Tidal Hights

[Brian_B]

Anyone know why the tidal predictions are different on Easytide and the YBW site and which is correct?

http://www.ybw.com/weather/home.html

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[alldownwind]

Probably worked out using different algorithms. Neither is 'correct'. Both are simply predictions. affected by wind, barometric pressure, and lordy knows what else. If you have 2 different predictions, best to take the average!

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[Guest]

UKHO is the Adm body and should in all be the master to refer to ....
but since copyright instistence by UKHO on data used for calculation -various other bodies have been used to determine tides ..... his brings in differences.

I am not sure which body is used by YBW .... but this may account for it .... now I'm comparing with my PDA and PC based data ....


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[Guest]

Now this is very interesting ........ as I have two sets of data - both stating clearly as using UKHO Harmonics and data. So they should have same answers ? Nope .....

All in same time zone - UTC (GMT)

25th Aug
Belfield
0623 1156 1859

Easytide
0546 1135 1825

Now onto NOAA based :

0558 1135 1830

PBO

0601 1135 1840


Mmmmmmmmmm !! Actually as posted on another thread I have noticed that the differences seem to increase and decrease roughly in line with moon phases .... but why ? Surely it would make sense that physics etc. is near constant - subject of course to loacl / atmospheric effects at time ... but calcs shouldn't differ so variedly ????

<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[Oldhand]

I think there are differing sophistications of UKHO harmonic calculation. They probably only release a "wide area" model for external use but use a more refined "coastal model" for their own published data. This is only a guess but would explain different predictions.

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[Guest]

As I understood it ....from the old DOS days .... now thats a story in itself ! ..... NP159 Simple Harmonic Constant provides basis for Tidal calcs.

Now the old DOS program said .... s used by Admiralty for its predictions etc.- so NP159 I assume is for all UKHO ...... Belfield whose figures are compared below post with UKHO Easytide claim Simple Harminic Constant basis ..... ????


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[ChrisE]

Where are you trying to predict? Places like Lymington have double highs in one tide cycle, especially around springs, and some algorithms quote one and others the other, if you see what I mean.

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[aitchw]

Wouldn't it be more prudent to take the worst case scenario for your intended action. So if low tide is critical then use the lowest suggested figure and vice versa?

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[Brian_B]

I understand what you mean but I'm looing at Plymouth (Devonport).

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