The recent raised water levels brought on by flooding along the east coast has prompted Skipper Shick to ask a question that all of us have faced when approaching a bridge, be it a fixed or closed vertical clearance: Where is the highest spot? And, as you know, the height boards are notoriously inaccurate, especially wooden ones on older bridges. Let us hear what goes through your mind as you look up at that immovable object ahead.

I have never seen a definitive answer to the question of where exactly the height is to be read on the clearance gauges, e.g., where does the water lap at (say) exactly 63′. It appears that the exact-foot locations are at the center of the digits, so if the water cuts the bottom of the `63’ digits (assuming every foot is noted), then you’d have 63’6’³. The few gauges that have index marks put the index marks at the center of the digits. Can you confirm or correct?

Also, we should remind people that different bridges use different measurement points. Some signboards say `to center of span,’ some say `minimum clearance,’ some say `low iron.’ You have to look at each one.
Larry Shick