Arise of the NAP
More than three centuries ago a standard ordnance datum was established in Amsterdam, based on the average high water mark of the Zuiderzee, which was Amsterdam’s connection with the North Sea. This ordnance datum was – and is – the referral point for all constructions of any kind in the Netherlands. It was also adopted as the standard for the European Union.
The NAP was established in Amsterdam in the 17th century. In that century floods and waterlogging occurred frequently. In 1675 a flood disaster initiated a huge effort to better protect vulnerable low-lying areas and their inhabitants against floods. The high tide water was henceforth measured systematically. Between 1683 and 1684 the high tide water mark was measured daily. The average was called the “Amsterdam level” (acronym A.P.). This average high tide sea level was adopted as the zero level (reference plane). It was used to improve and where needed to heighten existing dykes and water barriers to a safe level. In 1684 the mayor of Amsterdam, Johannes Hudde, had eight marble stones cemented in the sluices around the IJ. Every stone had a horizontal groove in the middle which represents the height. The inscription on the stone worded:
ZEE DIJKS HOOGHTE ZIJNDE NEGEN VOET VYF DUYM BOVEN STADTSPEYL
(sea dyke height being nine foot five thumbs above town-level)
Mayor Johannes Hudde and the only remaining stone benchmark (Huddesteen) in the Unicorn sluice
Converted that is 2.67 meters above A.P. These marble stones (nowadays named “Huddestenen”, named after mayor Hudde), were meant to indicate how high the sea dykes must be above A.P. (Amsterdam Level).
Amsterdam Level (AP) was renamed Normal Amsterdam Level (NAP) two centuries later. This name change was the result of a new norm for precision measurements which emerged from 1875. There is therefore no difference between AP and NAP. From then on, much more accurate level measurements were taken.
The NAP is a reference plane. This means that all heights of land, water, dykes and tunnels are measured with respect to this plane.
In the past this reference plane was determined with the help of simple instruments. Nowadays they use several modern techniques such as automatic level instruments and GPS (satellite Navigation).
Would you like to see all these instruments, measurements techniques and history of the NAP? Come and immerse yourself in the NAP exhibition!