Vertical clearances of bridges and overhead cables are measured in feet above what reference point?

Vertical clearances of bridges and overhead cables are measured in feet above the Mean High Water mark. This reference point indicates the average height of the water at high tide over a specified period, which is crucial for ensuring that boats and vessels can safely navigate under bridges and cables without the risk of collision.

Measuring vertical clearances from Mean High Water provides a standardized reference that accounts for tidal changes, which is essential for safe navigation. It ensures that the clearance is adequate even during higher water conditions, thereby protecting vessels from potential damage or accidents.

Using Mean Low Water or Low Water Datum would not adequately account for rising tides, while the National Geodetic Vertical Datum relates more to land elevation rather than navigational clearances in a tidal context. Therefore, Mean High Water serves as the most reliable and relevant benchmark for measuring vertical clearances in maritime settings.