Tidal time differences

Daydream believer said:
On the day it might well be different. But why are they calculating it different in advance. I accept that issues might arise with Osea esp as it is up a river & a degree of adjustment have to be applied, but they should NOT apply to a major port like Dover

Still does not explain why 4 different sets of tables cannot agree on a prediction for Dover though, does it.
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Because the factors that cause tide heights, times and flows are hugely complex.

We can only approximate (albeit rather impressively) what the tide will be doing at a particular point in time and geography by developing models which more or less match the historical record of what tides actually did in past years. Different models will be better or worse at doing that at particular different times and places. Then once you've devised a model, you have to decide how much computing power are you going to apply to the model to produce the result.

Imagine you want to devise your own model of the tides. First you need a historical record that you are going to try to get your model to match. Even for somewhere like Dover, where there will be a long historical record of actual results, you first have to make adjustments to take account of any changes in the calendar, clocks, datums, exact location of measurements, changes the coastline or harbour walls, etc.. Now you have to somehow compensate for the the wildly different weather conditions that would have influenced each and every one of those measurements. To do that you will have to make important assumptions that will affect the final predictions. And that's before you've even started to try to develop a model!

The problem is not the variation in tidal predictions, which are perfectly good enough for sailing purposes, but in your unrealistic expectations of them.
 
You can download Walton data from the PLA in CSV, XLSX, XML (PDF as well but that's no use). If you use the xlsx format then apply some 'Excel' for your own Osea Tide Table. Yes, I hate Excel's use of time but it works (obviously, hence all the tables in the book!).
 
tillergirl said:
You can download Walton data from the PLA in CSV, XLSX, XML (PDF as well but that's no use). If you use the xlsx format then apply some 'Excel' for your own Osea Tide Table. Yes, I hate Excel's use of time but it works (obviously, hence all the tables in the book!).
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As i understand it, PLA do not allow down load for a third party website
 
Perhaps I will.
I have just spent an hour loading & editing the file suggested in post #9 & find - for example when comparing Walton tides with Reeds -the following differences for 01 June 2020. (This was just one at random. I have not looked much further.)
LW- 1 hour 35 mins
HW 46 mins
LW 52 mins
HW 27 mins
I think even for simple things, like laying our boats up ,- let alone nipping over sand banks in the Thames estuary, -1 hour 35 mins is pretty much useless.
 
Well..... Comparing that web site v the PLA makes different values:

LW 37 mins
HW 47 mins
LW 55 mins
HW 35 mins.

Reeds is close to the PLA! The first variation looks suspiciously like a GMT/BST issue?

The source of that website looks suspiciously like from the U.S. I would suggest you don't use that!
 
LittleSister said:
Because the factors that cause tide heights, times and flows are hugely complex.

We can only approximate (albeit rather impressively) what the tide will be doing at a particular point in time and geography by developing models which more or less match the historical record of what tides actually did in past years. Different models will be better or worse at doing that at particular different times and places. Then once you've devised a model, you have to decide how much computing power are you going to apply to the model to produce the result.

Imagine you want to devise your own model of the tides. First you need a historical record that you are going to try to get your model to match. Even for somewhere like Dover, where there will be a long historical record of actual results, you first have to make adjustments to take account of any changes in the calendar, clocks, datums, exact location of measurements, changes the coastline or harbour walls, etc.. Now you have to somehow compensate for the the wildly different weather conditions that would have influenced each and every one of those measurements. To do that you will have to make important assumptions that will affect the final predictions. And that's before you've even started to try to develop a model!

The problem is not the variation in tidal predictions, which are perfectly good enough for sailing purposes, but in your unrealistic expectations of them.
Click to expand...
Plus, in most ports, there may not actually be all that much data being gathered to work on the differences.
And in the Thames Estuary area, I suspect that movement of the banks etc may make old data invalid?

My scepticism (if that's the right word?) comes from racing in the Solent. I've seen a lot of tidal atlases and spent a lot of time going slowly towards buoys thinking 'we were here last month at 10 minutes before HW, here we are again and the tide's @@@@ different'....
 
tillergirl said:
Well..... Comparing that web site v the PLA makes different values:

LW 37 mins
HW 47 mins
LW 55 mins
HW 35 mins.

Reeds is close to the PLA! The first variation looks suspiciously like a GMT/BST issue?

The source of that website looks suspiciously like from the U.S. I would suggest you don't use that!
Click to expand...
I have deleted it. I had looked at the BST issue but that made it worse.
There are others sites that are poor & i think that navigators who download these tables off the internet or chart plotters should be aware.
" Not to be used for navigation" really does mean that:confused:
 
Walton on the Naze is the standard port for both Osea and Harwich. For planning beyond 7 days, Harwich tides are published annually by the Harwich Harbour Authority and free to download (for non-commercial purposes). Harwich tide times are very little different from Walton (HTs <+7mins, LW <-12 mins).

Adding/subtracting, as appropriate, the Harwich and Osea Island differences from Walton (as given in Reeds) suggests HT at Osea will be between +43 and +50 minutes on Harwich (depending on the time of day of the tide), while low tides will be between +19 and +60 minutes (ditto).

If you say on your site the tide at Osea is about 40 minutes after that published for Harwich, that will give a time accurate (extreme weather aside) within about 30 minutes (theoretically 20), which is plenty good enough for planning more than 7 days ahead.

Alternatively, you could write yourself a spreadsheet which would convert the published Harwich times to Osea ones, and publish that on your site (with suitable acknowledgements and disclaimers).

Link to the Harwich 2020 tide tables (despite what it says in link address!) here - https://hha.co.uk/wp-content/uploads/2019/11/TT-individual-pages-Low-Res.pdf
 
LittleSister said:
Walton on the Naze is the standard port for both Osea and Harwich. For planning beyond 7 days, Harwich tides are published annually by the Harwich Harbour Authority and free to download (for non-commercial purposes). Harwich tide times are very little different from Walton (HTs <+7mins, LW <-12 mins).

Adding/subtracting, as appropriate, the Harwich and Osea Island differences from Walton (as given in Reeds) suggests HT at Osea will be between +43 and +50 minutes on Harwich (depending on the time of day of the tide), while low tides will be between +19 and +60 minutes (ditto).

If you say on your site the tide at Osea is about 40 minutes after that published for Harwich, that will give a time accurate (extreme weather aside) within about 30 minutes (theoretically 20), which is plenty good enough for planning more than 7 days ahead.

Alternatively, you could write yourself a spreadsheet which would convert the published Harwich times to Osea ones, and publish that on your site (with suitable acknowledgements and disclaimers).

Link to the Harwich 2020 tide tables (despite what it says in link address!) here - https://hha.co.uk/wp-content/uploads/2019/11/TT-individual-pages-Low-Res.pdf
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Fair point. Thanks
Harwich Haven have already given me permission to publish certain info to the website rather than do links. However, in spite of them saying they were going to update their website Nov 2019 it is still old data. When they do I will be altering what I show. I just wanted to avoid doing it twice
 
Daydream believer said:
No good for planning beyond 7 days
Besides I prefer to use links as little as possible so to keep the browser on the site.
Hopefully they will click on other parts & consider following up on taking a mooring then possibly joining our group activities
Click to expand...
Why don't you do as I do on my site - Put the Standard port Table on your site and a Difference Table or leave the difference for the user to calculate.
As for keeping the browser on your site, set any link to open in a new Tab.
Is your Site live, how about a link :)

PS Difference Table

Harwich Difference.png
 
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Daydream believer said:
However, in spite of them saying they were going to update their website Nov 2019 it is still old data. When they do I will be altering what I show. I just wanted to avoid doing it twice
Click to expand...

I'm not sure exactly what data you're saying needs updating, but the 2020 tide tables are definitely already there, either via the link I posted (despite HHA mistakenly leaving 2019 in part of the address) or by navigating from the HHA home page.
 
LadyInBed said:
Why don't you do as I do on my site - Put the Standard port Table on your site and a Difference Table or leave the difference for the user to calculate.
As for keeping the browser on your site, set any link to open in a new Tab.
Is your Site live, how about a link :)

PS Difference Table

View attachment 84059
Click to expand...
You miss the point i made. What data to use to start with? it has to be something i can legally down load & as I have laready pointed out some of the sources vary by quite a lot. I cannot just take , say, data from imray , as it is copyrighted
 
LittleSister said:
I'm not sure exactly what data you're saying needs updating, but the 2020 tide tables are definitely already there, either via the link I posted (despite HHA mistakenly leaving 2019 in part of the address) or by navigating from the HHA home page.
Click to expand...
It is now. A short while ago it was not.( although i may have been mislead by the 2019 note) I have been watching every 14 days. I have now removed some of the old 2019 stuff & will put the new 2020 info on shortly.
 
Daydream believer said:
You miss the point i made. What data to use to start with? it has to be something i can legally down load & as I have laready pointed out some of the sources vary by quite a lot. I cannot just take , say, data from imray , as it is copyrighted
Click to expand...
:ROFLMAO: I think you had better define legal and copyright!
My Dover tides are an image of an HTML file copied from another site with superfluous data edited out, but I am flattered that you have reproduced them :love:
 
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