NAP:

as you can see behind the dike it is 2,2 m above NAP:

The normal Amsterdam level (usually abbreviated as NAP) is the reference altitude to which Germany, Netherlands, height measurements in Norway, Sweden and Finland are related.
(copied from Wikipedia)


For convenience, the NTP is often equated to the average sea level. Historically however, the NAP is closer to the mean high water level in the river IJ for closure in 1872.