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. Chart Datum is shown on charts as the zero metre contour
Most authorities use a datum called chart datum (CD), which to all intents and purposes is the lowest astronomical tide (LAT) for that length of coastline or that particular port.For navigation purposes any water depth has to be referenced to a common datum for it to have any meaning.
However, MLLW may be used as the tidal datum for tidal curves in the United States of America and is not as low as lowest astronomical tide.
The reason for using LAT is that there will always be at least the depth shown by the soundings, even at low water. Any higher datum will inevitably mean that sometimes there will be less water than the charted depth.
- There is a common misunderstanding that chart datum is constant for the whole of a chart, but this is not so.
- Coastal effects could cause the lowest astronomical tide to be significantly different at two places close together on the same chart.