INTERNATIONAL HYDROGRAPHIC REVIEW 
MAY 2020 
65 
1. Motivation 
The Baltic Sea is an international shallow, non-tidal area in the northern part of Europe with 
dense traffic. Accurate depth data and water level information is of absolute importance to pre 
serve the safety of navigation in the area. So far, numerous local reference levels were used in 
the Baltic Sea for nautical charts and water level information, such as water level observations, 
forecasts and warnings. The exact definition of the chart datum may vary between countries, re 
gions as well as over time. The matter of these existing Baltic Sea chart datums is a rather com 
plex issue, not only for the navigators, but also for the responsible hydrographic offices and other 
users of depth data in general. Thus, a unified vertical reference system is important to improve 
navigation in the Baltic Sea region. 
For centuries, the chart datums in the Baltic Sea were defined as the “mean sea level” (MSL), 
which was observed at tide gauges, i.e., a height obtained by a long-term average of the sea level 
readings measured with respect to the land at the particular tide gauge location. This implies that 
the chart datum is influenced by two different quantities - long term sea level changes as well as 
possible height changes of the land. Both quantities could not be observed separately. This has a 
great impact especially in the Baltic Sea region which is strongly affected by the postglacial 
Scandinavian land uplift phenomenon. While the climate-induced sea level rise has an order a 
few millimeter per year, the land uplift affects the heights by up to one centimeter per year. This 
results in a lowering of the depth in major areas of the Baltic Sea and is well observable by 
mareographs (see Figure 1). 
-2 -1 
■ m m/year 
10 11 
Figure 1: Land uplift in Fennoscandia according to the NKG2016LUmodel (Vestol et al. 2019)