Tide table discrepancies

[GTom]

I was just browsing the internet for a Dover tide table, found various sources. The interesting thing is an up to 20 minutes difference amongst them,
eg. for today, 6 May 2019:

"official" UKHO Easytide:
LW HW LW HW
06:59 11:43 19:13 23:57
0.9 m 6.6 m 0.9 m 6.6 m

A Dutch site escout.nl
06:58 12:00 19:16 00:16 (next day)
0.7 m 6.8 m 0.8 m 6.9 m

Smaller differences compared to UKHO is tides4fishing and what appears to be identical to tides4fishing is https://tides.mobilegeographics.com/.

Apparently other parties use other prediction models for copyright reasons.

 

[MoodySabre]

Predictions...........how can any be "official"

Belfield
0801 1258 2019 BST
0.7 6.8 0.9

 

[LadyInBed]

Assuming that all other factors are equal, It depends on what harmonics are put into the mix. I can live with a 20 min spread given that tables are predictions, not hard facts cut in stone.

 

[Thistle]

Predictions...........how can any be "official"

Belfield
0801 1258 2019 BST
0.7 6.8 0.9

Remember that Belfield always uses local times while most others use UTC.

 

[lw395]

Assuming that all other factors are equal, It depends on what harmonics are put into the mix. I can live with a 20 min spread given that tables are predictions, not hard facts cut in stone.

Exactly.
Also if you are observing the tide real time, it's often very hard to see exactly when HW is, the rate of change can be slow around HW and LW.
Look at some typical data, it's quite hard to be sure which dot is highest.
https://www.ntslf.org/data/realtime?port=Plymouth
You could say HW was half way between when it passes through say 5m going up and going down but you might get a different time if you chose 5.05m, as the curves are never symmetrical.
I think we see times given to the nearest minute and expect that sort of accuracy.

Mostly I need to know if HW is breakfast time, 11ses, Lunchtime etc etc.

 

[GTom]

Exactly.
Also if you are observing the tide real time, it's often very hard to see exactly when HW is, the rate of change can be slow around HW and LW.
Look at some typical data, it's quite hard to be sure which dot is highest.
https://www.ntslf.org/data/realtime?port=Plymouth
You could say HW was half way between when it passes through say 5m going up and going down but you might get a different time if you chose 5.05m, as the curves are never symmetrical.
I think we see times given to the nearest minute and expect that sort of accuracy.

Mostly I need to know if HW is breakfast time, 11ses, Lunchtime etc etc.

To find HW, you need to observe more points between HW and LW, fit a curve (ideally sine, but what is ideal...), that will give you a maximum somewhere. Anyway, one minute accuracy suggested by most tables is impossible, weather has more effect on the tides, sometimes much more...

Predictions...........how can any be "official"

Valid point! More to it, they claim to be certified, whatever that means. Is the weather forecast "certified"?

 

[LadyInBed]

By the way, if you want to put the tables on your phone / tablet as images you can download them HERE

 

[GTom]

By the way, if you want to put the tables on your phone / tablet as images you can download them HERE

I use the calendar link of mobilegeographics (bottom of the page, "subscribe") with google calendar. But true, images may be more handy.

 

[bikedaft]

By the way, if you want to put the tables on your phone / tablet as images you can download them HERE

thanks

 

[wombat88]

There can be a difference of at 20 mins or more between Hydrographic Office H/W at Itchenor and Premier Marinas lock info. I'd suggest 5 minutes would be have been OK.

Whether this matters is moot...and yet...

 

[LittleSister]

Anyway, one minute accuracy suggested by most tables is impossible, weather has more effect on the tides, sometimes much more...

Even without the weather, do not assume the tide tables are 'reality': they are only predictions based on models. Impressively accurate models, mind, but real life is more complicated than the models.

(Reminds me of doing 14+ knots SOG off Brittany in our 23 footer (hull speed 5 knots), when both the plotter and paper tidal atlas said it was slack water. )

 

[AntarcticPilot]

Valid point! More to it, they claim to be certified, whatever that means. Is the weather forecast "certified"?

When I see the word "certified" I always remember the classic line from St Trinians - during a staff-room discussion of the latest calamity, one teacher says something like "You're all wrong! I'm perfectly sane - AND I have a certificate to prove it!"

The exact timing of tides is subject to many influences, and as others have noted, the rate of change at high or low water is small. If you're relying on calculations that require an accuracy of minutes, you're going to get caught out sooner or later! The tidal constants are determined from long term tide-gauge observations. However, constants derived from different sub-sets of observations will almost inevitably differ, as local events such as wind and barometric pressure will affect different sets of reading differently. Further, local dredging or other harbour works can affect things.