The History of Dutch Ordnance Datum (NAP)

Frans J.P.M. Kwaad,
physical geographer

Home

By order of Johannes Hudde, Lord Mayor of the city of Amsterdam and famous mathematician, in 1683, visible marks of the already existing city's ordnance datum of Amsterdam (AP) were established. For that purpose Hudde had large, white marble stones installed in the walls of eight sluices (locks) in the sea dyke running along the southern waterfront of  "het IJ". "Het IJ" was, and still is, a major watercourse in Amsterdam; until 1932 it had an open connection with the North Sea. On each of the marble stones a horizontal line was cut representing the height of the top of the sea dyke. Only one of these so-called Hudde stones still remains in place, in the Eenhoorn sluice. The stone bears the following inscription: ZEE DYKS HOOGHTE, ZYNDE NEGEN VOET VYF DUYM BOVEN STADTSPEYL. In translation it reads: SEA DYKE'S HEIGHT, BEING NINE FEET AND FIVE INCH ABOVE CITY DATUM. This means that the horizontal line on the stone sits at an altitude of 2.676 m above Amsterdam Ordnance Datum. In this way Amsterdam Ordnance Datum (or AP) was represented by a mark for everyone to see. Since 1683 the definition of AP (later NAP) is: a datum level at 2.676 m below the horizontal line on the Hudde stones. It is important to realize, that when AP was renamed NAP in 1891, the original datum level was maintained.
The question arises, which stage or level of "het IJ" corresponded with Amsterdam Ordnance Datum in 1683. Was it mean sea level (half tide level) or a different stage? From daily measurements of high and low tide near the Haarlemmer sluice in 1683-1684, it is known that Amsterdam Ordnance Datum nearly coincided with mean summer high tide on "het IJ". There is a difference of only 1.8 mm between the two. It can be safely assumed, therefore, that mean summer high tide on "het IJ" was, in fact, selected as the city's ordnance datum in the seventeenth century. The mean difference between high and low tide on "het IJ" amounted to 33.6 cm in 1683-1684. This means that mean sea level (or half tide level) in those days was 17 cm below Amsterdam Ordnance Datum. By relative sea level rise since then, mean sea level along the Dutch North Sea coast at present is between 0 and 10 cm +NAP. This varies slightly from place to place.
During the 18th century, especially in the years 1797-1812, AP was transferred by geodetic levelling from Amsterdam to other locations in the country where also AP marks were installed. By Royal Decree of 1818 it was ordained that from then on AP should be used as Ordnance Datum for the whole of the Netherlands, henceforth being Dutch Ordnance Datum. In the years 1875-1885, a first geodetic precision levelling was carried out to check AP marks throughout the country. Five still remaining Hudde stones were used as a reference during that measurement campaign. It appeared that not all AP marks and land surface elevations based on those marks were at the right altitude, due to measurement errors and vertical movements of the marks. New AP marks were installed, old ones corrected and land surface elevations remeasured. AP (Amsterdams Peil) was renamed NAP (Normaal Amsterdams Peil). As a result, since 1891, terrain and water heights in the Netherlands are given in m  + or - NAP.
In 2005 another correction on NAP-heights was carried out, but again the original datum level was maintained. 

More detailed information on the history of Dutch Ordnance Datum (NAP) can be found on Normaal Amsterdams Peil

Below two pictures of the Eenhoornsluis with the last remaing Hudde stone are shown:

Measurements show that the water level of "het IJ" in Amsterdam gradually rose during the period of measurement (1700 to 1861). In 1683 mean summer high tide on "het IJ" was chosen as the city's datum level. In 1861 yearly mean summer high tide had risen by about 10 cm (the yellow line in the graph below). So had yearly mean low tide (the pink line) and mean sea level (the half tide line in blue). It is important to realize that Amsterdam ordnance datum was not affected by sea level rise since its introduction in 1683. It still is the level of summer high tide on "het IJ" as this was in 1683. Click on data to see the measurements.