Miscellaneous Notices to Mariners
6 Tidal Waters
Observed water levels, water level forecasts, tide tables and vertical reference levels for the Norwegian coast are available at Kartverket’s service Se havnivå
The water level has one contribution from the predictable tide, due to the moon and the sun, and one meteorological contribution (the surge) due to weather effects such as pressure and wind, The water level forecast combines the tide with forecast for the meteorological contribution.
The Lowest Astronomical Tide (LAT) and the Highest Astronomical Tide (HAT) refers to the lowest and highest tide level which can be predicted to occur under average meteorological conditions. LAT and HAT are determined by finding the lowest low water and the highest high water from 19 years of predicted tides.
Reference Level for Depths and Heights in the Tidal Tables
The vertical Chart Datum in Norwegian waters is based on the Lowest Astronomical Tide (LAT). In areas where the meteorological contribution can dominate the water level for longer periods of time, an extra safety margin has been added so that the datum is lower than LAT.
For Norwegian waters, the Chart Datum is defined as follows:
- From Utsira and north to the Russian border (including Svalbard) Chart Datum equals LAT.
- From the Swedish border to Utsira, Chart Datum is 20 cm lower than LAT.
- In inner Oslofjorden north of Drøbaksund, Chart Datum is 30 cm lower than LAT.
This definition of Chart Datum was introduced into the Norwegian charts in January 2000. The datum of charts made prior to this date refer to Mean High Water Spring.