NAP stands for Normaal Amsterdams Peil, the reference surface to which all heights in the Netherlands are related, on land and in water. A height of NAP 0.00 m lies approximately at mean sea level in the North Sea. Anything above carries a plus sign, anything below a minus.
The NAP network
The datum is not a single point but a national network of benchmarks: bronze bolts set into facades, bridges and viaducts. There are around 35,000 of them across the country. In addition, several hundred underground benchmarks are founded on a deep sand layer that barely settles; these form the backbone of the system. Rijkswaterstaat maintains the benchmarks and re-levels them periodically, because the ground surface in parts of the country is subsiding.
Comparable national datums exist elsewhere: Ordnance Datum Newlyn in Great Britain, NHN in Germany, and NGVD/NAVD in the United States. They are all close to mean sea level but not identical, so cross-border projects must always state which datum a height refers to.
Use in construction
- Site datum on drawings — a project defines a datum level, often the top of the finished ground floor. All dimensions are then given relative to it
- Tied to NAP — planning documents, drainage layouts and external works drawings give levels in NAP
- Setting out — the surveyor transfers the level from a known benchmark nearby onto profiles or a fixed reference on site
- Water management — polder water levels, embankment heights, drainage and sewer connection levels are always quoted in NAP
Points to watch
- Site datum and NAP are not the same thing. Record in the specification exactly how many metres NAP the site datum represents
- Check two benchmarks. A single benchmark may have settled or been damaged; a check level prevents the whole job being set out wrongly
- Subsidence. In peat and clay areas the ground level relative to the datum changes over the years; use current data, not an old drawing
- Floor level and flooding. New floor levels are deliberately set above road level and the expected flood level — that margin is the first line of defence against water ingress
Related terms
- Datum
- Ground level
- Levelling
- Setting out
- Building height
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