Module 11 - Tides & Tidal Streams
Tidal Observation Buoys and Beacons
What you will learn
Use an identified authoritative tide gauge, stream station or harbour display to challenge a prediction without confusing an ordinary buoy with a live sensor.
Observed data can reveal pressure, wind, surge or river effects, but only when the station, datum, timestamp, quality and location are understood.
A tide gauge, current meter, authoritative harbour display or published observation station can provide measured height or stream evidence. An ordinary navigation buoy or beacon does not automatically carry a live tide sensor; identify the actual service and variable before relying on it.
Compare observation and prediction only after checking that they use compatible datums and time bases. Read the station location, last update, quality flag and whether the measurement is height, stream or another sea-state variable. A remote gauge may not represent a shallow bar or narrow race.
Use disagreement as a trigger to increase margin and reassess the passage. The observation supplements the passage plan; it does not remove the need for charted depth, current publications, visual/radar evidence or a safe alternative.
Challenge a shallow-bar prediction
An identified harbour gauge reads 0.35 m below the prediction, but it is ten minutes old and several miles inside the estuary.
1. Check comparability
Confirm datum, timestamp and whether the station is representative of the bar.
2. Use the adverse difference
Do not average the lower observation away; test the transit against the more conservative credible water level.
3. Reassess or wait
Seek local information and direct evidence, increase margin or use the alternative if uncertainty remains.
Sense check: A number labelled live is not useful until its zero, age and location are known.
Common mistake or limitation
- A navigation buoy or beacon is not evidence of a live tide measurement unless the stated service provides one.
- Two height values on different datums cannot be compared directly.
Recap
- Verify the station before using its number.
- Compare compatible datum, time and location.
- Let adverse disagreement reduce confidence and widen the safety margin.
Optional quick check
Section 8 of 13
What must be checked before comparing a gauge observation with a tidal prediction?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Tides & Tidal Streams
This public lesson is part of Revision Module 11. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.