Navigation General · Tides
The chart datum used on NOAA nautical charts for depth soundings is:
- AMean High Water (MHW)
- BMean Sea Level (MSL)
- CMean Lower Low Water (MLLW)✓ Correct
- DLowest Astronomical Tide (LAT)
Explanation
NOAA uses Mean Lower Low Water (MLLW) as the chart datum for soundings in US waters. Depths on the chart are referenced to MLLW — the average height of the lower of the two daily low tides. This means actual depths at high tide are greater than charted, providing a safety margin.
Authority: NOAA Tide Tables; Chart No. 1
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