Why are THESE tide times different ?

[chockswahay]

Can anyone shed some light for me please?

I am using NavPlanner (3.1) on my laptop with 24G (2005) chart. When I use the tide predictor to check the tides at Portsmouth I find they are quite different from other published times.

I list below the HW times for Monday 26th May 2008.......

Source HW HW

NavPlanner 04:01 16:01

EasyTide 03:28 16:13

YBW 03:37 16:22

All times are 'local'

Does any one have any thoughts/ideas/suggestions please?

At times the NavPlanner/24G times are nearly one hour different from Easytide (adjusted for local time).

Cheers

Chox /forums/images/graemlins/confused.gif

 

[VicS]

Add to those

UK Hydrographic Office <span style="color:white"> ....... </span> 03:29 <span style="color:white"> ... </span> 16:13

P.O.L <span style="color:white"> .............................. </span> 03:37 <span style="color:white"> ... </span> 16:22

XTide <span style="color:white"> .............................. </span> 04:23 <span style="color:white"> ... </span> 16:53

XTide (2) <span style="color:white"> ......................... </span> 03:42 <span style="color:white"> ... </span> 16:22


and you begin to realise they are only predictions based on different calculations. Even XTide give two differing tables for Portsmouth and 3 for some ports.
They are not carved on tablets of stone. Don't forget they are also affected by meteorological conditions

The best advice would be to use the least favourable for your planning but since there is pretty good agreement between UKHO, POL and Easy Tide I'd stick with those.

It looks as though YBW probably use the Proudman Oceanographic Laboratory tide tables

 

[Searush]

Careful Vic, it is possible that the ones giving agreement simply use the same assumptions or perhaps same variables.

POL is (allegedly) the premier authority in tidal predictions - and they have a real cute head office on Bidston Hill. I use Laver's paper booklet based on POL predictions & it works well for the Liverpool Bay/ North Wales areas - but then it should I guess as that's their home waters!

 

[Spars]

Could it be the double high tide caused by the IOW?

 

[VicS]

There is no double high at Portsmouth. There is further west but not at Portsmouth.

 

[wotayottie]

Simple - tide times are predictions just like the weather. The accuracy depends on the algorithm used - some are more complex and precise than others. But none are totally accurate, so the wise sailor doesnt count on them to the nearest nats genitalia. /forums/images/graemlins/grin.gif

 

[chockswahay]

Thanks for the replies guys, so you don't think the Navionics software is too unreliable then?

Chox

 

[VicS]

I think I would compare the predictions from Navionics with those from either the UKHO or POL and with the observed tidal data and form an opinion as to the reliability/accuracy of the predictions from Navionics.

This page will give you access to a comparison of the POL predictions with the recorded data. You could add in your Navionics figures to see how they compared with the recorded data.

Given a boring day in the office you could make it even more boring by collating all the figures for say the year so far and drawing your conclusions from that.