Marine Glossary

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chart sounding datum

The tidal datum to which soundings and drying heights on a chart are referred. It is usually taken to correspond to a low water stage of the tide. Often shortened to CHART DATUM, especially when it is clear that reference is not being made to a horizontal datum (that affects the GPS) or HEIGHT DATUM which is the reference for charted light heights and bridge clearances. .

US West Coast charts use Mean Lower Low Water for the sounding datum, which is also the same now for the Gulf Coast and East Coast, although up until sometime in the 90s they had used Mean Low Water, because they do not have much distinction between the two lows each day. Canada uses Lowest Normal Tides, which is a slightly lower reference plane.

The concept of chart datum is only used to understand how the charts were made. There is no practical need for this information because the soundings on a chart are always relative to zero tide height, regardless of what sounding datum was used. Thus not knowing what datum is in use for the chart you have will never affect your navigation.

However, it remains crucial to know that there is a HEIGHT DATUM (typically mean high water on US charts) that is used to reference light and structure heights, and bridge clearances.

Referring to the green foreshore region on a chart (part that covers and uncovers with tide level), we see tan land on one side and blue water on the other. The depth contour where green meets blue has a value of 0.0; the depth contour where green meets tan = -MHW. It is minus because this is referring to a "depth" relative to the sounding datum.

See also chart datum and sounding datum