Nautical chart zero

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The chart zero (abbreviation SKN; English chart date ) or chart zero for short is a reference surface for water depths in seafaring. Information on water depths in nautical charts and tide tables refer to it. A specific nautical chart zero is assigned to each map. In the case of tidal waters , the aim is to define the deepest occurring low water - the extreme low spring water - as chart zero. As a result, minimum water depths are always displayed for depth information, even in tidal areas.

Sea chart zero, sea level, coastline

Nautical chart zero must not be confused with normal height zero (NHN), which is used for maps. Since the sea chart zero as the lowest tide level (Lowest Astronomical Tide (LAT), see below) is determined by the tidal range, its difference to sea level differs from place to place. This difference is about half of the respective maximum tide amplitude .

Since the nautical chart zero is lower than the mean low water (MNW, international MLW = Mean Low Water ), the zero height line in nautical charts can deviate from the tidal limit in other maps in such a way that the tidal areas are larger in the nautical chart. In sea maps and maps, the coastline is drawn in at mean (tidal) high water (MHW), i.e. it is about half the tidal amplitude above sea level and accordingly further inland, but below or seaward of the water's edge at maximum tidal high water (HAT = Highest Astronomical Tide). The elevation data in watts, differentiated in (German) nautical charts from the water depths in the fairways by underlining the place in front of the decimal point, reach values ​​that correspond to the average of the maximum and mean tidal range.

Jumping low water

MSpNW and LAT

Traditionally, map zero is defined as mean spring low water (MSpNW) - that particularly low water level left by the ebb tide at full or new moon. So it is approximately half the tidal range below mean sea level , the geoid . The MSpNW thus differs from the height reference of the national survey by amounts that can exceed several meters.

For the sea depths or depth lines given in sea ​​charts , this definition has a significant advantage over normal height zero (or meters above the Adriatic Sea or other gauges): the map indicates the minimum water depth, i.e. the water depth that remains even when the water is particularly low .
The disadvantage of equating SKN = MSpNW is that the altitude reference differs from state to state when the tides are different. When issuing the nautical charts, however, it is customary for the original data applicable there to be adopted from foreign waters, so that your own and third-party charts also indicate the same depths for the same area.

Lowest Astronomical Tide

In Germany and the North Sea states, the nautical chart zero has been defined as the locally "lowest possible tidal water level" or "Lowest Astronomical Tide (LAT) " since 2005 . LAT is a little deeper than MSpNW and is hardly undercut even by extreme spring tides , so that the depth information on the nautical charts leads to greater safety for shipping. Furthermore, the new nautical chart zero rules out negative values ​​in the tide tables .

Its deviation from the geoid is, however, dependent on the locally variable, maximum tidal range , so that chart zero does not represent a general level surface . On the German North Sea coast is just over half a tidal range lower than mean sea level (MSL), which z. B. in Cuxhaven is 2.1 m. On the coast of the Baltic Sea, normal altitude zero and chart zero coincide.

In the past, chart zero was also defined in the German North Sea as the local mean low spring water (MSpNW). Since 2005, the nautical charts in all North Sea countries have been converted to a uniformly defined nautical chart zero, the LAT, which is calculated as the locally astronomically lowest possible tidal water level. In Germany, LAT is 0.3 to 0.6 m below the previous SKN (MSpNW), depending on the location.

LAT was introduced as a new SKN to standardize the depth information in worldwide electronic nautical charts ( ECDIS ), first in the North Sea by the "North Sea Hydrographic Conference". This idea was adopted by the IHO and made a worldwide recommendation for all member states. All waters with a tidal range greater than 30 cm should refer their chart zero to LAT. For waters with a tidal range smaller than 30 cm, the mean water level is considered to be SKN.

Mean water level

For coastal waters without the influence of tides (tidal range less than 30 cm), the mean water level (MW) is considered to be chart zero. This means the arithmetic mean of all water levels over an averaged period (19 years in Germany).

Also for nautical charts of the Baltic Sea , as an almost tide-free body of water, the mean water level is usually chosen as chart zero. A switch to LAT is not planned.

SKN in rivers

In river mouths and the lower course of inland waterways , SKN is defined as the horizontal reference area. This refers to different reference values ​​depending on the state. In Germany, LAT is used for inland waterways on the North Sea coast and MW for those on the Baltic Sea coast.

SKN in Europe

There is no international or European authority that determines the chart zero. The IMO has been recommending the worldwide introduction of LAT since 1993 . The “Tidal Working Group” prepares proposals for the member states with the aim of standardization to simplify the application in electronic nautical charts ( ECDIS ). The table shows the status of the information and deviations that the German BSH has collected.

country SKN LAT since Δ to LAT earlier, Δ to LAT Δ to WGS 84
Germany North Sea LAT 2005 --- MSpNW
Germany Baltic Sea MW
Great Britain LAT ---
France Atlantic LAT ---
France Mediterranean
Belgium LAT MLstLWS, +10 to +25 cm
Netherlands LAT --- GLLWS
Denmark West LAT --- MSpNW
Denmark East MW MLWS
Norway north, center LAT --- NGzW
Norway south LAT --- ESLW, −20 to −30 cm
Sweden MW --- MW
Spain Atlantic LAT ---
Spain Mediterranean
Portugal LAT --- 0 to −20 cm
Italy LLWS
Croatia LLWS
Greece MSpNW
Poland
Russia NNW
Estonia
Latvia
Lithuania
Finland
before 2005 each country had a different SKN

Before 2005, most of the European countries had different definitions for chart zero. In the meantime, all states bordering the North Sea use LAT as a nautical chart zero. For Germany, the complete implementation in all nautical charts lasted until 2012. In the Baltic Sea region it is planned to switch to Baltic Sea Chart Datum (BSCD) 2000. BSCD 2000 is based on the Amsterdam level , which is implemented in Germany as normal height zero . Sweden plans to complete the transition by 2020.

SKN in other continents

country SKN Deviation from LAT Deviation from WGS 84
United States MSpNW MLLW
Japan ~ LLW

literature

Web links

Individual evidence

  1. ↑ The nautical chart zero in tidal areas (explanations on nautical chart zero from the German Federal Maritime and Hydrographic Agency).
  2. ↑ Swedish Maritime Administration: Mean Sea Level
  3. ↑ sjofartsverket.se: new reference level