60th NHC Meeting Agenda item C6 11 – 13 April 2016 Baltic Sea Chart Datum 2000 Stavanger, Norway Sweden

Baltic Sea Chart Datum 2000 and possible impact in the North Sea Summary This paper decribes the possible upcoming need for harmonisation efforts into the Northern Sea area (especially for Denmark, Norway and Sweden) as a result of the establishment of the Baltic Sea Chart Datum 2000. It is suggested that the Nordic Hydrographic Commission recognises this task and takes action to study the topic of harmonising chart datums and propose solutions.

Background The Balic Sea Chart Datum 2000 (BSCD2000) Based on decision made by the Baltic Sea Hydrographic Commission (BSHC), a new reference level; the Baltic Sea Chart Datum 2000 (BSCD2000) is now being introduced in the Baltic Sea charts (paper charts and ENCs). A common reference level means that national solutions are replaced by a Baltic Sea joint reference. Most of the countries have presented their plan for the transition. Russia has no time schedule presented for the transition but is positive in principal to the concept. Denmark has already implemented the new height system DVR90 in the mid-1990s and is using it also as chart datum for the Baltic coast northwards to the latitude of Skagen Lighthouse (57°45,0’N 10°35,8’E) and due eastward to the Swedish coast (along 57°45’N)1. Bornholm is located within these limits. The Swedish realization of BSCD2000 is based on the Swedish height system RH2000 and all nautical charts are planned to be updated latest 20202. In the Baltic Sea, Germany using MSL as CD based on national height system. Estonia and Latvia is updating their national height system intended as a base for their new chart datum. A common geoid model for the Baltic Sea is now produced within the FAMOS3 project.

GNSS From the very start, GNSS affected rapidly the community in many ways and will continue to affect more areas of expertise. Our image of how the world is described has changed and GNSS has inspired the use of maps. It did not take long after GPS was introduced before the charts around the world had been

1 Bag om søkortet udgave 2, ajourført til den 11. juni 2013, chapter 10.1.3, Miljøministeriet, Geodatastyrelsen Denmark; ”Udenfor Skagen anvendes Middelspringtidslavvande som kortdatum, men gradvist vil kortdatum udenfor Skagen blive ændret til Laveste Astronomiske Tidevand (LAT). Udenfor Skagen defineres ved en linje, der forbinder Skagen Fyr på positionen 57°45,0’N 10° 35,8’E langs med breddeparallellen 57°45’N til den svenske kyst.” 2 Does not include charts in inland waters of Sweden. Maybe some charts covering inland waters gives priority before some offshore charts. All charts planned to be updated latest 2022. Inland charts will refer to RH2000 with a specified offset. 3 Finalising Surveys for the Baltic Motorways of the Sea, co-financed by EU.

adapted to the global reference system, WGS84. For quite a long time the focus was on the use of the horizontal positioning. But the use and benefits of altitude information grows with accuracy and access to reliable models of reference levels. IHO Resolution 3/1919 Datums and Bech Marks An updated resolution 3/1919 Datums and Bench Marks have been circulated several rounds between the responsible within the IHO community; TWLWG, IHB, IHO Circular Letter to MS and HSSC. Paper HSSC7-05.8C submitted by IHB refers to related documents and include also the draft proposed by TWLWG. HSSC7 endorsed the amendments. In relation to BSCD2000 this text is an especially important section in the updated resolution:

In geographical areas where the tidal range is negligible and in non-tidal areas It is resolved that depths, and all other navigational information should be referred to Mean Sea Level (MSL) or other level as closely equivalent to this as is practically acceptable to Hydrographic Offices.

Note: The adopted level may be a well-defined geodetic datum as used for heights in land survey applications or an observed local Mean Sea Level (MSL) based on long series of water level observations.

In order to support other non-navigational applications and also to indicate the characteristics in the area, it is recommended to adopt the mean of yearly lowest/highest water levels, or as a suitable percentile of lowest/highest water levels, observed over a long time period.

CD (Chart Datum) reference level in nautical charts Chart Datum in Norwegian waters. Norwegian waters are connected to Danish and Swedish waters in Skagerrak. From the island Utsira (N 59.32°) and northward CD consist of LAT at Norwegian waters. At the southern coast of Norway the astronomical tide is small compared to the meteorological surge. Here CD is lower than LAT for safety reasons. LAT minus 20 cm is used from Utsira to the Swedish border, and LAT minus 30 cm in the inner part of the Oslofjord.4

4 Source: EU Coastal Mapping project WP2.1 questionnaire, updated/distributed February 29, 2016.

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Chart Datum in Danish waters. As already stated, Denmark has already implemented the new height system DVR90 in the mid-1990s and is using it also as chart datum for the Baltic coast northwards to the latitude of Skagen Lighthouse (57°45,0’N 10°35,8’E) and due eastward to the Swedish coast (along 57°45’N). Outside Skagen is used Middle Low Spring Water (MLSW – Middelspringtidslavvande) but will be changed to LAT outside Skagen. See note 1 at page 1 for more details. Chart Datum in Swedish waters. Today Swedish charts refer to MSL from many different years, which in combination with the postglacial uplift makes it confusing for the end users. As already stated, BSCD2000 is now introduced and all charts are planned to be updated until 2020.

The definition of the Baltic Sea Chart Datum 2000 (BSCD2000) The Baltic Sea Chart Datum 2000 (BSCD2000) is a geodetic reference system adopted for the Baltic Sea nautical charts and publications. It is based on the definitions for the European Vertical Reference System (EVRS) as well as the European Terrestrial Reference System 89 (ETRS89a, b, c). The reference epoch for height changes due to the postglacial land uplift in Fennoscandia is 2000.0. According to this definition:

a) The height reference surface of BSCD2000 over the sea area is an equipotential surface of the Earth’s gravity field. The zero level of BSCD2000 is in accordance with the Normaal Amsterdams Peil (NAP).

b) The vertical coordinate is specified by normal heights. The normal potential is defined by the Geodetic Reference system 1980 (GRS80).

c) Corrections of the permanent solid earth tides are made so that the normal heights are in the zero tide system.

d) Temporal height changes due to the postglacial land uplift will be reduced to the epoch 2000.0.

e) The unit of the normal heights is meter.

The realization of BSCD2000 shall make use of the existing national geodetic infra-structure, i.e. the official national vertical and spatial reference frames and corresponding services. The official national reference frames are in agreement with the official realiza-tions of the European vertical and spatial reference systems EVRS and ETRS89 within the level of a few centimeters.

The Baltic Sea is not much affected of diurnal tide, or by meteorological effects. So the new chart datum (the geoid) is close to the mean sea level for the year 2000. Due to the land uplift the epoch 2000 is important to state.

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Conclusions The consequences and implications of the existence of different chart datums in the Skagerrak area are complicated and the situation is not optimal from a user perspective. There is a need to further study and clarify the situation. There are also connections to existing and future establishments of bathymetry portals, such as EMODnet Bathymetry and the Coastal Mapping portal, where a harmonized vertical reference level is always an issue. Sweden proposes to establish an NHC Working Group (mainly trilateral Denmark-Norway-Sweden) with the task to draft proposals for a more homogeneous chart datum solution in the Skagerrak area.

Actions requested from NHC60

The NHC60 is invited to

- Note this report - Discuss and decide on the establishment of a NHC WG as proposed

Appendix

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