Definition
Vertical clearances and the elevations of lights, hilltops and other conspicuous objects on Admiralty charts are referenced to Mean High Water Springs, whereas soundings and drying heights use chart datum. US charts commonly use mean high water instead. Because MHWS is well above chart datum, actual clearance under a bridge at a given moment equals the charted clearance plus MHWS minus the present height of tide, and only equals the charted figure at spring high water.
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