Amsterdam Ordnance Datum or Normaal Amsterdams Peil (NAP) is a vertical datum in use in large parts of Western Europe. Originally created for use in the Netherlands, its height was used by Prussia in 1879 for defining Normalnull, and in 1955 by other European countries. In the 1990s, it was used as the reference level for the United European leveling Network (UELN) which in turn led to the European Vertical Reference System (EVRS).
Mayor Johannes Hudde of Amsterdam is credited with beginning the first works for the creation of this vertical datum after he expanded the sea dike following a flood in Amsterdam in 1675. Between 1683 and 1684, he had daily measurements taken of the water level of the adjacent sea arm, Het IJ, during high tide. The calculated averaged was called the Amsterdams Peil ("Amsterdam level", AP) and used to calculate the minimum height of the sea dykes, which he set at "9 feet and 5 inches" (2.67 m) above AP.
