USCG Exam Practice · Chart symbols and abbreviations (Chart No. 1)
A bridge clearance figure is shown on a US nautical chart. To which water level is that clearance referenced?
- AMean Lower Low Water (MLLW), the same datum used for soundings.
- BMean Sea Level (MSL), the standard vertical datum for all US charts.
- CEither Mean High Water or High Water Spring, depending on the chart.✓ correct
- DMean Low Water (MLW), to provide the most conservative clearance value.
Why C is correct
Overhead clearance heights for bridges and power lines are referenced to either Mean High Water or High Water Spring depending on the chart, not to MLLW. This is the worst-case (least clearance) water level, giving the mariner the minimum available clearance. Light List — Overhead Clearance.
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