Normaal Amsterdams Peil
Normaal Amsterdams Peil (NAP) or Amsterdam Ordnance Datum is a vertical datum in use in large parts of Western Europe. Originally created for use in The Netherlands, it was adopted by Prussia in 1879 under the name Normalnull, and in 1955 by other European countries.
Originally the zero level of NAP was the average summer flood water level (not mean sea level) in the IJ in the centre of Amsterdam, then still connected with the open sea, in 1684. Currently it is physically realized by a bench mark in brass in the centre of Amsterdam, which is a popular tourist attraction.
Currently NAP is close to mean sea level at the Dutch coast.
- History of NAP van der Weele, 1971, Dutch Royal Society for Sciences, Governmental Commission for Geodesy
-  (In Dutch)
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