Tide Times

[Thistle]

I've just checked the tide details for this afternoon (21 Jan 2022) at Rosyth (a primary port for tide purposes)
Easy Tide (Admiralty) HW 5.4m 1709
Forth Ports (who run Rosyth) HW 5.2m 1731
Belfield Tide Plotter (commercial) HW 5.6m 1651

I find the 40 minute and 0.4m difference in predictions both surprising and concerning and note that that's before taking any account of prevailing wind and atmospheric pressure let alone any offset for secondary ports. I guess the safe thing to do is to look at whichever happens to be the worst case scenario for whatever you are trying to do but it does seem to add another - unwelcome - complexity to tidal calculations.

How would you go about resolving such inconsistencies?

Update: the actual figures, as reported on the Forth Ports website, seem to agree pretty much with Easy Tide but with a surge of -0.12m which can probably be explained by the high atmospheric pressure.

 

[Aeolus]

I would recognize that tidal predictions are an imprecise science because of the many factors that can influence both the timing and the height. Different systems use different calculations resulting in different predictions. As you point out, wind and air pressure are not taken into account and can have a significant impact. The key conclusion is that all of the predictions are only indicative and one must expect reality to be different.

 

[Plum]

I've just checked the tide details for this afternoon (21 Jan 2022) at Rosyth (a primary port for tide purposes)
Easy Tide (Admiralty) HW 5.4m 1709
Forth Ports (who run Rosyth) HW 5.2m 1731
Belfield Tide Plotter (commercial) HW 5.6m 1651

I find the 40 minute and 0.4m difference in predictions both surprising and concerning and note that that's before taking any account of prevailing wind and atmospheric pressure let alone any offset for secondary ports. I guess the safe thing to do is to look at whichever happens to be the worst case scenario for whatever you are trying to do but it does seem to add another - unwelcome - complexity to tidal calculations.

How would you go about resolving such inconsistencies?

I have been using the Absolute Tides app for many years and it nearly always agrees with actual observations. That also shows the same as Easy Tide at: 17:09 5.4m (probably the same source). I never try the resolve differences and stick with the one I am comfortable with.

Www.solocoastalsailing.co.uk

 

[Pye_End]

I have been using the Absolute Tides app for many years and it nearly always agrees with actual observations. That also shows the same as Easy Tide at: 17:09 5.4m (probably the same source). I never try the resolve differences and stick with the one I am comfortable with.

Www.solocoastalsailing.co.uk

Same here. There are some very strange numbers around when it comes to tides.

 

[LadyInBed]

These should be correct
Tides for Rosyth, Scotland - BBC Weather

 

[Skylark]

Worth pursuing to see if you can resolve ?

Do the three sources list lat and long for their”Rosyth” location?

Also, perhaps the three sources use a different barometric and/or other monthly/seasonal variables?

 

[bikedaft]

The spread of times seems fair. Tide tables are not that exact, I don't think they can be, with other factors as you have mentioned.

(From comparing tide tables to what actually happens at tidal gates on the W coast for a long time ?)

 

[Aeolus]

This explains how tidal predictions are calculated. Note that the 37 different harmonic constants used to calculate the tides are derived from a series of actual measurements. If models use different sets of measurements (e.g. taken at different times or over different periods), then they will produce different predictions.

About Harmonic Constituents - NOAA Tides & Currents

 

[AntarcticPilot]

At Rosyth, tides will be influenced by the river flow; further, the bridge works immediately downstream will have influenced tidal constants. So, it will depend very much on what periods the tidal constants were calculated over. This could be a case where taking the longest possible time series is not the best option!

 

[Thistle]

At Rosyth, tides will be influenced by the river flow; further, the bridge works immediately downstream will have influenced tidal constants. So, it will depend very much on what periods the tidal constants were calculated over. This could be a case where taking the longest possible time series is not the best option!

Many thanks: that looks likely. I'd have thought that unusual river flow would be in the same bracket as prevailing wind and atmospheric pressure, ie for the application of intelligent guesswork after the various harmonics had been applied to produce a prediction. I guess some of the predictions I looked at could use harmonics derived before the latest bridge was added but I remain disappointed and a little concerned that there is such a wide discrepancy between the predictions. With the tidal curve being fairly flat at HW and LW, I can probably live with the difference in timing but in the Port Edgar mud at the other side of the river a difference of 0.4m in tidal height could have unwelcome consequences.

 

[bikedaft]

I just don't think tide tables can be that precise, I allow an hour either side, it's probably a bit less than that. Celestial bodies can be predicted, but neither rainfall, wind direction, barometric pressure can be, much in advance anyway. Does swell affect tide too?

 

[AntarcticPilot]

Many thanks: that looks likely. I'd have thought that unusual river flow would be in the same bracket as prevailing wind and atmospheric pressure, ie for the application of intelligent guesswork after the various harmonics had been applied to produce a prediction. I guess some of the predictions I looked at could use harmonics derived before the latest bridge was added but I remain disappointed and a little concerned that there is such a wide discrepancy between the predictions. With the tidal curve being fairly flat at HW and LW, I can probably live with the difference in timing but in the Port Edgar mud at the other side of the river a difference of 0.4m in tidal height could have unwelcome consequences.

If the tidal curve is flat at HW and LW, the other thing is that quite minor shifts in the tidal coefficients could alter the time of nominal HW and LW quite significantly. People have described similar problems in the Solent, where there is a double HW.

As others have noted, there are many confounding factors. We can predict the astronomical contribution to tides very well, and fixed things like basin resonances - as long as they remain fixed. But non-astronomical contributions, such as meteorological contributions and varying river flow mess things up, and of course in the upstream parts of an estuary, the basin geometry can change - especially when they build whacking great bridges!

 

[[70521]]

How would you go about resolving such inconsistencies?

Taking the boat out and see what the depth sounder said.

It's a prediction. A bit like the weather. Not an exact science. As long as the time is within 15 minutes and depth within 0.50 metre I am really not that fussed as I plan to go places with enough water to float or take appropriate action when drying out.

 

[Ningaloo]

The tidal predictions are theoretical and cannot take local weather effects into account.
Having written software to calculate tidal curves I would say the the prediction IS very scientific but, as already pointed out, the calculation depends on a number of angles and factors to predict the gravitational forces plus a number of harmonic constants for each port. In addition there are shallow water and other effects.
I am surprised that there is much discrepancy as anyone predicting tides for the UK has to pay UKHO for the privilege and I would have thought they would all use the base data provided by them. That said, there is more than one way to calculate the predicted heights, and some of the lesser harmonics may be omitted for simplicity or speed of calculation.
For all the above reasons best to be generous in allowing a little extra under your keel when anchoring or in shallow waters.

 

[Rhylsailer99]

also dont forget the atomspheric presure can change the tide height by approx 1cm for every mb

 

[LadyInBed]

also dont forget the atomspheric presure can change the tide height by approx 1cm for every mb

Tidal Height related to Pressure : Monty Mariner

 

[Rhylsailer99]

Tidal Height related to Pressure : Monty Mariner

being almost a full time bait digger i know a big pressure combined with a north wind = £££££ and a low pressure combined with a south wind is game over.

 

[NormanS]

Tidal Height related to Pressure : Monty Mariner

When I first started paying attention to these matters, atmospheric pressure was measured in inches of mercury, and sea level differences in feet. We always reckoned that one inch of mercury corresponded to one foot of water, and it is reassuring to see that although the terms have changed, the effect is still the same.

 

[Spirit (of Glenans)]

Unless you are doing something requiring exacting precision, you could just go for the middle one, or take an average. Then, as you get the opportunities, check the online info with your leadline, (you do have a leadline?) . You should also calibrate your sounder using the leadline.

 

[DipperToo]

I have been using the Absolute Tides app for many years and it nearly always agrees with actual observations. That also shows the same as Easy Tide at: 17:09 5.4m (probably the same source). I never try the resolve differences and stick with the one I am comfortable with.

Www.solocoastalsailing.co.uk

I appreciate that the results may not be 100% precise, but I am trying to get Cherbourg set up as a secondary port with Absolute Tides using Dover as the reference port. Looking at the Almanac I can see the various constants I need such as extracting the MHWS, MHWN etc plus the tide times for Dover vs Cherbourg. However, I cannot find a reasonable explanation within the help section for entering the data in Absolute Tides in creating the secondary port. It keeps giving tide times errors.
Has anyone else managed to get a secondary Port created easily? I think the trouble is that Cherbourg is a standard port and as such the Almanac does not give offsets to another port!
Just trying to do this so I can use the Reeves Fowkes tidal atlas if I do not have the Cherbourg times with me but can use Absoluter Tides as a reasonable guide.
Thanks for any assistance.