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. Mayor Johannes Hudde of Amsterdam in a way came up with the idea after he expanded the seadike after a flood in Amsterdam in 1675. Of course a dike should be storm-resistant to protect a city against flooding, and in this case a margin of "9 feet and 5 inches" was deemed enough to cope with rising water. So he measured the waterlevel of the adjacent sea-arm, Het IJ and compared it with the waterlevel in the canals within the city itself. He found that the water level at an average summer flood in the sea arm (when the water level reaches its maximum, not counting storms) was about the same as the level on the other side of the seadike, plus the margin of 9 feet and 5 inches. The quite constant waterlevel in the canals of Amsterdam, called Amsterdam Peil (AP), equaled the level at summer flood at sea in the sea-inlet, which changes throughout the year. The Amsterdam level (AP) was in 1860 carried over to other areas in the Netherlands to replace locally used levels. In this operation an error was introduced which was corrected (normalised) between 1885 and 1894, resulting in the Normaal Amsterdam Peil (NAP). Originally the zero level of NAP was the average summer flood water level in the IJ just north of the centre of Amsterdam (which was at the time, in 1684, the main shipping area, then still connected with the open sea). Currently it is physically realized by a benchmark in brass in the Opera Building cum City Hall of Amsterdam, which is a popular tourist attraction.