DIY Tidal Curves

[obmij]

How does one go about plotting a tidal curve i.e how does the almanac go about it?

Got on to this line of thinking because Holyhead tidal curve is too small to accommodate some of its secondary ports, so when working out Conwy for example you have to use a bodge, such as halving the scale for Holyhead or using a similar curve with a bigger range.

Both workable of course (as is a quick mental 12ths calc) but I would like to do it properly.

It would also be interesting to have proper tidal curves for some secondary ports, printed and ready to go.

This is more for interest rather than practical purposes I'm aware there are 'workarounds' (and software)

Cheers

 

[lpdsn]

Do you have a source of data other then the tidal curve for the primary port? That's usually what you use to determine intermediate heights, even though it is not a really accurate method for all secondary ports.

If you really do have a separate source of data, use Excel and fit a curve to it.

 

[FWB]

If you have an iPhone or iPad then Tides Planner by Tucabo is your friend...

 

[prv]

If you have an iPhone or iPad then Tides Planner by Tucabo is your friend...

+1

Can feel a little overcomplicated at first, but once you get used to it it's indispensable. Does tidal streams too, either fully dynamic or (my preference) extracts from the Admiralty atlases with the times set up for the given day.

Pete

 

[[70521]]

How does one go about plotting a tidal curve i.e how does the almanac go about it?

This might help with some background.

http://en.wikipedia.org/wiki/Tide-predicting_machine

 

[AntarcticPilot]

This might help with some background.

http://en.wikipedia.org/wiki/Tide-predicting_machine

Fascinating!

Fourier analysis is the way to do it nowadays; it is A-level maths, and is easy to implement on a computer (look up Fast Fourier Transform).

 

[NormanS]

Yes, but does the actual water do what the calculations say it will?

 

[[70521]]

Fascinating!

Fourier analysis is the way to do it nowadays; it is A-level maths, and is easy to implement on a computer (look up Fast Fourier Transform).

I have a love/hate relationship with Fourier after all the suff I did with in on my degree

 

[AntarcticPilot]

Yes, but does the actual water do what the calculations say it will?

Mostly, except when there are unusual meteorological conditions causing the water to be higher or lower than it should be.

 

[AntarcticPilot]

I have a love/hate relationship with Fourier after all the suff I did with in on my degree

I have used it a lot for image analysis, and can't understand why people insist on doing things badly in the Space domain when they are easy and precise in the Fourier domain. I've even developed a technique for doing shape from shading in the frequency domain; sadly it turned out someone else had done it before, but i was still proud of my mathematical skill!

 

[NormanS]

Mostly, except when there are unusual meteorological conditions causing the water to be higher or lower than it should be.

Like wind or a change in atmospheric pressure. Unusual?

 

[ronsurf]

Yes, but does the actual water do what the calculations say it will?

Well, it's better than guessing.

The Admiralty book has all the values for the equation in the Wiki link. Just plug them in, solve the equation for every 15mins and plot the line. Easy!

 

[AntarcticPilot]

Like wind or a change in atmospheric pressure. Unusual?

If it's enough to significantly change the sea-level, it is unusual (and probably cruel as well!). No prediction is reliable to the level of tens of centimetres, for a multitude of reasons, and attempting to do tidal calculations to that accuracy is not worth the bother. You can attempt to correct for barometric pressure (there is a fairly good relationship), but the effect of wind is so dependent on the local configuration of the sea-bed that it isn't feasible to do it reliably.

 

[Daydream believer]

Put the tidal chart on a photo copier & scale up the image & you have a larger scale one. You can then do a lot & you can have one for every secondary port nearby that you want.

 

[prv]

attempting to do tidal calculations to that accuracy is not worth the bother.

Indeed - hence why the pilots round here regularly ask VTS for a tide gauge reading when bringing the big ships in - to see how much water there actually is, rather than a mere prediction.

Pete