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Sounding Datum

All depths indicated on nautical charts are reckoned from a selected level of the water called the sounding datum (sometimes referred to as the reference plane). For most NOAA charts of the U.S. in coastal areas, the sounding datum is Mean Lower Low Water. (MLLW). In the Great Lakes, the Plane of Reference is the International Great Lakes Datum (1985).

Depths shown on charts are the least depths to be expected under average conditions. Since the chart datum is generally a computed mean or average height at some state of the tide, the depth of water at any particular moment may be less than shown on the chart. For example, if the chart datum is MLLW, the depth of water at lower low water will be less than the charted depth as often as it is greater.

This is a legend that appears on NOAA nautical charts that shows 'Soundings in Feet' at Mean Lower Low Water.  In addition, tidal information is shown in a table.  The table shows Mean Higher High Water Levels, Mean Lower Low Water Levels as well as Mean High Water and Mean Low Water ranges.
This graphic shows the relationship between various water level datums and charted features.  Mean Higher High Water, Mean High Water, Mean Sea Level, Mean Low Water, and Mean Lower Low Water are all shown.  Charted depths are referenced to MLLW.  Vertical clearances are referenced to MHHW.
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