Chart datum
The chart datum (abbr. SKN; . Engl Chart Datum) or short cards zero is a reference surface for water depths in shipping. Information on water depths in nautical charts and tide tables refer to it. Each card is assigned a specific chart datum. In tidal waters, the aim is the deepest occurring low water - the extreme low water spring - set it as the chart datum. This always minimum water depths are displayed in the depth data in tidal estuaries.
Chart datum, sea level, shoreline
Chart Datum should not be confused with mean sea level ( MSL ), which is used for maps. Its difference from the normal place of zero is because the chart datum as the deepest tide level ( Lowest Astronomical Tide LAT s, u ) is determined by the tidal range is different to place. This difference is about half of the respective maximum tidal amplitude.
As the chart datum is lower than the mean low water ( MNW, International MLW = Mean Low Water ), the zero - height line on charts may differ from the watt limit in other card in such a way that the mudflats are larger in the chart. In lake - like maps of the coast line at the middle ( tidal ) flood ( MHW ) is drawn, so is approximately half the tidal amplitude above sea level and correspondingly further landward, but below or seaward of the water's edge at maximum Tidenhochwasser HAT ( Highest Astronomical Tide ). The amount of information in watts, divided into ( German ) charts of the water depths in the navigable waters by underlining the point before the decimal point, thereby extend to values that correspond to the average of the maximum and the mean tidal range.
Spring low water
Traditionally, as a card zero the mean low water Spring ( MSpNW ) (English: Mean Lower Low Water, MLLW ) defined - those particularly low water level, the expiration of the low tide leaves at full or new moon. It is therefore almost by half the tidal range below mean sea level, the geoid. The MSpNW thus differs from the reference amount of land surveying by amounts that can exceed several meters.
For items included in charts depths or deep lines that determination has compared to the mean sea level (or meters above the Adriatic or other levels ) a significant advantage: the map indicates the minimum depth of water, so that water depth, which still remains even at very low low tide. A disadvantage of equating SKN = MSpNW that the height reference of state differs to state, when the tides are at different heights. However, it is common in the publication of nautical charts that from foreign waters, the appropriate local original data is transferred, so that own and others' cards for the same area also specify the same depths.
Lowest Astronomical Tide
In Germany and the North Sea States, the chart datum " lowest possible tidal water level " or " Lowest Astronomical Tide ( LAT) " since 2005 as a locally defined. LAT is a little deeper than MSpNW and is rarely below also of extreme spring tides, so that the depth values lead to the charts improving the safety of navigation. Furthermore, the new chart datum excludes negative values in the tide tables.
Its deviation from the geoid is, however, from the locally variable, maximum tidal range dependent, so that chart datum is not a general level surface. At the German North Sea coast, it is a good half a tidal range lower than mean sea level ( MSL ), which eg accounts for 2.1 m in Cuxhaven. Consistent mean sea level and chart datum on the coast of the Baltic Sea.
Previously, the chart datum was also in the North Sea than the local mean low water Spring ( MSpNW ) defined. Since 2005, the charts are on a defined uniform chart datum, the LAT, changed in all North Sea countries, which is calculated as a local astronomical lowest possible tide water level. LAT is located in Germany depending on the location from 0.3 to 0.6 m below the previous SKN ( MSpNW ).
LAT was introduced as a new SKN to standardize the depth data in global electronic charts ( ECDIS), first in the North Sea " North Sea Hydrographic Conference" by the. This idea was adopted by the IHO and pronounced as the world's recommendation to all Member States. All waters with a tidal range greater than 30 cm are to refer her to chart datum LAT. For waters with a tidal range of less than 30 cm of water Medium sized businesses are the SKN.
Average water level
For coastal waters Tideneinfluss ( tidal range less than 30 cm) of the Mean water level (MW ) is considered as chart datum. This refers to the arithmetic mean of all water levels averaged over a period of time ( in Germany 19 years old).
For nautical charts of the Baltic Sea, as a nearly free tidal water body, usually the mean water level is chosen as the chart datum. A switch to LAT is not provided.
SKN in rivers
In estuaries and the lower part of inland waterways SKN is defined as a horizontal reference surface. This refers, depending on the state in different reference values . In Germany is used for inland waterway on the North Sea coast LAT, for those on the Baltic coast MW.
SKN in Europe
There are no international or European authority, which specifies the chart datum. Since 1993, the IMO recommends the worldwide launch of LAT. The " Tidal Working Group " makes recommendations to the Member States with the aim of standardization for ease of use in electronic charts ( ECDIS). The table below shows the status of the information and deviations that the German BSH has collected.
Prior to 2005, most European states had differing definitions for the chart datum. Meanwhile, use all North Sea states uniformly LAT than chart datum. For Germany, the complete reaction lasted in all charts until 2012.