Tide Table Differences

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tarwyn

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Recently I bought a tide plotter software package so that I could do my first-cut passage planning in the winter months for the coming season. The software package I bought uses UK Hydrographic Office data and the simplified harmonic method of prediction NP159 is used to make calculations.

Previously, I've used Reed's Almanac for advance planning and Easytide 7 days before casting off and these sources have always been reliable and hardly have any differences in their tidal data.

I was prompted to check the data in tidal software with Easytide and the tide tables provided by YBW.
For today for Portsmouth, the 3 sources give for the first HW & LW:

YBW: HW 0421 4.4m, LW 0929 1.3m

Easytide: HW 0402 4.4m, LW 0935 1.3m

Software package: HW 0430 4.3m, LW 0951 1.5m

As you can see, there is hardly any difference between the predicted heights of tide but what does worry me is the difference in the predicted times. 20 minutes difference is not a huge worry for a standard port like Portsmouth but doing the same exercise for a secondary port such as Chichester Harbour gives a difference of nearly an hour. This does cause problems for deciding which is the best time to arrive at Chichester Marina which has shallow approaches and a sill.

Does anyone know why there is such a big difference in the times given that the source data is supposedly the same i.e. from UKHO?

Thanks, Tarwyn
 
Are you sure about the original source? For instance the Dover tide tables in the RNLI diary are from the Proudman Oceanographic Laboratory, not UKHO.
 
Easytide and the software package definitely use UKHO data. Don't know what the source data is for the YBW tide tables in the weather and tides section. Did look but couldn't see a source other than Met Office - which I presume just pertains to the weather info.
 
I had the same problem with Belfield Tide Plotter software. This is the response I got when I e-mailed the software authors:
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Accuracy of tides

Tide Plotter uses a method of computation to predict the tide which has been derived by the Admiralty and can be found in the front of the Admiralty Tide Tables, the simplified harmonic method. The harmonic constants, angles and factors data used also is derived, under license, from the Admiralty Tide Tables.

There can be variations in the predicted data from different sources, even when the same method of prediction is being used. For example Macmillan’s and the Admiralty Tide Tables can show differing times for some ports. This can be caused by interpretation of the harmonic curves and the fact that additional harmonic data can be used for the production of standard port publications.

Tide Plotter uses the simplified harmonic method to calculate the tide at one-minute intervals and then determines the (usually) two high points and two low points during a 24-hour period to give HW and LW. On occasions there can be stands, periods when the tide is a the same height and this is the first point at which there can be variations on when exactly is HW, is it the first point at which it reaches the highest point, the average, median, etc., and then there can be double tides etc. Determining one definitive point is not always straightforward. The graph is a far more useful tool for interpreting the tides.

The method Tide Plotter uses, in common with other computational methods of calculation uses the simplified harmonic method to calculate both standard and secondary ports, this is not normally used in practice because of the complexity of the calculations, (possibly university level mathematics) and therefore the method of adding or subtracting time from the standard port is used. The simplified harmonic method will give far more detail than the method of adding and subtracting time.

The harmonic data used in the calculations comes from the UK Hydrographic office who in turn obtains the data from either the Admiralty or other national hydrographic departments.

There can be variations and it is most important to remember whatever the source it is only a prediction.

Meteorological conditions will causes differences in tide. In the main barometric pressure and prolonged wind will cause differences in height. Differences in time are mainly caused by wind. Again there is more detailed information available in the Admiralty Tide Tables.

--------------------------------------------------------------------

Not wholly clear but I think it means they use a simplified formula which will produce 'less accurate' (my words) results. I suppose that in practice, unless you calculate to the nearest inch and don't mind scraping your keel when a wave goes by, the differences are not too great.

Jonathan
 
Lets put this into perspective. Nothing is going to give you accurate tidal heights wherever you go, as no tide prediction software I know takes into account barometer readings and tidal surge heights?

What you get is a 'prediction' not a guaranteed height.

Predictions are purely that, and they come by various sources.

Treat them as predictions, and you won't go far wrong.

Treat them as gospel, and you will have problems.

Most people accept that there is a % discrepancy in such data, and just make allowance for it
 
Predictions are of course only predictions as said above and lots of thing can affect what you actually get on the day.

However, if you look closely at todays predictions for Portsmouth from EasyTide, there seems to be an inconsistancy between the tabulated figures and the curve.

HW = 04:52 but the curve seems to have its peak centered around 05:20 ish. It's certainly later than 05:00. The next LW it better but seems a little late on the curve. The peak of the next HW is just before 18:00 while the tabulation shows 17:16. This is obviously loads out.

I overlayed the curve on a curve predicted by Belfield. Interestingly the curves match quite well.

Let's see what a mail to the UKHO will provide.
 
To add to the confusion XT Tides Europe gives two tables for Portsmouth (there are three tables for some ports, London Bridge for example).
One gives HW 04:36, 4.51m and LW 09:53, 1.50m
the other HW 04:39, 4.33m and LW 09:24, 1.32m

As Brendan says they are only predictions not guaranteed heights. The more tables you consult the more different figures you will find. Combined with the effects of wind and barometric pressure these are the reasons you must allow a good margin for error. If you have several sets of figures available take the "worst" figures for any particular situation and still allow a safety margin.
 
Agree.

I had this proved when we moored up in Wootton Creek. Reeds and C map chartplotter gave -1hr and -1.25 HW respectively but as we seemed to stop swaying I looked at water going past boat. It was going out rather rapidly.

Decided that water had not read the almanac properly as it was definately going OUT. Quickly ploughed our way out through the silt. Checked the figures again and we were right and the water wrong!

Since then always assume they are predictions and allow +/- 1hr. Its not until it happens to you that you fully understand the meaning of the word predictions!!
 
Re: Vote of confidence in Tarwyn

Thanks Twister Ken...and yes, you have guessed correctly: I did have a hand in organising the ESC Christmas lunch.

And thanks to all who have replied on the topic of tidal predictions. If I'd just stuck with Reed's Almanac, and not been so nosy, I would have accepted times of tide - plus or minus 30 minutes and taking the weather and sea state into account.

I like the comment about the water not reading the Almanac in Wootton Creek /forums/images/graemlins/smile.gif Will watch out with particular interest next time I'm there.

So the moral seems to be: don't sail up small creeks in a 2.0m draught fin-keel yacht anywhere near low water especially when the barometer is pushing 1030 and wind speed is 21 knots plus.

cheers, Tarwyn
 
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