Poland: What is the meaning of „at mean water level 500”?

Often, when we provide information on port restrictions, the question arises as to the meaning of the term „at mean water level 500”? I will answer this question briefly.
To begin with, let us distinguish between water level and water depth:

Water depth means the depth of the water between the surface and the seabed.

Water level refers to the height or elevation of water above or below a reference point.

It should be noted that water depth changes with changes in water level. Therefore, the water level serves as more practical information, assuming that the topography of the seabed in a given location remains stable over the longer term.
When providing data related to the water depth, it’s necessary to establish a universal vertical datum reference point to define the zero level. Since the Earth isn’t a perfect sphere, geodetic and cartographic needs require fixed reference points to enable height comparisons over larger geographical areas, extending beyond individual countries or regions. From 1 January 2024 onwards, the only reference point for vertical measurements (H=0.000) applicable in Poland will be the point determined by the mareograph in Amsterdam, based on measurements from 1683 and 1684, known as „Normaal Amsterdam Peil” (N.A.P.) in Dutch or „Normal Null” (NN) in English.
So, if the zero level for Poland matches the zero level in Amsterdam, what does the value „500” stand for? Well, it is a purely technical trick. After subtracting 500 cm from the NN level, it is possible to define the „500 level” as the new name for the „zero level”, so in case of a periodic drop in the water level, negative values do not appear in the readings. For example, the 490 level means that the water level is 10 cm below the reference level (i.e., zero level).

Source: Own elaboration