3-10. The charted depth is the vertical distance from the reference plane, called datum, on which soundings are based (usually, but not always, mean low water), to the ocean bottom. As discussed earlier, the actual depth of water can be less than the charted depth or below the reference plane. This number is shown by a minus (-) sign placed before the height of tide in the tide tables. The depth of water is equal to the algebraic sum of the charted depth and the height of tide.