Amsterdam Ordnance Datum orNormaal Amsterdams Peil (NAP) is avertical datum in use in large parts ofWestern Europe. Originally created for use in theNetherlands, its height was used byPrussia in 1879 for definingNormalnull, 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 theEuropean Vertical Reference System (EVRS).[1]
MayorJohannes Hudde ofAmsterdam is credited with beginning the first works for the creation of this vertical datum after he expanded the seadike following aflood in Amsterdam in 1675. Between 1683 and 1684, he had daily measurements taken of the water level of the adjacent sea arm,Het IJ, duringhigh tide. The calculated averaged was called theAmsterdams 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.[2]
In 1850, the datum was used at several places inBelgium, and in 1874 theGerman government adopted the datum for first-order levelling.[3] AP was carried over to other areas in the Netherlands in 1860, to replace locally used levels. In this operation, an error was introduced which was corrected (normalised) between 1885 and 1894, resulting in theNormaal Amsterdams Peil.
Currently it is physically realised by a brass benchmark on a 22-metre (72 ft)pile below theDam square in Amsterdam.[4] The brass benchmark in the AmsterdamStopera (combined city hall and opera house), which is a tourist attraction, is no longer used as a reference point.
52°22′23″N4°53′34″E / 52.37306°N 4.89278°E /52.37306; 4.89278