Tidal Differences

[Portland Billy]

I recently bought 'World Tides HD' for i-pad - the lay-out is easy to read and gives a real time display of the chosen place. Seemed to be the dog's proverbial.
BUT I've been planning a run down to St Kats for the weekend of 24th/25th/26th August and compared with the PLA tide tables the i-pad timings for Richmond and Tower Pier are way out ! (Yes - I have allowed for BST !).
Has anyone any thoughts/experience of this ?

I think I'll stick with my old fashioned way of doing things - Lock through Richmond (about 0630 on 25th) as soon as there's enough water, and then push the tide for the first hour and a half, and hope there aren't too many rowers to slow me down. Last lock at St Kats is 0920, so we should have time in hand.
I'll have the tribe plus grandsons for their first trip into the 'big river' so don't want too many hiccoughs.

 

[Captain Coochie]

How far out ? You need to add time on the PLA tide table for Richmond .

 

[Portland Billy]

Richmond

??
The PLA table for Richmond is GMT, as usual - so add an hour.
eg PLA low water 28th July AM is 0341 GMT (0441 BST)- PLA tables.
I-pad gives 0542 (BST).
There are descrepancies all over the place. Heights are also at variance.
Something wrong somewhere.

 

[TrueBlue]

??
The PLA table for Richmond is GMT, as usual - so add an hour.
eg PLA low water 28th July AM is 0341 GMT (0441 BST)- PLA tables.
I-pad gives 0542 (BST).
There are descrepancies all over the place. Heights are also at variance.
Something wrong somewhere.

Doesn't some adjustment have ot be made for restricted water flow up the Tideway?

 

[John Barry]

The PLA tide tables are created from the PLA's own harmonics. Other prediction sources will use other harmonics, or corrections from standard ports (which may also use 'someone else's' harmonics).

Using the UKHO harmonics for Richmond Lock (Port 116, 51°28'N, 0°19'W) and SHM (about the top 30 harmonics, data from ATT2011) I get 28/7/12 as (times UTC):
UKHO 0435 -0.05 0930 +3.66 (I only calculate to the nearest 5 mins)
PLA 0341 -0.14 0936 +3.87

Using a different harmonic set will always give different results - as will using a different number of harmonic. Unless you know the source of the harmonics used on the iPad, the number of harmonics used, how up to date they are, and what calculation method was used (I doubt you'll be able to find any of these out) you can only be comforted by the fact that the water has its own set of rules, that doesn't match the iPad's numbers, the PLA's or the UKHO's or anyone else's.

The ATT comments that the upper reaches of the tidal Thames are markedly affected by river water, and that UKHO predictions are based on a flow rate at Teddington of 18 cumecs. Higher flow rates normally give more water.

Hope this helps.