Module 11 - Tides & Tidal Streams
Tidal Definitions and Datum
What you will learn
Put soundings, drying heights, tidal heights and overhead clearances on their stated vertical references before making a depth or clearance decision.
A correct calculation can still be unsafe when its numbers use different zeros. Depths and tidal heights relate to Chart Datum; an overhead clearance uses the high-water datum named on the chart.
Chart Datum is the reference for charted soundings and predicted tidal heights. In appreciably tidal UK waters it is normally Lowest Astronomical Tide or a closely related low-water level, but the chart title block is authoritative. Add tidal height to a sounding to estimate water depth.
A drying height is measured upward from Chart Datum. Subtract it from the tidal height to estimate water over the feature. If the drying height exceeds the tide, the positive difference is exposed height, not a negative water depth.
Overhead clearances use a different high-water reference. IHO S-4 edition 4.10.0 prefers HAT or a closely equivalent datum where tides are appreciable, while permitting stated national alternatives. HAT and LAT exclude meteorological effects and are not guaranteed limits on observed water.
Worked example
A drying rock is 1.8 m above Chart Datum. The yacht draws 1.5 m and the chosen under-keel allowance is 0.5 m.
- 1Required water over the rock = 1.5 m + 0.5 m = 2.0 m.
- 2Minimum predicted height = 1.8 m drying height + 2.0 m water requirement = 3.8 m.
- 3The skipper still adds suitable allowances for pressure, wind, waves, squat and source uncertainty.
Sense check: A required tide lower than the 1.8 m drying height would leave the rock exposed, so 3.8 m has the correct direction and scale.
Label every vertical reference
The plan includes a 2.4 m sounding, a 3.1 m tidal height and a bridge clearance printed against a high-water datum.
1. Separate the decisions
Use sounding plus tide for depth; do not reuse that arithmetic for overhead clearance.
2. Read the chart note
Identify the clearance datum and any safe-clearance or power-cable qualification.
3. Add operational margins
Include draught, squat, wave response, pressure, wind and measurement uncertainty as relevant.
Sense check: If two values use different zeros, combining them without conversion cannot produce a defensible clearance.
Common mistake or limitation
- Chart Datum is not automatically identical to LAT everywhere; use the datum stated on the chart.
- HAT and LAT are astronomical reference levels, not absolute limits on the sea that may be observed.
Recap
- State the datum beside every vertical value.
- Add tide to a sounding and subtract a drying height from tide.
- Treat overhead clearance as a separate high-water-datum calculation.
Optional quick check
Section 3 of 11
A feature dries 1.8 m; required water over it is 2.0 m. What minimum predicted tidal height is needed before extra margins?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Tides & Tidal Streams
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