Chart Navigation & Plotting · Chart Basics — Soundings and Datum
Charted soundings on a U.S. NOAA chart are referenced to which datum?
- AThe deepest observed tide
- BMean sea level
- CMean high water
- DMean lower low water (the sounding datum), giving a conservative least-depth value✓ Correct
Explanation
U.S. charts reduce soundings to mean lower low water (MLLW), so charted depths are typically the least you can expect; actual depth is usually greater by the height of tide. Vertical clearances, by contrast, are referenced to mean high water for a conservative overhead figure.
Authority: NOAA charts; Bowditch (Pub. No. 9), chart datums
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