Eygthene
Well-Known Member
In the 2013 Reed's Nautical Almanac, Swanage differences are given with reference to standard port Poole Harbour, as you suggested.
Secondary port time differences.
- Thread starter laika
- Start date
In the 2013 Reed's Nautical Almanac, Swanage differences are given with reference to standard port Poole Harbour, as you suggested.
dedwards
Well-Known Member
Aha, its just my old almanac. Sounds like I need to go out and buy a new one. Thanks
laika
Well-Known Member
To follow up on this...I sent a mail to a few organisations asking why we calculate secondary port time differences with respect to time of high water at a standard port rather than point in the spring/neap cycle as we do for other tidal calculations (including those in tidal atlases and diamonds on charts). I acknowledged that time of high water would, as far as I knew (but could they advise me otherwise if I was missing something), be determined by the same underlying astronomical factors as produce other tidal variations.
One learned body replied with a lot of information on how to calculate secondary port tides, but didn't reply when I thanked them but emphasised the point I was interested in. UKHO replied on the second attempt with the info sent to alant, but including the graphics which didn't render in his post, e.g:
View attachment 31477
I thought hard about this then asked them whether:
* Secondary port time differences are tabulated with respect to time of high water at the standard port (rather than point in the spring/neap cycle) because the dataset analysed consists of times of high water at standard port and time of high water at secondary port. In other words, it's a statistical artefact
* If this was the case, have the statistics been done to see whether springs vs. neaps is as good, or a better predictor of this time difference than time of high water at the standard port. After all, that's what we use for other tidal flow calculations.
They replied that the reasons were "in line with" my assumptions about data being analysed within a dataset, and that they weren't aware of any studies investigating other methods of predicting time differences.
My conclusions are therefore (until someone proves otherwise):
* We calculate secondary port HW times against time of HW at the standard port because this is the way the statistical analysis has been done, not because it is a better predictor of the time difference than springs vs. neaps.
* Method of calculation doesn't imply any gross differences in astronomical influences between this and any other tidal calculations we do (which are based on springs/neaps).
* It's all a very broad and inaccurate brush anyway (which we knew)
* People who spend too much time with algorithms stress about different things to the rest of the population (like having to use 2 methods of calculating the same interpolation factor for a day's navigation when they might equally well use one).
http://xkcd.com/974/
One learned body replied with a lot of information on how to calculate secondary port tides, but didn't reply when I thanked them but emphasised the point I was interested in. UKHO replied on the second attempt with the info sent to alant, but including the graphics which didn't render in his post, e.g:
View attachment 31477
I thought hard about this then asked them whether:
* Secondary port time differences are tabulated with respect to time of high water at the standard port (rather than point in the spring/neap cycle) because the dataset analysed consists of times of high water at standard port and time of high water at secondary port. In other words, it's a statistical artefact
* If this was the case, have the statistics been done to see whether springs vs. neaps is as good, or a better predictor of this time difference than time of high water at the standard port. After all, that's what we use for other tidal flow calculations.
They replied that the reasons were "in line with" my assumptions about data being analysed within a dataset, and that they weren't aware of any studies investigating other methods of predicting time differences.
My conclusions are therefore (until someone proves otherwise):
* We calculate secondary port HW times against time of HW at the standard port because this is the way the statistical analysis has been done, not because it is a better predictor of the time difference than springs vs. neaps.
* Method of calculation doesn't imply any gross differences in astronomical influences between this and any other tidal calculations we do (which are based on springs/neaps).
* It's all a very broad and inaccurate brush anyway (which we knew)
* People who spend too much time with algorithms stress about different things to the rest of the population (like having to use 2 methods of calculating the same interpolation factor for a day's navigation when they might equally well use one).
http://xkcd.com/974/
BruceDanforth
Well-Known Member
you just need a couple of points to interpolate between it doesn't really matter when they are
Kawasaki
Well-Known Member
I teach this stuff and it still is not an exact Science
Working out secondary port times is not accurate anyway
Take the Menai Strait where I am its a mystery!
So will other places with strange tidal flows be
Cripes we can have a metre or two more than predicted between Port Dinorwic and Menai Bridge 3 miles apart
Atmospheric pressure can have a real effect as to when high or low water occur and at what time!
Wind direction too
Working out Standard Ports to Secondary Port's difference ,goes in the bin anyway when they might be 20 mins or so predicted .
There are only two Standard Ports in Wales
Holyhead and Milford Haven
So if I want to 'adjust' to the Secondary Port of Menai Bridge I use Holyhead
9 times out of 10 it just doesn't work!
If we get a North Easterly the tide at Menai is early ' tother way round if it's sow west!
Don't get me wrong, I realise there has to be a 'calculation' and a system to work out Secondary Ports etc
But wherever I have been in the UK on pleasure trips or doing Commercial work the 'locals' always say 'It's different around here etc etc etc!
Don't believe what it says in 'Reeds'
So in answer to the OP's question
Just ask a Local
Working out secondary port times is not accurate anyway
Take the Menai Strait where I am its a mystery!
So will other places with strange tidal flows be
Cripes we can have a metre or two more than predicted between Port Dinorwic and Menai Bridge 3 miles apart
Atmospheric pressure can have a real effect as to when high or low water occur and at what time!
Wind direction too
Working out Standard Ports to Secondary Port's difference ,goes in the bin anyway when they might be 20 mins or so predicted .
There are only two Standard Ports in Wales
Holyhead and Milford Haven
So if I want to 'adjust' to the Secondary Port of Menai Bridge I use Holyhead
9 times out of 10 it just doesn't work!
If we get a North Easterly the tide at Menai is early ' tother way round if it's sow west!
Don't get me wrong, I realise there has to be a 'calculation' and a system to work out Secondary Ports etc
But wherever I have been in the UK on pleasure trips or doing Commercial work the 'locals' always say 'It's different around here etc etc etc!
Don't believe what it says in 'Reeds'
So in answer to the OP's question
Just ask a Local
Tomahawk
Well-Known Member
In Ramsgae.. if it is high water at lunch time it is springs
If HW is early morning late afternoon it is neaps
There will be similar local effects around all the coastal areas
The tide flow is different at neaps to springs.. so there will be a different time delay fromstandard to secondary at springs to neaps..
the time of HW is a simplified way of working out if you are looking at neaps or springs.
If HW is early morning late afternoon it is neaps
There will be similar local effects around all the coastal areas
The tide flow is different at neaps to springs.. so there will be a different time delay fromstandard to secondary at springs to neaps..
the time of HW is a simplified way of working out if you are looking at neaps or springs.
