Question 373Navigation General

The correct answer is D) distance from the water to low steel of the bridge. Clearance gauges at bridges indicate the vertical clearance, or the distance from the water surface to the lowest point of the bridge structure. This allows vessels to determine if they have enough overhead clearance to safely pass under the bridge. The other answer choices do not accurately reflect the purpose of clearance gauges. Option A refers to water depth, which is not what clearance gauges measure. Option B refers to the charted vertical clearance, not the actual clearance shown on the gauge. Option C refers to the tide height, which may affect the clearance but is not directly indicated by the gauge itself.