Chart datum
From BAWiki
Definition
Reference surface for the depth indication of waters in maps and plans. Note: The reference surface is regularily defined so that it is rarely below the water level. The reference surface for depth indications on charts (chart datum, SKN) corresponds in Germany for the North Sea outside the tidal rivers to the local MSpTnw. For the Baltic Sea the reference surface corresponds to sea level (approximately MW). For tidal rivers, the particular chart datum is adopted for the charts.