Absolute vertical motion of the Amsterdam Ordnance Datum (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. However, little is known about absolute vertical movements of the primary network. This information is indispensable for the interpretation of water level measurements at the tide gauges along the Dutch coast. It may be provided by gravity measurements. Here we present a time-series analysis of more than twenty years of gravity measurements at the stations Westerbork, Epen, Zundert, and Radio Kootwijk. It reveals that only station Epen shows a statistically significant movement of -0:252±0:066 μGal yr-1, which corresponds to an uplift of 1:3±0:5mmyr-1. For the other stations, the trends are statistically not different from zero at a significance level of 0.05. Corrections for water table variations are found to be indispensable; peak-to-peak amplitudes range from 4 μGal (Westerbork) to 28 μGal (Radio Kootwijk). Depsite some fundamental objections, corrections for instrumental offsets reduce the data scatter. First experiments with 7 years of soil moisture data acquired at station Radio Kootwijk reveal that the gravity signal of soil moisture variations has a standard deviation of 2:2 μGal, which is comparable to the noise standard deviation of measured gravity.