Tidal info - almanac vs. navionics

[Tim Good]

Excuse my ignorance on this matter but why are there differences in the tidal data predications in various places? Surely everyone uses a single formula to give the most accurate preditctions?

For example if you take the 2012 Reeds almanac then the high tide time for today in Avonmouth is 12 minutes and 0.5m different to that of what Navionics electronic charts say. I also realised that another tidal tables book gave different figures again.

Does anyone know why there are such differences. I mean 12 mins is not the end of the world but half a M is quite substantial.

Cheers

Tim

 

[LadyInBed]

Different sets of (and number of) harmonics used.
Admiralty harmonics are copyright so cost money to use
pol.ac.uk harmonics are free to use

 

[Tim Good]

Different sets of (and number of) harmonics used.
Admiralty harmonics are copyright so cost money to use
pol.ac.uk harmonics are free to use

Is there a general consensus as to which ones are most accurate?

 

[ffiill]

Try www.mdr.co.nz
A tidal prediction programme which for UK tides uses the Proudman Oceanographic Institutes data(University of Liverpool)
Some of the stations are slightly different-for example they have a measuring station in Tobermory although I have some data which I believe origionally was sourced from the Admiralty of standard ports.
The programme currently runs up to about 2060 and is a worlwide prediction programme.
Oh and whilst a contribution is asked for its basically free.

 

[LadyInBed]

Is there a general consensus as to which ones are most accurate?

Admiralty harmonics are as they have been at it a long time and put a lot of (our) cash into tweaking the harmonics.
EasyTide is our access to them but I don't know how many of the harmonics are used to produce the prediction.
One thing you can bet your boots on is that more of the harmonics are used for the MOD predictions

 

[maxi77]

When I wore a Blue Suit the tide tables I used were the same ones sold to the general public, merchant navy etc. Strangley the more complex the formulae used to predict tides the closer to reality the predictions are. Also a generic worlwide system will not give good results in some places where as local formulae will take into effect local conditions and thus be consistantly better. Hence many hydrographers charge for the use of their data and formulae to recoup some of the costs of maintaining their database and updating of their formulae.