Garmin Bluechart tide data problem

[greenalien]

A few weeks ago, I was sailing from Poole to Yarmouth. I used the tidal data from the Bluechart G2 Vision on my Garmin 4012 chartplotter to do the passage planning, aiming to arrive at Yarmouth at slack high water.
To my surprise, as I approached the Bridge buoy, I noticed that my SOG was decreasing rapidly, and next thing, I found myself in rough, shallow water - I had been swept onto the end of the Shingles bank. It quickly became obvious that the tidal data on the Garmin was inaccurate, and the tide had turned over an hour before the chartplotter was saying; after quickly starting the engine and getting clear of the Shingles, I then had to spend the next 2 hours motoring hard against a foul tide to get to Yarmouth.
I contacted Garmin about this, and here is their reply:

"All of the tidal data that that Garmin supply on the Bluechart products
are within the standard error tolerance for simplified tides. The UKHO
who supply this data tells us that the simplified predictions should be
within one hour and one meter at the peaks (high and low). It can also
vary from day to day. If it is off by 30 minutes today, it might only
be off by 5 minutes tomorrow. Although they don't guarantee that an
occasional prediction will not exceed the stated tolerance, we have
found that it is rare. Because of the simplified data that the UKHO
supply the tidal curve shown on the chart plotter wont show second high
tides as the curve will take into account both high tides as one.

The UKHO will not license us the full harmonic information that you see
on that website. Garmin are only allowed to use the simplified tidal
constituents that approximate the full harmonic predictions. This has
not changed since our BlueChart products were first produced, and it is
the same for every marine cartography supplier. To my knowledge, they
do not license the full harmonic predictions to anyone.

The Garmin units are designed as an aid to navigation not primary source of data. "

I think this is a totally unsatisfactory response - supplying inaccurate data is dangerous, there's no other way to describe it - if they can't provide accurate data, then they shouldn't supply it at all. Accurate tidal data must be available, after all, it's published in all the almanacs, so why are Garmin apparently unable to get it licensed to them?

Anyhow, lesson learned, nearly the hard way. I'll use the almanac data in future.

 

[Sandyman]

Depends on what you interpret as accurate as there are so many changing variables which effect the tides.

 

[greenalien]

Depends on what you interpret as accurate

Given that travelling along the South Coast involves navigating through a series of tide gates, accurate information on high and low water times seems to me pretty essential, and that info shouldn't be changed by high and low pressure, for example.

 

[Sans Bateau]

Last year in May when I got the 2010 G2 Vision chart update, I noticed some errors, errors I knew about because they were local to me. For instance the position of West Pole at the entrance to Chi harbour had been moved, several years and UKHO chart updates earlier. When I complained to Garmin, they sent me a similar response to yours, basically saying they use the latest chart scans. Which of course could not be true.

Garmin hardware may be good but I believe the charts and software are a little suspect.

I like modern electronic nav kit, but only as a guide. Paper charts, tidal flow charts and tables for me.

 

[Sandyman]

''Accurate tidal data must be available, after all, it's published in all the almanacs''

But that data is only a prediction. A calculation. It is only a prediction because it does not & cannot take into account the variables such as pressure & wind.
Seems what you are after is real-time tide information which is constantly updated.

 

[greenalien]

Seems what you are after is real-time tide information which is constantly updated.

That would be nice! However, all I really want is the data on the electronic chart to be the same as in the paper almanacs. Is this really too much to ask?

 

[Sandyman]

That would be nice! However, all I really want is the data on the electronic chart to be the same as in the paper almanacs. Is this really too much to ask?

All I can say is that there must be a real big difference from you lecky chart to the paper one for you to end up in the predicament you did. I know it sounds daft but your lecky plotter is set to BST isn't it ?

I have a C80 and have noticed slight variations on that compared to written tables but nothing as bad as it would seem on yours.

 

[greenalien]

your lecky plotter is set to BST isn't it

Yes, that was the first thing I checked.

 

[Sandyman]

Greenalien can you give us some idea of the differences in tide times from your plotter compared to the paper tables ??

The largest difference I have sen on my plotter is something like 30mins

 

[greenalien]

The date that I had problems was 30 June - Tidal data for Hurst and Yarmouth was wrong, but as I'm not on board at present, I can't quote the exact data.

 

[Sans Bateau]

The date that I had problems was 30 June - Tidal data for Hurst and Yarmouth was wrong, but as I'm not on board at present, I can't quote the exact data.

I have my G2 chart at home, and I have just checked it in my 'Home Port' against UKHO tide times for the date you mention, and I'm sorry to say that there is only a 10 mins difference between the two. If you tell me what time you were trying to enter Hurst I can do a more detailed comparison for you.

 

[greenalien]

Just got back on board, and double checked everything.

1) The Bluechart G2 Vision chart that I'm using is VEU705L dated Jan08
2) The time zone set on my Garmin 4012 chartplotter is WET (Western European Time) with Daylight Saving set to Auto. (There is no option for GMT or UTC) - in any case, it seems to be the same as BST, as are all following times.
3) Checking today - HW Southampton shows as 11:43 on the Garmin - ABP Tidetables say 10:53
Also for today, HW Yarmouth is shown as 11:31 on the Garmin - Yarmouth Harbour tide tables say 10:38

Checking for 1st July, which was when I had the 'problem' - Garmin says Yarmouth HW is 12:40; Yarmouth tide tables say 11:17

I have a feeling that the errors stem from an over-simplified tidal curve, which tries to average-out between first and second high water - in any case, the double high water in the Solent isn't shown on the curves from the plotter.

 

[Stu Jackson]

Given all that, why ever use the Garmin for tides ever again? While I have them on my GPSMap 76 Cx, I always use the printed local tide and current tables.

Much appreciated that you noticed this and posted their reply. Thanks a lot.

 

[earlybird]

double checked everything in any case, it seems to be the same as BST, as are all following times.
3) Checking today - HW Southampton shows as 11:43 on the Garmin - ABP Tidetables say 10:53
Also for today, HW Yarmouth is shown as 11:31 on the Garmin - Yarmouth Harbour tide tables say 10:38

My Belfield shows Southampton HW to-day as 11:40 BST, are you reading your tables correctly?

 

[greenalien]

My Belfield shows Southampton HW to-day as 11:40 BST, are you reading your tables correctly?

From http://www.southamptonvts.co.uk/highlowwater.asp

"Predicted Times and Heights of High and Low Tide at Dock Head, Southampton. ALL TIMES ARE GIVEN IN LOCAL TIME.

12/08/2011
Second Tide 00:12 4.1m
Low Water 04:20 1.1m
High Water 10:53 4.3m
Second Tide 12:43 4.3m
Low Water 16:42 1.2m
High Water 23:06 4.4m"

 

[Billjratt]

Is anybody sitting in the marina looking at a pile - or a rock or whatever, checking the ACTUAL high/low tide?(against their plotters)
I can't believe there's so much debate about this subject from this particularly busy area...

 

[greenalien]

I can't believe there's so much debate about this subject from this particularly busy area...

This isn't a debate - it's a discussion of the large inaccuracies in the Solent area tidal data supplied by a major manufacturer. Accurate tidal data is essential when passage planning, or you could be swept off course, or miss a tide gate and spend hours having to motor against a foul tide - or get caught in overfalls.

 

[Sans Bateau]

greenalien

I think the problem is the issue date of your G2 chart. 2008/9 charts had a fault on them and were replaced FOC by Garmin. I had mine replaced in May (?) 2010. The earlier versions of the charts had some errors, it could be the problem you describe is one of them. Also check with Garmin web site that you have the latest version of software on your plotter, updates are FOC.

I'm going to the boat later today, I will double check what I said earlier from my home port software, I'll check it on the plotter.

 

[greenalien]

I think the problem is the issue date of your G2 chart. 2008/9 charts had a fault on them and were replaced FOC by Garmin.

I have the latest software on my plotter, and Garmin won't replace the chart, it isn't one of the ones that they recalled.

The tide data is derived arithmetically, it is this algorithm that doesn't work for Solent tides, as far as I can tell, because the 'simplified' data set on which it is based has the wrong info for 1st High Water.

 

[bignick]

"All of the tidal data that that Garmin supply on the Bluechart products
are within the standard error tolerance for simplified tides. The UKHO
who supply this data tells us that the simplified predictions should be
within one hour and one meter at the peaks (high and low). It can also
vary from day to day. If it is off by 30 minutes today, it might only
be off by 5 minutes tomorrow. Although they don't guarantee that an
occasional prediction will not exceed the stated tolerance, we have
found that it is rare. Because of the simplified data that the UKHO
supply the tidal curve shown on the chart plotter wont show second high
tides as the curve will take into account both high tides as one.

The UKHO will not license us the full harmonic information that you see
on that website. Garmin are only allowed to use the simplified tidal
constituents that approximate the full harmonic predictions. This has
not changed since our BlueChart products were first produced, and it is
the same for every marine cartography supplier. To my knowledge, they
do not license the full harmonic predictions to anyone.

Definitely an unsatisfactory response.

It sounds like you queried the differences between your Garmin and Easytide? Does the Garmin show double high water curves? Navionics on my phone certainly does, which makes me wonder about the validity of their claim. I used it (in conjucntion with an almanac/charts before I get flamed) taking my boat from Soton to Poole the other weekend, and the tidal stream info and tide height info all seemed pretty close to Reeds / Admiralty Tidal Atlas info.

Also if the info supplied to all the chart companies is exactly the same then why are there differences between them? I always see difference between the times and heights (to a lesser extent) given by "WorldTides" and "Navionics" Pottery Pier in Poole Harbour shows a differnce of 25mins and 4cm in the LW times this morning.

Could it be down to local tide info being referenced to different major ports eg. to Portsmouth or Dover, and there being variation in difference of the tide times of these major ports?

Another idea - areas like Poole are referenced to LW times as they're more accurate, but I guess that chartplotters always reference to HW times, which may lead to some errors?