NAP. 'Normaal Amsterdams Peil' (NAP) or 'Amsterdam Ordnance Datum' is used as the reference water level since the end of the 19th century. Originally the zero level of NAP was the average summer flood water level in the IJ just north of the centre of Amsterdam, then still connected with the open sea. Nowadays zero meter NAP is about the same as sea level. NAP is an official and very important measurement system for our country, and its data are used for protection against floods, water management and all kind of studies.
The mural shows the Texel lighthouse on the left, where we were standing on 33,6m above NAP. The lowest point is the village of Nieuwerkerk aan den IJssel (6,76m below NAP), and the Vaalserberg (322,2m above NAP) is the highest point.