Module 11 - Tides & Tidal Streams
Tidal Diamonds and Stream Atlases
What you will learn
Select the correct atlas hour from the named reference-port high water, read set and rate, and interpolate spring/neap rate only by the prescribed method.
Diamonds and atlases describe horizontal water movement. Their clock is high water at a named reference port, not necessarily high water at the vessel's location.
A charted tidal diamond identifies a position whose table gives set and rate by hour relative to HW at a named reference port. Set is the direction towards which the water flows in degrees true; rate is speed in knots. Mean spring and neap rates are listed separately.
A tidal-stream atlas shows a wider area's flow on separate chartlets before and after reference-port HW. ADMIRALTY atlases provide direction and mean spring/neap rates, commonly in tenths of a knot. The stream need not turn at local HW or LW.
When the exercise or source instructs interpolation, compare the day's range with the stated mean neap and spring ranges and interpolate the rate. Divide a long leg across relevant hourly pages or use the prescribed averaging method instead of applying one vector indefinitely.
Worked example
Mean neap range is 2.6 m with 1.2 kn rate; mean spring range is 5.2 m with 2.4 kn rate. Today's range is 3.9 m and the exercise specifies linear interpolation.
- 1Range fraction = (3.9 - 2.6) ÷ (5.2 - 2.6) = 1.3 ÷ 2.6 = 0.50.
- 2Rate increase = 0.50 × (2.4 - 1.2) = 0.60 kn.
- 3Interpolated rate = 1.2 + 0.60 = 1.8 kn.
Sense check: Today's range is halfway between the stated neap and spring ranges, so the interpolated rate should lie halfway between 1.2 and 2.4 kn.
Build a time-labelled stream vector
Reference-port HW is 1430 and the leg begins at 1230 on a page labelled HW -2.
1. Align the clock
1230 is two hours before the named reference-port HW, so HW -2 is the starting page.
2. Read set and rate
Take the local arrow direction and the appropriate mean or interpolated rate.
3. Review the leg
Split or update the vector when the passage crosses an hourly boundary or area.
Sense check: A set of 090° flows towards east; plotting it west reverses the set convention.
Common mistake or limitation
- Using local harbour HW instead of the named reference-port HW selects the wrong atlas page.
- A spring/neap interpolation is a model specified by the source, not a guarantee of the exact stream encountered.
Recap
- Read reference port and hour before the arrow.
- Plot set towards and rate in knots.
- Update the stream evidence across the duration of the leg.
Optional quick check
Section 9 of 11
A stream set is 090°T. In which direction does the water flow?
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.