Matching units is not enough. Heights also need a compatible vertical reference.
A height needs a zero
“Two feet of tide” is incomplete without the reference level. NOAA publishes several tidal datums, including Mean Lower Low Water (MLLW) and Mean High Water (MHW). They describe different reference levels, not alternative spellings for the same zero. Keep the station and datum together in every note or screenshot. A unit conversion from feet to metres does not convert one datum into another.
What BoatWaypoint actually displays
The Tide Window Planner requests high/low predictions with the MLLW label and the time-zone information supplied by its data contract. It does not transform those predictions into a bridge-clearance forecast. The bridge checker instead compares a direct air-gap observation with your boat height and buffer. Keeping these workflows separate prevents an unstated datum conversion from appearing to be a measured answer.
A deliberately hypothetical reference example
Suppose two verified reference levels at one hypothetical location differ by 4 feet. A value of 1 foot above the lower reference would be 3 feet below the higher reference. The 4-foot offset is invented solely to illustrate reference arithmetic; it is not a transferable local datum. In a real calculation, the applicable location, reference levels and transformation must all be established independently before using an offset.
Read the whole station record
The station ID, reference epoch, available datums and units belong with the number. NOAA’s datum pages explain the references and provide station-specific information. Do not borrow an offset from another harbour because the names look similar, and do not assume an offshore or nearby station represents a confined waterway. Where the relevant relationship is not established, mark it unknown rather than deriving a passage estimate.
A useful cross-check before departure
Write the bridge’s reference exactly as shown in the current official material, then write the tide product’s datum on the next line. If you cannot explain how those references relate at the crossing, stop the conversion there. Continue with official local information or a relevant direct measurement where available. This is why our checklist asks about source references separately from the numerical margin: a tidy subtraction can still answer the wrong question.
Explore NOAA tide predictions
Keep the inputs, source references and unresolved dependencies together.
Explore NOAA tide predictions