A chart attribute, a sensor observation and your remaining margin are three different things.
Name the number before using it
A bridge search may return a vertical-clearance attribute from a chart-derived GIS feature. An air-gap station returns a measurement at a defined bridge reference point. Your planning margin subtracts the boat height and buffer from that measurement. Label each of these distinctly in your notes. Neither an attractive card nor a decimal place makes one type of information interchangeable with another.
What a direct observation contributes
NOAA describes air gap as the distance between a defined point beneath a bridge and the water surface below it. That definition matters: the reference point is not a promise about every track beneath the span. BoatWaypoint keeps the station ID, observation time and preliminary-data context alongside the value. Use the official station page to understand the particular measurement before treating it as relevant to your intended passage.
Worked example: the three inputs
Assume an illustrative measured gap of 30 feet, a recorded boat air draft of 24 feet and a chosen buffer of 3 feet. The remaining planning margin is 30 − 24 − 3 = 3 feet. The boat-to-reference difference before reserving the buffer is 6 feet. Calling both values “clearance” would conceal the reserved allowance. A positive 3-foot result is arithmetic for this example, not an instruction to pass.
Use chart discovery for identification
Our ENC finder helps locate candidate bridge features around a waypoint and shows available chart attributes. NOAA states that ENC Direct to GIS is a converted product that is not certified for navigation. Follow the original chart reference and current notices; verify the crossing, span, relevant track and any operational restrictions. An absent attribute is unknown information, not a zero-height bridge and not an unlimited clearance.
When the answers appear to disagree
First compare the identity, location, reference level, units and timestamps. Two sensors on one crossing may represent different designated points, and a chart reference need not describe the same instant as a water-level observation. Do not choose whichever number produces the larger margin. Keep the discrepancy unresolved until an authoritative source explains it. In the checklist, separate “bridge identified” from “clearance reference verified”; completing the former does not complete the latter.
Check a NOAA air-gap observation
Keep the inputs, source references and unresolved dependencies together.
Check a NOAA air-gap observation