The vertical reference for depths and drying heights on a nautical chart. It is set low enough that the water seldom falls far below it, so a mariner adds the predicted tide to the charted depth and any error falls on the safe side. Many countries use Lowest Astronomical Tide; the United States uses Mean Lower Low Water. Survey depths reach chart datum through tide observations or an ellipsoid separation model.
Depths are reduced to chart datum, so the 10.2 in the channel is exactly what the pilot sees on the ENC.
Chart datum is a local tidal level that slopes and steps along the coast, not one flat surface, so a long survey crosses datum zones that each carry their own reduction.
Also heard as
- CD
- sounding datum
Related terms
Tide correction
The adjustment that removes the height of the tide from measured depths so they refer to chart datum.
ERS
Ellipsoidally referenced survey: depths taken from GNSS ellipsoid heights and converted to chart datum with a separation model.
Separation model
A grid giving the height difference between the GNSS ellipsoid and chart datum across an area.
Drying height
Height above chart datum of a rock, bank or flat that dries at low tide, shown on charts as an underlined figure.