The table says 1.0 metre at 16:10. Your boat draws 1.4. Something has to give, and usually it's your understanding of the number rather than the seabed. Almost every confusing figure in a UK tide table comes back to one question: a height above what? Answer that and the whole table turns into a straightforward tool.
Chart datum is a fixed reference level, and it is deliberately set low. For most Admiralty charts covering UK waters it is Lowest Astronomical Tide — the lowest level the sea is predicted to reach under average weather conditions over a long run of years. It is not the seabed, not mean sea level, and not something you can see. It's a line drawn beneath the water and labelled zero.
Everything on the chart hangs off that zero. Soundings — the numbers scattered across the blue — are depths below chart datum. Drying heights, usually printed with a small line underneath, are heights above it for features that uncover at low water. Tide table predictions are quoted against the same baseline, which is why those three sets of figures can be added and subtracted without any conversion.
Because chart datum sits near the very bottom of the range, the real sea is usually deeper than the chart suggests. That is the point. A chart is a promise about the minimum generally available, not a measurement of what is under you at this moment.
When a table tells you high water is 3.8 m, it means the surface will be 3.8 m above chart datum at that time. It does not mean there will be 3.8 m of water at your berth.
To get the depth of water, add the charted depth to the height of tide. If the approach channel is sounded at 2.5 m and the tide is 1.0 m, you have roughly 3.5 m. Take off a draught of 1.5 m and about two metres remains beneath the keel. Run the same sum at low water with a predicted 0.2 m and you are down to 2.7 m — still fine, but half a metre of margin thinner than you might have assumed.
Some tables print minus signs. A predicted height of −0.3 m is not a printing error: it means the surface is expected to sit 30 cm below chart datum. It happens, and under a spring low with high pressure overhead it can happen by more than the table suggests.
The phrase is loose, and loose phrasing costs people money. A one-metre tide might mean the water surface is one metre above chart datum at that moment. Or it might mean the tide rises one metre between low and high — a range, not a level. Two very different things, and the gap between them can be the whole of your clearance.
There is a second wrinkle. Chart datum is set per port or chart area, not globally. Lowest Astronomical Tide at one harbour is not the same physical level as Lowest Astronomical Tide twenty miles along the coast, because the range and the seabed differ. A reading of 1.0 m in one place and 1.0 m in another do not describe the same depth of water unless the charted depths match. Compare total depths, not tide heights.
Tide tables predict astronomical tides. They assume average weather, and average weather is not what you get. Air pressure and wind move water around, and both can shift the real level away from the printed figure.
A rough guide for pressure: about 1 cm of sea level for each hectopascal. High pressure pushes the surface down; low pressure lets it rise. Sustained onshore winds can pile water against a coast, and in places such as the southern North Sea or the Bristol Channel a surge can add considerably more than the barometer alone would suggest. Offshore winds do the opposite and drain a harbour.
So the same table entry can turn into a level noticeably lower than predicted on a calm, bright, high-pressure day, or noticeably higher when a deep low tracks across. Departures tend to be largest in winter. Either way, "above predictions" and "below predictions" are not errors — they are the weather talking.
Chart datum is not the only zero in circulation, and this is where a lot of quiet confusion lives.
Ordnance Datum Newlyn is the land-mapping baseline for Great Britain. Chart datum sits below it, but the gap varies from place to place, so a height above Ordnance Datum and a height above chart datum are not interchangeable.
Local port datums still appear in some harbour authority publications and older sailing directions. MHWS and similar levels turn up in bridge clearances, overhead cable heights and some pilotage notes — a clearance quoted above mean high water springs is not a depth above chart datum.
Units catch people out too. Metres, feet and
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