What is Chart Datum?
Chart Datum is the plane below which all depths are published on a navigational chart. It is also the plane to which all tidal heights are referred, so by adding the tidal height to the charted depth, the true depth of water is determined. By international agreement Chart Datum is defined as a level so low that the tide will not frequently fall below it. In the United Kingdom, this level is normally approximately the level of Lowest Astronomical Tide. The highest and lowest predicted tides that can occur are deemed Highest Astronomical Tide (HAT) and Lowest Astronomical Tide (LAT) respectively. These levels are the highest and lowest levels which can be predicted to occur under average meteorological conditions, and under any combination of astronomical conditions; these levels will not be reached every year.
