How to use a tidal curve
A standard port’s tidal curves show how the tide rises and falls around high water, more accurately than the rule of twelfths. There’s usually a curve for spring tides and one for neap tides, and on most days the tide is somewhere between the two.
The curve gives you a factor: 0 at low water, 1 at high water. Multiply it by today’s range, add the low water height, and you have the height of tide.
What you need
- HW time and height, and LW height, at the standard port
- The port’s mean spring and neap ranges
- The tidal curves
The method, step by step
- Work out today’s range: HW height − LW height.
- Compare it with the spring and neap ranges: how far is today from neaps towards springs? (today − neap range) ÷ (spring range − neap range).
- Work out your time relative to HW. Before HW is minus.
- Read the spring curve and the neap curve at that time.
- Interpolate: factor = neap reading + (spring reading − neap reading) × the fraction from step 2.
- Height of tide = LW height + factor × range.
If today’s range is bigger than the spring range, use the spring curve; if it’s smaller than the neap range, use the neap curve.
Make sure you read the correct side of HW: the rising (flood) side before HW, the falling (ebb) side after.
Many almanacs have you draw a line between the HW and LW heights across the curve’s height scale instead of using a factor. It’s the same calculation done graphically.
Worked example
Question 222-223U · fictional practice data
| Port | Kestrel Haven (standard port) |
|---|---|
| High water | 0958 4.8 m |
| Low water | 1.4 m |
| Springs / neaps | 4.7 m / 2.2 m |
| Find height at | 0648 |
Using the Kestrel Haven tidal curves, find the height of tide at 0648.
- Today's range 4.8 − 1.4 = 3.4 m
- Springs or neaps? (3.4 − 2.2) ÷ (4.7 − 2.2) = 0.48The fraction of the way from the neap curve to the spring curve.
- Time from HW 0648 − 0958 = −0310 (HW−3h 10m)
- Read the curves springs 0.37, neaps 0.46
- Factor 0.46 + (0.37 − 0.46) × 0.48 = 0.41
- Height of tide 1.4 + 0.41 × 3.4 = 2.8 mLW + factor × range, rounded to 0.1 m.
Answer: Today's range 3.4 m · 0648 relative to HW −0310 · Factor from the curves 0.41 · Height of tide at 0648 2.8 m
Common mistakes
These are the slips TideTutor catches in this exact question, in the words it uses to point you back on track:
- That's the HW height. The range is the difference between HW and LW.
- The range is HW minus LW, not HW plus LW.
- Is 0648 before or after HW at 0958? Before HW is minus.
- That's the spring curve. Today's range is between springs and neaps, so interpolate between the two curves.
- Check which side of HW you read. 0648 is before high water.
- That's the height above low water. What do you add to it?
Questions people ask
What is the factor on a tidal curve?
How far up the range the tide is: 0 at low water, 1 at high water. Height = LW + factor × range.
Why are there two curves?
The shape of the tide changes between springs and neaps. When today’s range is in between, you interpolate between the two curves.
Should I use the rule of twelfths or the tidal curve?
The curve, when you have one. The rule of twelfths is a quick estimate that assumes a regular six-hour tide.
Practise until it’s easy
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All ports, charts and tidal data in these guides are fictional, for practice only. Never use them for navigation.