Secondary ports puzzle

[glynd]

Almost posted this in East-Coast, but thought it would appreciate a wider audience.

Just sat down exercising the grey cells a little, and trying to remember how do do these things, but crosschecking I came across a bit of a puzzle.

Looking at the calculations for Chatham Lock & Approaches has high and low water lagging behind the Standard Port of Sheerness by between 10-18 minutes depending on the situation.
This is out of Reeds for 2021

Looking at Wednesday 5th May, the BBC / UKHO shows a 33 minute difference between the Standard port and Secondary at low-tide.
This was just a quick spot, and I haven't dug any deeper - it does make me wonder how accurate I can expect a secondary port calc to be - and indeed any tidal predication at all.

Low tide at Sheerness 14:20
Low tide at Chatham L 14:53.

Sheerness Wed
Tides for Sheerness, South East England - BBC Weather

Chatam L
Tides for Chatham (Lock Approaches), South East England - BBC Weather

Reeds agrees with UKHO for the Sheerness predictions, but I can't see why the Chatham one is so much further out?

As a final note, Chatham MDL marina has the LW at 14:10 BTW, so their prediction for Chatham is 40 mins out from the BBC / UKHO.

 

[TernVI]

What is the shape of the tidal curve?
You may be quibbling over a very small amount of water.
It can be hard to tell exactl when LW is, you can see when it's gone up or down a couple of feet when you're an hour away in either direction, but sometimes it doesn't move much in 20 minutes.

No data for Chatham, but the tide gauge network is interesting and shows when the sea is not following the predictions:
Real-time/near real-time data display for Sheerness | National Tidal and Sea Level Facility.
Not too far out today, despite the weather!

I think many peope expect far too much accuracy from tidal predictions.
A lot of people also don't understand how different the sea level change can be in just a couple of miles, particularly in rivers and harbours.
For instance the predictions for Portsmouth are for QHM's tide gauge, in the dockyard. They don't give the right answer outside the harbour entrance or up at Port Solent.

 

[glynd]

Nothing fancy on the tidal curve, and there does seem little movement around then.

I take it as a prediction. and reality will obv be affected by wind in the north sea / surges / baro-pressure

My wondering is really why the predictions are differ, rather than why reality is different to the prediction!

 

[Mark-1]

Given that Reeds uses the *time* of HW as an approximation of tidal *range* I would trust pretty much any other source of secondary port tidal data over Reeds.

Clever and useful in 1921. The opposite in 2021.

 

[Concerto]

You may prefer to use the Peel Ports tide tables to do more checks.
https://www.peelports.com/media/6069/official-tide-tables-for-chatham-2021.pdf
https://www.peelports.com/media/6070/official-tide-tables-for-sheerness-2021.pdf

 

[glynd]

You may prefer to use the Peel Ports tide tables to do more checks.
https://www.peelports.com/media/6069/official-tide-tables-for-chatham-2021.pdf
https://www.peelports.com/media/6070/official-tide-tables-for-sheerness-2021.pdf

Quick compare shows Sheerness generally matches within 5mins of UKHO
Chatham is close on HW - 5 mins or so, generally but LW also has a 40min disparity at neeps (5th)