Relative sea level riseis the rise in sea level relative to a reference plane, which is connected to a fixed point in the subsurface. The relative sea level rise is the sum of the actual sea level rise, due to the increase in and expansion of sea water resulting from temperature rise, and the movement of the reference plane.
Since 1891 the NAP (Normal Amsterdam Level) has been the national reference plane in the Netherlands. Mainly due to land subsidence this reference plane is not stable and has to be remeasured periodically. The last national campaign resulted in the 2005 NAP adjustment.
Rijkswaterstaat has been measuring the water levels along the coast for over decades and compared them to this reference level. The relative sea level rise based on water level measurements over the period 1890 to 2017 shows a constant rise of 18.6 ± 1.2 cm per century. Baart et al. estimated that subsidence is responsible for approximately 4.5 cm of the relative sea level rise that has occurred this last century. The absolute rise in sea level is therefore estimated at 18.6 - 4.5 = 14.1 cm per century (Baart et al., 2018).
Figure 1 Tidal station Hoek van Holland and example of NAP reference bolt in measurement station
In the method of sea level rise calculation, described in Dilling et al. in 2010, the time series of water levels are corrected for the 2005 NAP adjustment. They state that the reference height has not been corrected before 2005. Due to archive work it has become clear that the reference height has been altered several times before.
Assuming that water levels historically have been accurately measured in relation to NAP, it is probably better to report the sea level rise in respect to this national reference plane instead of the currently used methods.
Baart et al., 2018, Zeespiegelmonitor
Dilling et al., 2010, definitie-zeespiegelstijging-voor-bepaling-suppletiebehoefte