A HARMONIZED 
VERTICAL REFERENCE 
SYSTEM FOR THE 
BALTIC SEA 
Jyrki Mononen, Wilfried Ellmer, Thomas 
Hammarklint, Lars Jakobsson 
RH 2000(ml 
♦0.10 
*0 00 
Chart datum: MSI 1*00 (Chart SC41S1) 
— Sea surface at any time 
Survey datum: MSI 2000 
Water level datum: MSI 2011 
Land survey datum: RH 2000 2 predicted MSL 202) 
AI measurements in meters 
MSL = Mean Sea Level (Year) 
Datum : Reference level 
RH 2000 >s the Swedish national realisation of 
lVPS (luropeen vertical Reference System) 
* I and uplift 0 0 rm/year 
** Land upkftO.S cm/year 
Because there is no common vertical reference 
system for hydrographic or navigational tasks, 
Baltic Sea Hydrographic Commission (BSHC) 
has considered to be important to harmonize chart 
datums in the Baltic Sea. Countries have their own 
national systems differing from each other. Systems 
are based principally on mean sea level (MSL), but 
national realizations of MSL are different. This 
kind of situation is inconvenient for navigators, for 
data transfer between Hydrographic Offices and for 
other use of depth and water level data. 
BSHC decided in 2005 to establish a Chart Datum 
Working Group (CDWG) to study and foster the 
harmonization of vertical reference systems. 
CDWG members include representatives from 
all Baltic Sea Hydrographic Offices, permanent 
representatives and observers from national 
geodetic and oceanographic organizations and 
BOOS. 
The working group studied first the feasibility of 
the European Vertical Reference System (EVRS) 
as a principal alternative for a harmonized 
vertical reference system for Baltic Sea nautical 
charts. It also studied possible time schedules and 
necessary preconditions necessary for each Baltic 
Sea country to move to use harmonized datum on 
their nautical charts. The working group develops 
recommendations and guidelines on how the 
transfer period could be implemented. 
The EVRS has been found to be feasible as a 
harmonized vertical reference system for the Baltic 
Sea. The change to the EVRS based reference 
system would have practically no effect in the 
southern parts of the Baltic Sea, but in the northern 
parts it would have around 15 - 20 cm effect on 
depths. There is good commitment among the 
Baltic Sea countries to take this harmonized datum 
in to use. 
Fig. la. Present situation. 
RH 2000 (m) 
_ 
Î 
Chill datum - Land lurvay datum i 
Water laval datum = RH 2000 
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AB mtifunminH in m«t#>i 
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Datum s Reference level 
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1 
RH 2000 is Ihf Swedish national realisation of 
r 
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EVRS (European Vertical Reference Syitem) 
u 
Seabed 
Fig. lb. Situation with harmonized chart datum. 
There are a lot of benefits which can be achieved 
with a well-defined, international EVRS based 
harmonized vertical reference system when all the 
depth and water level information can be provided 
in the same datum within the whole Baltic Sea. 
E.g. elimination of confusion between different 
chart datums, and safer and easier data transfer 
between national Hydrographic Offices and other 
organisations. Further on a harmonized vertical 
reference system enhances wider and easier use of 
the data in accordance with the INSPIRE directive 
and enables the full utilization of future enhanced 
navigation systems based on International 
Hydrographic Organization (IHO) S-100 standards. 
Other benefit for the EVRS is that it is a common 
European vertical reference system to which many 
national height systems are based on. Thus depths 
on sea and heights on land will be referenced to 
the same reference system. The common levelling 
network around the Baltic Sea, Baltic Levelling 
Ring (BLR), computed and adjusted in 2006 has 
given the possibility to tie all mareographs to this 
common datum. 
Figure la illustrates the present complex situation 
with different MSL based chart datums. Figure lb