INTERNATIONAL HYDROGRAPHIC REVIEW 
MAY 2020 
72 
This change of chart datum to a zero level that is a little offset from MSL may at first glance ap 
pear unnatural to the mariner. However, the advantage of the geoid-based definition of the chart 
datum is that the geoid is virtually independent from changes of sea level and can be determined 
to high accuracy without sea level observation (see next paragraph). Thus, as motivated in Sec 
tion 2, the geoid provides the required unified reference surface in order to monitor absolute and 
relative sea level changes by combination of space-geodetic techniques (GNSS, satellite altime 
try) and classic local sea level observations along the coast (water level stations, geometric level 
ling). This holds true also for the Baltic Sea where global sea level rise (i.e., dynamic ocean) is 
contrasted by postglacial land uplift (i.e., dynamic Earth), since the latter effect can be modelled 
with high accuracy. 
Geoid-based HRS are realized by so-called geoid models, i.e., gridded values of the geoid height 
with respect to the reference ellipsoid used for GNSS applications. They can be computed from 
gravimetric measurements with a typical internal standard uncertainty of 1-2 cm in areas with 
good gravity data coverage. For practical use, the gravimetric geoid models are fitted to the ellip 
soidal and physical heights of co-located GNSS and levelling benchmarks in order to become 
compatible with the national height system realizations. The HRS for the BSCD2000 will be an 
adopted quasigeoid 3 model referenced to a common zero level. 
In this respect, BSCD2000 replaces the so-far heterogeneous and inconsistent historical MSL 
realizations between countries, or even between map sheets or harbors within the same country. 
The computation of this geoid model was stimulated by preparation works co-financed by the EU. 
Acquisition and measurement of the necessary data and the computation of interim (i.e. prelimi 
nary) geoid models were organized and performed within the project “Finalizing Surveys for the 
Baltic Motorways of the Sea” (FAMOS). Unfortunately, the last phase of the FAMOS project (2019 
-2020) could not be realized, so that the finalization of the BSCD2000 HRS is now coordinated by 
the CDWG and planned for the end of 2022. 
The second most important characteristic is the new and unified zero level. BSCD2000 
will be linked to the Normaal Amsterdams Peil (NAP). 
NAP is the zero level for the European Vertical Reference System (EVRS) but also for most 
countries around the Baltic Sea. All states formerly referring to the Kronstadt datum have 
changed their national height systems or are in transition to change to the EVRS, except for 
Russia 4 . Numerically, these national realizations of the NAP coincide at the centimeter level, as 
demonstrated in Figs. 2a-e and Table 1 in comparison with EVRF2019, the last official release of 
the European height system EVRS (Sacher et al. 2019). Consequently, the respective national 
height systems can be considered as ready-to-use realizations of the BSCD2000 within its 
specification (overall standard uncertainty better than 5 cm, sufficient for the typical precision of 
charted depths; see further Agren et al. 2019). 
3 
- The conceptional difference between geoid and quasigeoid is a geodetic subtlety that is only relevant on land at high 
altitudes. 
4 
- Only for Russia, simple transformation models between national heights and BCSD2000 will be necessary, probably 
by means of local datum shifts for the areas of Oblast Kaliningrad and the Gulf of Finland/St. Petersburg.