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sounding datum The datum used on nautical charts to correlated the values of the soundings with the tide level. The charted soundings are the depth of the water when the tide is zero. At higher tide we have more water (sounding + tide) and at negative tides we have less water (sounding - tide). The actual average tide level used to define zero (the sounding datum) does not really matter for anything we do. For US charts, this happens to be MLLW. Canada uses a slightly lower datum (lowest normal tides), which results in fewer negative tides, but likewise has no effect on navigation or chart reading. It is valuable to know that when the tide is zero, the water meets the land where the blue meets the green on the chart. At higher tide there will be less green than plotted, and at negative tide there will be more green, ie more dry foreshore. Unlike the other datums we use in chart navigation, this is the one we do not need to know. See chart datums, especially the vertical datum, as that tells us the height of the tide when the water meets the tan of the land.

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