Tide prediciton websites discrepancy

[Lionel]

Afternoon Folks

In checking some tide times on Easy Tide for Tobermory -

Easy Tide - Tobermory

I then came across another website giving tide predicitons (www.pol.ac.uk) so punched for Tobermory again and got -

POL ac.uk - Tobermory

They are different. You may or may not argue they are substantially different but from this one example there are discrepancies over time of high and low tide (typically about 20 mins) and heights differ by about 300mm.

Which one is right? Or are they both just approximations?

Thanks, /forums/images/graemlins/confused.gif

 

[sarabande]

Such discrepancies are not uncommon between software solutions. The formulae used to calculate tidal heights are closely guarded commercial secrets, and wind and weather will affect the heights and times much more than the mathematics.

Take an empirical average of what you have in hand.

 

[NealB]

The one whose prediction for a particular tide on a particular day coincides with actual high water is the correct one (for that tide on that day).

Another tide, another day, the other maybe correct.

Or maybe they'll both be 'wrong'.

They're just estimates based on simple(ish), man-made mathematical models.

 

[Steve Clayton]

[ QUOTE ]
Take an empirical average of what you have in hand.

[/ QUOTE ]
Somewhere between 9 to 11" then? /forums/images/graemlins/smile.gif

 

[bedouin]

Yes they are both approximations - but they are close enough for all practical purposes. If your planning is impacted by which is correct then you are putting too much faith in the predictions.

 

[Athene V30]

Don't forget to make a correction for atmospheric pressure and to take account if the wind is helping the tide in or holding it back.

There again is it really that critical?

 

[Lionel]

Thank you all for your kind input.

So it's wet finger out the porthole and boy on front with plumb line /forums/images/graemlins/grin.gif

It's not overly critical as I generally give a wide margin of error with a draft of 1.4m. However, I was suprised to see 300mm difference on heights.

 

[ebbtide]

Re: Tide prediciton discrepancies

You should add Belfield's software into the pot as well.

A few weeks ago both that and Easyside were in accord, pressure was near normal at 1015 and winds had been light for days.

Yet the height of tide was 0.6m MORE than predicted.
Any ideas?

 

[mormargh]

They are two different models. Easytide is Admiralty Hydrographic Office and POL is Proudman Oceanagraphic Laboratory. Results also differ in the small booklets sold/given away by chandlers, yacht clubs, marinas etc depending on whose data they have used (usually stated somewhere).

 

[rogerdog]

Remember they are tidal *predictions* -ie an educated guess - not a German train timetable (anyone who lived there will understand this joke).

In addition why on the Yachtmaster theory calculate tidal height etc to the nearest 10cm; in all fairness to the instructor as he said that's what the RYA want and certainly could see the futility of this.

 

[Bilgediver]

boy on front with plumb line

******************************************
Please use the right tool for the job......Plumb line your wife uses when hanging wallpaper and no place on a boat afloat where nothing is plumb!!!!! You need a sounding line...different animal altogether . You could do a flounder a terrible injury with a plumb line

/forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

 

[Lionel]

Och Bilgediver,, my humble apologies and thank you for correcting my erroneous ways.

I guess this also rules out using a plumb line to hang the wife with. Soddit /forums/images/graemlins/smile.gif

 

[brianrunyard]

They're only predictions.
The only accurate tide tables are the ones published after the event!!!