The result is important for the interpretation of the water level measurements along the Dutch coast. The data data set that we used for this research concerns gravity values, sorted by date, measured with the Micro-g FG-5 absolute gravity meter, with instrumental offset values on these measurements, obtained from comparison campaigns. This data is further supplemented with uncertainties about the instrumental offset and gravity measurements. Furthermore, the groundwater levels at the time of gravity measurements have also been added.