Definition
Charted depths are soundings reduced to chart datum, the level below which the tide seldom falls, taken on modern British and international charts as Lowest Astronomical Tide. Actual depth equals charted depth plus height of tide, so a 3.2 m sounding with 1.8 m of tide gives 5.0 m. Drying heights above chart datum are shown underlined. Low barometric pressure and strong offshore winds can depress the real level below prediction, so a margin should always be retained above the calculated figure.
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