Secondary port time differences.

[Buck Turgidson]

Buck, everything you say is true, but as Nigel says, it is only the Sun's harmonic that is a constant to the 0000 0600 1200 1800

But not all tables use 0000 0600 1200 1800 so it isn't just the suns harmonic in play.

 

[LadyInBed]

But not all tables use 0000 0600 1200 1800 so it isn't just the suns harmonic in play.

No matter what four equi-spaced times you choose they are still only relative to the Sun's position somewhere in the sky!

 

[nigelmercier]

But not all tables use 0000 0600 1200 1800 so it isn't just the suns harmonic in play.

But the sun is the only influence that would have a similar effect at the same time each day.

 

[alant]

Last time this was asked on Scuttlebutt (googling turned up an old thread), lots of people explained how to do secondary port calculations and no-one answered the question the OP had actually asked. Let's be clear: I know how to do secondary port calculations and do them every time I go sailing. That's not the question. What is the question *is* is the one I was asked the other day:

What causes the difference in the difference of high or low water time at the secondary port according to what time of day high/low water occurred at the standard port.

If I was the sort of person who doesn't like to admit when they don't know something I would have done some hand waving, mumbled something about the gravitation effect of the sun and which way the earth was pointing at high/low water, and distracted everyone with an amusing youtube video of a kitten. Actually that roughly *was* my response but with an additional acknowledgement that it wasn't a real answer and I'd research a better explanation. Remarkably, of all the resources I've found about secondary port calculations, all explain how to use tabulated data without explaining how the content of the tables is calculated.

Anyone want to have a go at explaining?

Again: THIS IS NOT A QUESTION ABOUT HOW TO DO SECONDARY PORT CALCULATIONS!

I had a similar question posed, whilst running a Day Skipper Course, so asked the Hydrographic Office.

This is an attachment they sent me (some may not copy) -

"Explanation of the Time and Height Differences published in Part II of Admiralty Tide Tables
Extract from Admiralty Tide Tables Volume 1, NP201

The time differences shown have been derived originally from a non-harmonic analysis of tidal observations at both the Standard Port and the Secondary Port.

Brief description of non-harmonic analysis
In order to derive the time and height differences at the Secondary Port with respect to the times and heights of High (and Low) waters at the Standard Port, the differences need to be calculated by comparing the available tidal data at each location, as follows:-





4 plots are then required as follows:-

· High Water Times at the Standard Port vs. High Water Time Differences at the Secondary Port
· Low Water Times at the Standard Port vs. Low Water Time Differences at the Secondary Port
· High Water Heights at the Standard Port vs. High Water Height Differences at the Secondary Port
· Low Water Heights at the Standard Port vs. Low Water Height Differences at the Secondary Port

Examples of these plots are shown as follows:-

Time Differences:-


Height Differences:-





Time Differences
On looking at the Time Difference curve diagrams it can be seen that there is one ‘peak’ and one ‘trough’ along their length, which effectively refer to the maximum and minimum time differences tabulated for the Secondary Port in the Admiralty Tide Tables. The times at which these maxima and minima occur at the Standard Port being the headers used in bold in the Standard Port header information (e.g. 0000 & 1200, 0600 & 1800 etc. etc.)

The above explanations of how the time differences have been originally derived at the Secondary Ports also account for the variations seen in the Standard Port header information, for example as follows with Plymouth.


and


Essentially the differences are caused by the specific relationships between the Standard and Secondary Port(s) during the period of observed tides collected at both locations, i.e. the specific turning point of the maxima and minima has shifted slightly along the length of the time difference curve referenced above.

Reference to Spring and Neap Tidal Conditions
The ‘maxima’ and ‘minima’ referred to above could also be thought of in terms of Spring and Neap tidal conditions.

For example, looking at the Plymouth predictions for a Spring High Tide on a Spring gives the following prediction:-



So High Water Springs on this day occur at 06:37 and 18:57, which is relatively close to the columns shown above at 0600 and 1800 (remember this is only one day’s Spring Tide in isolation).

Similarly, Low Water Springs occur at 00:29 and 12:50, again relatively close to the Low Water column of 0000 and 1200 (and the ‘alternative’ column of 0100 and 1300).



The Neap Tide gives similar confirmation:-



High Water Neaps on this day occur at 01:19 and 13:42, which is relatively close to the columns shown above at 0100 and 1300 (and quite close to the ‘alternative’ column of 0000 and 1200).

Similarly, Low Water Neaps occur at 07:21 and 20:04, again relatively close to the Low Water column of 0600 and 1800.

Again this is one Neap tide in isolation. The non-harmonic ‘time stamps’ were derived from a much longer period of tidal observation.

Conclusion
The above information gives a general feel for the way in which the time and height differences have been established initially and how the Secondary Port data relates to the Standard Port data accordingly.

It is not an exact science!"

Christopher Jones
Head of Tides
United Kingdom Hydrographic Office

Tel: +44 (0)1823 337900 x3504

Fax: +44 (0)1823 423892

E-mail: christopher.jones@ukho.gov.uk or tides@ukho.gov.uk
Web: www.ukho.gov.uk


What a job title - "Head of Tides".
I'm sure he will answer any other queries from forumites!

 

[laika]

I had a similar question posed, whilst running a Day Skipper Course, so asked the Hydrographic Office.

This is an attachment they sent me (some may not copy) -

Many thanks for that. excellent info. It could be just me being thick (probably is) but although that clarifies how they derive the data, I'm still not clear in my mind about what causes the data to look the way it does, but this post tells me who to ask for clarification and I'll report back with anything I discover (even if it's just that I'm as thick as I feared..)

 

[Buck Turgidson]

Many thanks for that. excellent info. It could be just me being thick (probably is) but although that clarifies how they derive the data, I'm still not clear in my mind about what causes the data to look the way it does, but this post tells me who to ask for clarification and I'll report back with anything I discover (even if it's just that I'm as thick as I feared..)

Lol, I'm with you. It would appear that the cause is not stated but the differences "come" from measured data.

 

[Eygthene]

Webcraft has given you the answer. Taking the example of Plymouth/Starcross, given by Nigel Mercer, you will find that H W springs at Plymouth always occur at about 0600 and 1800, whilst H W neaps always occur at about 0000 and 1200.

So the different corrections at those times are really related to time corrections at springs versus neaps.

I'm sure you already know that springs occur when the moon and sun are aligned or opposed (new moon and full moon) whilst neaps occur at first quarter and last quarter. Thus the times are of course, related to the sun as so many folk have pointed out.

Finally, we have the question of why would the time correction be different at springs from at neaps. Here I think it would be the great difference in the volume of water that flows up the channel and back at springs, compared with neaps, but I'm not sure.

 

[Buck Turgidson]

Webcraft has given you the answer. Taking the example of Plymouth/Starcross, given by Nigel Mercer, you will find that H W springs at Plymouth always occur at about 0600 and 1800, whilst H W neaps always occur at about 0000 and 1200.

So the different corrections at those times are really related to time corrections at springs versus neaps.

I'm sure you already know that springs occur when the moon and sun are aligned or opposed (new moon and full moon) whilst neaps occur at first quarter and last quarter. Thus the times are of course, related to the sun as so many folk have pointed out.

Finally, we have the question of why would the time correction be different at springs from at neaps. Here I think it would be the great difference in the volume of water that flows up the channel and back at springs, compared with neaps, but I'm not sure.

So you agree that it isn't just the sun that causes the difference in time offsets but the combination of sun and moon and the difference in mass and velocity of flow this causes. i.e. my post 13 and 16?

 

[Eygthene]

Yes Buck

 

[laika]

Webcraft has given you the answer. Taking the example of Plymouth/Starcross, given by Nigel Mercer, you will find that H W springs at Plymouth always occur at about 0600 and 1800, whilst H W neaps always occur at about 0000 and 1200.

So the different corrections at those times are really related to time corrections at springs versus neaps.

I'm aware of the causes of tides (the non-astrophysicist's version at least and I also see how the factors BlowingOldBoots describes in post #2 would cause variation in difference of time between standard port and secondary port according to rate of flow and height of tide changing between springs and neaps (which is what I believe you are saying). Makes perfect sense, and if the time difference corrections were a function of how springy or neapy the tide was I would be happy and not have asked this question in the first place. But they're tabulated as a function of time, not tidal range.

I think you're saying that time is being used as an approximation of where we are in the tidal cycle. But we don't *need* an approximation: We've got the info right there in the almanac. Moreover we use it in interpolating secondary port tidal height. Why use a time-based approximation when you could use the day's range?

Also, as mentioned in post #14, high water springs doesn't always occur at the same time in a given location. Don't have an almanac with me but HW portsmouth is usually within 30 mins either side of 00:15 this year. But sometimes it's within 30 mins of 05:15. which is at the other end of the time corrections scale

 

[Mark-1]

I'm glad you're asking the question, I think we may all learn something.

I think you're saying that time is being used as an approximation of where we are in the tidal cycle.

It's always seemed blatently obvious to me that it was a rough approximation of where we are in the tidal cycle. That assumption might be wrong of course, but most sailors share it. (Those that don't can't say what it actually is, merely that it isn't an rough approximation of where we are in the tidal cycle.)

But we don't *need* an approximation: We've got the info right there in the almanac. Moreover we use it in interpolating secondary port tidal height. Why use a time-based approximation when you could use the day's range?

I've always wondered that, I wonder if it's quicker to use time for some methods of interpolating? Frankly I'd have thought it was slower.

Also, as mentioned in post #14, high water springs doesn't always occur at the same time in a given location. Don't have an almanac with me but HW portsmouth is usually within 30 mins either side of 00:15 this year. But sometimes it's within 30 mins of 05:15. which is at the other end of the time corrections scale

It's a rough approximation.

 

[nigelmercier]

Taking the example of Plymouth/Starcross, given by Nigel Mercer, you will find that H W springs at Plymouth always occur at about 0600 and 1800, whilst H W neaps always occur at about 0000 and 1200...

Errr ... This is a wind-up, right? [See my Post #38]

 

[webcraft]

Errr ... This is a wind-up, right?

I think it is actually closer to 08.00/20.00 and 02.00/14.00 for Plymouth

- W

 

[Cruiser2B]

What causes the difference in the difference of high or low water time at the secondary port according to what time of day high/low water occurred at the standard port.

The time of day determines whether the sun and moon are in conjunction (springs) or opposition (neaps). Obviously if HW is near noon, the sun is more or less above you, and the moon is either above you or directly below you - with both gravitational forces acting together. If HW is closer to 0600 or 1800 then the moon is above ( or below) you, while the sun is on the horizon - the gravitational forces are acting in opposite directions, with the effect that the sun's pull "drags" on the more powerful moon's pull, thus slowing the horizontal movement of the water, so greater time difference. As has been said, it's just a better approximation.

 

[Eygthene]

Errr ... This is a wind-up, right?

Not sure what you meant by that! If you check the almanac, you will see that for Plymouth, HW springs falls between 06:20 and 07:06 on 51 of the 52 morning ocurrences of HW springs this year and between 18:31 and 19:30 on evening occurences. The corrections given for the secondary port (Starcross) are of course an approximation, as others have pointed out, so in the almanac they round off the corresponding HW/LW times to the nearest of 06:00/18:00 (or 00:00/12:00) when giving the corrections

 

[nigelmercier]

If you check the almanac, you will see that for Plymouth, HW springs falls between 06:20 and 07:06 on 51 of the 52 morning ocurrences of HW springs this year ...

I don't have an almanac, but interesting to note.

[I retract the rest of this post, see Post #38]

I'm pretty sure that the base times (such as 0600) are not related to these though, because they seem to be around these times for every port in the world.

 

[rgarside]

I don't have an almanac, but interesting to note. I'm pretty sure that the base times (such as 0600) are not related to these though, because they seem to be around these times for every port in the world.

I don't have the information to hand to check widely around the world, but those times should relate to the times of HW & LW given at the associated Standard Port. If you look at the Standard Port times for ports around the world, are they expressed in their local zone time? In the areas covered by Reeds, this would appear to be the case. If this is so, the times will relate to the Spring/Neap cycle in the way described in earlier posts discussing tides around the UK. [ie gravitational fields of Sun & Moon in or out of phase, differing volumes of water to flow to get from HW to LW at Springs and Neaps, and the relative constancy of the times of HW & LW at Springs and at Neaps].

As an example of different times to 1200 / 0600, look at Leith and its associated secondary ports. The times given for the corrections fit with the times of HW & LW at Springs and Neaps.

I thought the information from UKHO about how they derive them in detail based on their harmonic analysis was very useful.

 

[nigelmercier]

I don't have the information to hand to check widely around the world, but those times should relate to the times of HW & LW given at the associated Standard Port. If you look at the Standard Port times for ports around the world, are they expressed in their local zone time? ...

That is a very good point, and I retract my earlier statement about the times not being related. I was thinking in terms of "springs" being a week-long event, but now realise the point being discussed is the spring tide. From what we have discussed earlier, it makes sense that these occur at roughly the same time of day.

 

[dedwards]

I agree with others that it is likely due to the habit of springs happening at roughly the same time of day. As the UKHO post hints at, this seems to not have been done on purpose but was simply a function of somebody plotting the time differences and noticing that at one time of day they seemed to be larger than at anther time of day.

I would be interested to hear if calculating the time differences using the actual tide times as suggested elsewhere would be any more difficult and any more accurate.

On a side note, I recently had cause to take a look at the swanage tides and noticed that although it is a secondary port of portsmouth, its tides bare only a slight resemblance to portsmouth's tides. Does anybody know a good reason why it is not a secondary port of Poole instead seeing as its resemblance is much closer?

 

[Mark-1]

If this is so, the times will relate to the Spring/Neap cycle in the way described in earlier posts discussing tides around the UK. [ie gravitational fields of Sun & Moon in or out of phase, differing volumes of water to flow to get from HW to LW at Springs and Neaps, and the relative constancy of the times of HW & LW at Springs and at Neaps].

I was struggling with causality, or otherwise.

So I've just drawn it on a bit of paper, and it's obvious that local high tide at Springs has to be with the face of your bit of the earth facing (more or less) directly at or directly away from the sun. Approx Midday and midnight local time.

If there were no other factors at play it would be so at any point on the Earth and I bet it usually is.

Unless I've misunderstood, and I don't think I have.

Many thanks to the OP, I owe you one for posing this question.

Edit:
This is a more sophisticated version of my paper diagram:

I just asked myself 'what time is it at the points at HW and LW on this diagram and the causality became obvious...