Mayor Johannes Hudde of Amsterdam in a way came up with the idea after he expanded the sea-dike 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" (2.67 m - margin is defined in Amsterdam feet) was deemed enough to cope with rising water. So he measured the water level of the adjacent sea arm, Het IJ and compared it with the water level 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 sea-dike, plus the margin of 9 feet and 5 inches. The relatively constant water level in the canals of Amsterdam, called Amsterdams Peil ("Amsterdam level", AP), equalled the level at summer flood at sea in the sea-inlet, which changes throughout the year. 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 the Normaal Amsterdams Peil.