Module 11 - Tides & Tidal Streams
Tidal Definitions and Datum
What you will learn
Relate charted soundings, drying heights, tidal height and overhead clearance to the datum stated on the chart.
A safe depth or clearance calculation begins by putting every number on the same vertical reference. Soundings and drying heights relate to Chart Datum; overhead clearances use the high-water datum named on the chart.
Chart Datum is the zero from which charted depths and predicted tidal heights are measured. In areas with an appreciable ocean tide it is usually Lowest Astronomical Tide or a closely related low-water datum, but the chart title block is the authority. A sounding lies below Chart Datum, so add the predicted height of tide to estimate water depth.
A drying height is the height of a feature above Chart Datum. Subtract it from the height of tide to estimate water over the feature. If the drying height exceeds the predicted tide, the result is a height exposed above the water rather than a negative water depth.
Overhead clearances use a separate high-water reference. Current IHO guidance prefers Highest Astronomical Tide or a closely equivalent datum where tides are appreciable, while national alternatives are permitted and older charts may use another stated level such as MHWS. HAT and LAT are astronomical levels, not guaranteed observed limits: weather and surge can take water beyond them.
Worked example
A rock dries 1.5 m and the predicted height of tide is 3.2 m.
- 1Both values are measured upward from Chart Datum.
- 2Water over the rock = 3.2 m - 1.5 m = 1.7 m.
- 3Apply the vessel's required under-keel margin and allow for forecast uncertainty before treating the rock as passable.
Sense check: As the tide rises, water over the rock must increase; subtraction gives that sensible direction of change.
Label the datum beside every number
A plan contains a 2.4 m sounding, 3.1 m predicted tidal height and a bridge clearance printed against a high-water datum.
1. Separate depth from clearance
Use the sounding and tidal height for depth; do not reuse that arithmetic for the bridge.
2. Read the clearance note
Identify whether the charted clearance is referred to HAT, MHWS or another stated datum.
3. Add operating margins
Allow for draught, squat, wave action, pressure, wind and the reliability of the prediction as appropriate.
Sense check: If two numbers use different zeros, adding or subtracting them directly 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 exclude meteorological effects and are not absolute limits on observed sea level.
Recap
- Put every vertical measurement on its stated datum.
- Add tidal height to a sounding; subtract a drying height from tidal height.
- Overhead clearance uses a separately stated high-water reference.
Optional quick check
Section 3 of 8
A feature dries 1.5 m and the predicted height of tide is 3.2 m. What is the estimated water over it before margins?
Continue studying Tides & Tidal Streams
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