Module 11 - Tides & Tidal Streams
Using Tide Tables
Tide tables list the predicted times and heights of high water (HW) and low water (LW) for standard ports, for every day of the year. They are published in the nautical almanac (or the dedicated Admiralty Tide Tables).
To find the height of tide at a given time, you need: the times and heights of the HW and LW bracketing your required time, and the shape of the tidal curve for that port. You then use the tidal curve method to interpolate.
Remember that predicted tides assume normal atmospheric conditions. Strong onshore winds or low barometric pressure can raise actual water levels above predictions, while high pressure or offshore winds can lower them. A rough rule of thumb is that a 1 mb departure from standard pressure (1013 mb) changes the water level by about 1 cm.
- Tide tables give HW and LW times and heights for standard ports
- Check whether the product shows UT/UTC or local civil time before applying BST
- Heights are measured above Chart Datum
- Use tidal curves to find height at intermediate times
- Actual levels vary with wind and barometric pressure
Watch for this: Check the time basis printed by the tide product. Add one hour during BST only when the source is in UT/UTC; do not add it again when the source already displays local civil time.
Continue studying Tides & Tidal Streams
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