Chart Navigation & Plotting · Chart Basics
Vertical clearances of bridges and overhead cables on a U.S. chart are normally referenced to:
- AMean high water✓ Correct
- BMean lower low water
- CMean sea level
- DThe chart sounding datum
Explanation
Overhead clearances are given above mean high water, the conservative high-tide reference, so a vessel knows the least clearance to expect. Depths use a low-water datum and heights use a high-water datum — opposite ends, both conservative.
Authority: Chart No. 1; NOAA charting standards
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