Reply
 
Thread Tools Search this Thread
 
Old 14 March 2013, 11:57   #21
Member
 
Country: UK - England
Boat name: ShaarkBait
Make: Zodiac 3.6 FR
Length: 3m +
Engine: Mariner 9.9 4-stroke
Join Date: Nov 2012
Posts: 364
Thanks for the explanation Gerry. I think I understand it... EasyTide is an approximation due to 10cm resolution of data, AnyTide is to the 1cm, just difficult to pinpoint the exact time on the curve.

Would be nice to have the app say what time is HW/LW and what height without having to scroll over the graphs. It may be there and I missed it. Would be useful to avoid being on the same stretch of river as the large freighter that blocks the river every few high tides! If I was going out through the harbour mouth, I would hate to meet Mungo coming the other way.


Huge cargo ship carrying 1,000 tons of granite left pointing in the air after it runs aground on harbour wall | Mail Online

The different datums is a bit more important. Unfortunately I don't have my tide tables from Littlehampton with me so cannot be sure what I am looking to with regard to height. All I know, is that we have a sand bar that dictates when you can cross it and that its 0.9m above datum. I had always assumed that this was relative to the height of easytide and the harbour tide tables, however, I've got myself confused over what I am comparing to what - Easytide, AnyTide, Chart and Harbour Tide Tables. It will be my first season out on a boat at Littlehampton so I need to understand this.
__________________

__________________
IanH is offline   Reply With Quote
Old 14 March 2013, 15:48   #22
Member
 
Low Flyer's Avatar
 
Country: UK - England
Town: Portsmouth
Boat name: Cinsirrah III
Make: Oceans
Length: 8m +
Engine: Twin D3 160 Diesels
MMSI: 235089712
Join Date: Sep 2011
Posts: 462
Tucabo tide planner. Have a look.
__________________

__________________
Low Flyer is offline   Reply With Quote
Old 14 March 2013, 17:08   #23
Member
 
Country: UK - England
Town: NW& wherever the boat is!
Boat name: depends on m'mood!
Make: Humbers/15-24m cats
Length: 6m +
Engine: etec130/big volvos
MMSI: many and various
Join Date: Nov 2001
Posts: 1,815
I use Belfield tide plotter on a pc. Cost is a pittance when compared to boating costs generally, has always been highly reliable and we buy the one that covers europe as well as uk. If we are doing a delivery its always available and readable and of course still works when we cant get internet. We use it daily on the current job where we have 9-10metre tides and expanses of very shallow water plus a harbour bar that moves erratically. Take a reading every time we cross the bar and then recalculate the bar height from the tide plotter.
__________________
Dave M
www.wavelengthtraining.co.uk
wavelength is offline   Reply With Quote
Old 29 March 2013, 06:07   #24
Member
 
Country: UK - England
Boat name: ShaarkBait
Make: Zodiac 3.6 FR
Length: 3m +
Engine: Mariner 9.9 4-stroke
Join Date: Nov 2012
Posts: 364
Quote:
Originally Posted by IanH View Post
Gerry

Could you please advise on the discrepancy found between your app and easy tides? L/W 1 hour difference and 1m out? I can see that different height datum, but 1 hr difference?

Easy tide, low water 18:11 at 0.2m
I have two sets of printed tide tables in front of me (both produced for littlehampton harbourboard documentation) and they both agree with each other and one matches the easy tide website (and the other only shows high tides which also match). Heights also agree.

So there are four sets of tide tables of which three agree leaving the AnyTide app being the odd one out by significant margin in both height and time.

Yes, I can understand that the AnyTide app is probably scientifically correct but it would appear that it is at odds with local knowledge and understanding which would think make it pretty much useless when one has to manually convert between the two sets of data.
__________________
IanH is offline   Reply With Quote
Old 29 March 2013, 07:00   #25
Member
 
chris.moody's Avatar
 
Country: UK - England
Town: Daventry & Beaulieu
Boat name: Tigga2
Make: Ribcraft 4.8
Length: 4m +
Engine: Honda BF50
MMSI: 235900806
Join Date: Jun 2011
Posts: 984
RIBase
Quote:
Originally Posted by wavelength View Post
I use Belfield tide plotter on a pc. Cost is a pittance when compared to boating costs generally, has always been highly reliable and we buy the one that covers europe as well as uk. If we are doing a delivery its always available and readable and of course still works when we cant get internet. We use it daily on the current job where we have 9-10metre tides and expanses of very shallow water plus a harbour bar that moves erratically. Take a reading every time we cross the bar and then recalculate the bar height from the tide plotter.
I use Belfield Charts, which includes the tides as well, on my laptop for passage planning at home. I have tried it with a USB GPS dongle and it works well as a chartplotter but not found anywhere on a rib to mount a laptop From memory it cost £70 to £80.

When out on the water the Navionics app (iOS or android) is a good backup for charts and tides.
__________________
--
Chris Moody
Rib Tigga2 a Ribcraft 4.8 with a Honda BF50
chris.moody is offline   Reply With Quote
Old 29 March 2013, 07:05   #26
Administrator
 
John Kennett's Avatar
 
Country: UK - England
Town: Brighton
Length: 3m +
Join Date: May 2000
Posts: 6,830
Quote:
Originally Posted by Gerry View Post
for anyTide, we chose to use a datum that was, as close as possible, an accurate representation of the contemporary mean water depth for all prediction locations (which includes the observation sites like Littlehampton.) This decision may well be confusing to many users. We will consider using chart datum (or at least having the option of it) in future updates."
I've just noticed this, and hope I am confused! Are you really saying that you have a tide prediction program that uses your own datum and not chart datum?

Whether or not chart datum accurately represents current lowest astronomical tide is almost entirely irrelevant, provided the charts and tidal predictions use the same datum.

A predicted tide height in isolation is meaningless and is only any use combined with a chart. If the predicted heights use a different datum then what's the use of that?

Have your tidal scientists got much practical boating experience?
__________________
John Kennett is offline   Reply With Quote
Old 30 May 2013, 04:32   #27
Member
 
Country: UK - England
Boat name: ShaarkBait
Make: Zodiac 3.6 FR
Length: 3m +
Engine: Mariner 9.9 4-stroke
Join Date: Nov 2012
Posts: 364
If anyone heard the mayday yesterday morning from littlehampton harbour entrance, it makes you realise the importance of locally recognised data.

A sailing boat ran aground in harbour entrance at approx 08:00 BST.

Interestingly, the crew said that low tide according to their source was 08:25 however local published low tide is 08:57 BST. I checked an alternative source, an app on my iPhone, and found that the app did indeed predict low tide at approx 08:25. I wonder if they were using an app similar to this as their tide predictor and thus made their decision to enter the harbour narrows based on this?

What worries me, and it may explain why they decided that it was ok to enter/exit the harbour was the discrepancy in height. Official published low tide height was 0.3m but the app was stating a significantly higher 1m, the difference between their keel passing over the the bottom and it bedding in, a mayday being called, and the local lifeboat crew spending 3 hours in assistance.

This is all of course conjecture.
__________________
IanH is offline   Reply With Quote
Old 03 June 2013, 11:47   #28
Member
 
Country: UK - England
Town: Southampton
Boat name: Kingfisher
Make: Osprey Vipermax
Length: 6m +
Engine: Mercury Optimax 150
Join Date: Jan 2007
Posts: 89
Late response...

Sorry – been meaning to reply to this earlier and hopefully clarify any confusion from the earlier explanation. Apologies also for a rather long, detailed post, especially if I’m describing stuff that people already know well.

As I said before, our anyTide app uses two types of tidal predictions:

1. Based on known ports around the UK & Ireland
2. Models generated from a grid of triangles with 1.8km sides anywhere along the coast on the NW European shelf

The predictions at known ports are based on “real” data. i.e like other tide prediction engines, they’re based on a physical tide gauge sitting at a known location. The historical measurements from this are analysed and used to generate future predictions (via tidal harmonic constants). Since the tide gauge is sitting at a known location, the chart datum at that point is also known/can be calculated. Hence, the predictions at known ports are based on chart datum.

This is also true for our anyTide app – all the known/listed ports (shown as purple pins on the map) are referenced to chart datum by correlating the known mean sea level (MSL) with the known chart datum for that tide gauge. (MSL is a measurement of the sea level when high and low tides and wave peaks/troughs are all averaged out and is available directly from a tide gauge or derived from MHWS, MLWS etc).

However, as a general point, it’s really important to note that there can still be differences between the tide prediction for a tide gauge and the actual recorded tide at that point, e.g. for the tide gauge at Portsmouth (located on the south west corner of Victory jetty), here’s a chart showing the differences for yesterday and today:

Real-time/near real-time data display for Portsmouth | National Tidal and Sea Level Facility

The tide predictions are influenced by the number of harmonic constituents you choose to use for your calculation and are solely based on astronomical effects. I’d be extremely cautious about assuming any tidal prediction is accurate to the odd few cms and then being surprised when the “real” tide is sometimes considerably different. Meteorological effects can cause significant differences – e.g. all tide predictions assume an average atmospheric pressure (1016mb for the UK south coast). A 1mb difference in atmospheric pressure can cause a change in water height of around 1cm. So, yesterday in Southampton, the average barometric pressure was 1027.8mb, causing a possible lowering of water level by about 10cm. This will not be shown on a tide chart. The max meteorological effect is usually around 30cm, but can be worse for storms, etc. There’s a good summary at:

http://noc.ac.uk/f/content/using-sci...al_effects.pdf

I think the confusion about datums has arisen because of the 2nd type of prediction in anyTide – using models to predict the tide at any point around the coastline. In this case, the model is using a complex algorithm for a specific triangle of sea area without a tide gauge to generate a tide prediction and doesn’t have a known reference point with a record of historical tides or an associated chart datum. In this case we have access to detailed mean sea level (MSL) records and use these to generate a datum for each triangle. You can use the MSL to centre the tide predictions and then estimate a Lowest Astronomical Tide (LAT) based on a series of spring tide calculations. Since LAT approximates to chart datum, it gives a reference point for the modelled tides – with the important proviso that LAT is usually calculated over a very long period of time – years – to account for long term (and often more extreme) astronomical events. So the calculated datum is by no means perfect, but it's the best effort to produce a useful tide chart in these areas. We’re looking at adding precise chart datum to the modelled tides in a future app update, but it’s a non-trivial task since we’ll need to add reference data for every 1.8km coastal grid on the NW European shelf.

As regards Littlehampton specifically, I’m not sure why there’s a difference. I’ve correlated anyTide with our professional tide prediction engine, POLTIPS3 (sold to marine organisations and not cheap!) and it looks good. Littlehampton is a port with a tide gauge, so isn’t using the modelling side (to be precise the chart datum at the Littlehampton gauge is 2.74m below Newlyn Ordnance Datum, in case anyone’s interested).

e.g. today I’ve compared easy Tide online, anytide and POLTIPS and don’t see a great deal of difference. To quote the FAQ section on the UKHO easytide webpage about why different sources may estimate different tides:
“...the two sets of predictions are simply different, and neither is necessarily wrong. The best way to quantify the accuracy of any predictions would be to carry out a rigorous statistical analysis of the observed tide against the predicted tide.”

As regards the yachtsman running aground – always err on the side of caution. If they thought low tide was at 08.25 instead of 08.57, they should have actually had more water under the hull than they thought. However, the bottom of the tide curve is not going to show much change in water over about 30 minutes.

Best to use the tide prediction that you’re most comfortable with – but do so knowing what the inherent limitations are.

Gerry

P.S. NOC is based at Liverpool & Southampton and we run sea going research ships and coastal vessels. Lots of the people who work here enjoy boating and understand the practicalities...
__________________
Gerry is offline   Reply With Quote
Old 03 June 2013, 11:53   #29
Member
 
Country: UK - England
Town: Southampton
Boat name: Kingfisher
Make: Osprey Vipermax
Length: 6m +
Engine: Mercury Optimax 150
Join Date: Jan 2007
Posts: 89
Couldn't get three images of today's tides at Littlehampton attached to the last post, so here they are.

First is UKHO easytide, second is anyTide app, 3rd is the graph part of POLTIPS3.
Attached Thumbnails
Click image for larger version

Name:	easytide Littlehampton 030613.jpg
Views:	97
Size:	142.8 KB
ID:	80958   Click image for larger version

Name:	anytide Littlehampton 030613.jpg
Views:	71
Size:	112.2 KB
ID:	80959   Click image for larger version

Name:	POLTIPS3 Littlehampton 030613 graph.jpg
Views:	78
Size:	110.9 KB
ID:	80960  
__________________
Gerry is offline   Reply With Quote
Old 03 June 2013, 12:53   #30
Member
 
Country: UK - England
Boat name: ShaarkBait
Make: Zodiac 3.6 FR
Length: 3m +
Engine: Mariner 9.9 4-stroke
Join Date: Nov 2012
Posts: 364
Hi Gerry

I appreciate that it is sometimes all too easy to get sucked in to the nitty gritty of a problem. For sake of clarity and to further my understanding, omitting any reference to UKHO EasyTide, can you please explain the 0.7m disparity in tide height between the local published tide tables, ie as per the local harbourmaster printed tide table books and not other online references like EasyTide (even though from a non exhaustive sample EasyTide and local published data correlate perfectly).

Can I also draw you to Littlehampton Harbour website at Controlling Depths and Tidal Ranges – Littlehampton Harbour and explain how AnyTide relates to these figures in particular the statement "The controlling depth is the level of the Littlehampton bar at 0.9 metres above chart datum."

Also, Harbour Entry – Littlehampton Harbour

Quote:
The level of Littlehampton bar which extends 600 metres southwards from the end of the West Pier is 0.7 metres above chart datum. The depth of water covering the bar can be calculated by subtracting 0.9 metres from the height of water indicated on the tide boards situated within the harbour.
Also, from your comment below. How does AnyTide correlate with the heights of the tide gauges in the harbour?

Quote:
Littlehampton is a port with a tide gauge, so isn’t using the modelling side (to be precise the chart datum at the Littlehampton gauge is 2.74m below Newlyn Ordnance Datum, in case anyone’s interested).
I'm trying to deduce if/how AnyTide correlates to local knowledge in particular, knowing how much water will be above the bar when water levels are listed by various means.
__________________

__________________
IanH is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 15:40.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.