Chart datum (CD)

Chart datum is the reference plane to which charted depths and drying heights are reduced, chosen low enough that the actual depth of water rarely falls below the charted sounding. The modern international standard is Lowest Astronomical Tide, the lowest level predictable under average meteorological conditions, so a sounding plus the height of tide gives the depth of water available at any instant. Vertical clearances under bridges and cables, by contrast, are usually given above Highest Astronomical Tide or mean high water springs, a different datum.