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Old 05-01-2015, 00:42   #1
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Another foundering how do chart plotters get it wrong

Hi guys,

This morning there's been yet another grounding and loss of a sail boat in Tasmania. It's hit an island (I think) in the dark.

My question is, how does this happen? In relation to chart plotters.

Assuming they were following a chart plotter and not recording where they were on a map, how do experienced sailers like Vetus for example get it so wrong?

Are the c-ordinates coming from satalites wrong or are the chart plotters themselves misinterpreting the data?
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Old 05-01-2015, 05:09   #2
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Re: Another foundering how do chart plotters get it wrong

It would just be speculation at this point since we don't know what happened. But chart plotters usually are not sailing the boat (or even steering). I am willing to speculate the satellites were not "wrong". In almost all cases like this it's a human error and not an error by the machine.
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Old 05-01-2015, 05:16   #3
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Re: Another foundering how do chart plotters get it wrong

Nautical Charts & Pubs

NOAA has a large facility where I live and my next door neighbor works for them. His full time job is going all over the world on ocean mapping vessels, trying to update marine charts to make them actually match GPS coordinates, and to just be accurate. They are a long way from done. It's a big world.

I was amazed to find out how much of the world's current nautical charts, that are currently in use, were drawn by men who have been dead for hundreds of years. Needless to say, they didn't have GPS when they were doing it.
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Old 05-01-2015, 05:32   #4
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pirate Re: Another foundering how do chart plotters get it wrong

Hence the expression.. 'MK1 EYEBALL'...
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Old 05-01-2015, 05:40   #5
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Re: Another foundering how do chart plotters get it wrong

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Nautical Charts & Pubs

NOAA has a large facility where I live and my next door neighbor works for them. His full time job is going all over the world on ocean mapping vessels, trying to update marine charts to make them actually match GPS coordinates, and to just be accurate. They are a long way from done. It's a big world.

I was amazed to find out how much of the world's current nautical charts, that are currently in use, were drawn by men who have been dead for hundreds of years. Needless to say, they didn't have GPS when they were doing it.
I worked for a while in a chart store. When you looked at many charts of islands in the pacific, and started checking back the connotation woiuld read "Cook" or Bligh" yep, the charts were drawn by Captain Cook or Lieutenant Bligh and haven't been updated since.

Even here in Denmark most charts were surveyed and drawn in the late 1800's, using rowboats and casting leads. No real updates since then. Same is true for most other european countries and the US.
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Old 05-01-2015, 05:44   #6
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pirate Re: Another foundering how do chart plotters get it wrong

Must have been a singlehander..
I hear they are prone to this...
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Old 05-01-2015, 06:26   #7
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Re: Another foundering how do chart plotters get it wrong

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Originally Posted by Rustic Charm View Post
Assuming they were following a chart plotter and not recording where they were on a map, how do experienced sailers like Vetus for example get it so wrong?
Chartplotters are not infallible. In fact, I would guess that if they had been recording where they were on a map, this might not have happened, as they would have probably been keeping a better eye on what was outside the boat, instead of just staring at a chartplotter (of course, that's speculation on my part).

As to where the error comes in, numerous places. As mentioned, a lot of the charts are old. They do not always perfectly correspond with the GPS coordinates from the satellites. In fact, they hardly ever PERFECTLY correspond with the GPS coordinates from the satellites. They are, however, usually close enough to avoid a grounding.

Then, too, the signal from the satellites are subject to a variety of possible errors. They are, after all, just a radio transmission. As such, like any other radio transmission they are subject to interference, poor signals, and the like. The same things that can cause your VHF or SSB to have problems with reception and/or transmission can affect the GPS signal as well. All of these things can throw the location off by a bit.

All this is one reason the phrase "technology induced grounding" has come into being. Too many people stare at the chartplotter, assuming that it is never wrong, and drive their boat right into an obstacle of some sort.
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Old 05-01-2015, 06:31   #8
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Re: Another foundering how do chart plotters get it wrong

[QUOTE=boatman61; said Hence the expression.. 'MK1 EYEBALL'...:

He should have been "MK1 EYEBALL on the Plotter"

At night it can be disorienting to be on deck, tired and looking at indistinct shapes of what you mind decdes is land, to wit, the land you want it to be.

The eye is fallable. Be very careful with it at night.Chances are the plotter will be more accurate than you alone, or u with a hand bearing compass.

And stay further offshore so mistakes with eyes and plotters don't wreck your watch
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Old 05-01-2015, 06:51   #9
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pirate Re: Another foundering how do chart plotters get it wrong

[QUOTE=MarkJ;1715733][QUOTE=boatman61; said Hence the expression.. 'MK1 EYEBALL'...:

He should have been "MK1 EYEBALL on the Plotter"

At night it can be disorienting to be on deck, tired and looking at indistinct shapes of what you mind decdes is land, to wit, the land you want it to be.

The eye is fallable. Be very careful with it at night.Chances are the plotter will be more accurate than you alone, or u with a hand bearing compass.

And stay further offshore so mistakes with eyes and plotters don't wreck your watch [/QUOTE]

Yup... if you think you'll close W Tassie in the dark heave to/stand off till dawn... the one entrance is dodgey.. till you get behind the island.. or so I'm told..
If on the E coast... lotsa islands there but also lotsa shelter.. dropped the hook in the lee of one and got my head down till dawn and I could carry on again.
Which coast was it..?
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Old 05-01-2015, 06:59   #10
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pirate Re: Another foundering how do chart plotters get it wrong

Also.. folk forget.. the CP does not show where you are.. just where you've been.. and you've no way off knowing the time lag in signals.. so don't stuff up your night vision staring at screens constantly..
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Old 05-01-2015, 07:48   #11
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Re: Another foundering how do chart plotters get it wrong

I can't tell you the number of times, I've been fairly close to shore (inlets, rivers, etc) where the GPS showed my location as being onshore. I can see how it can easily happen the other way around. Also, if the GPS is not tied to a digital compass, then at slow speeds true course as represented by the GPS can be way off. At 4kts, I use the compass to ensure course is being held and not the GPS.
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Old 05-01-2015, 07:58   #12
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Re: Another foundering how do chart plotters get it wrong

novel concept---mark your place with gps/spot/wtf you are using for navigation. see how far OFF your coordinates you are placed....you willl be impressed with the inaccuracy you see.
then go to sea KNOWING you will go aground in rocks and be wary of your surroundings near shore.
here i have found i am 500 yards from my gps position marked. (to the north and east...)
use these tools wisely as they are only tools and are inaccurate in many locales far from us navy bases.....
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Old 05-01-2015, 08:08   #13
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Re: Another foundering how do chart plotters get it wrong

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Which coast was it..?
Tasmania only has 1 coast. It goes all around the island.



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Old 05-01-2015, 08:12   #14
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Re: Another foundering how do chart plotters get it wrong

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Hi guys,

This morning there's been yet another grounding and loss of a sail boat in Tasmania. It's hit an island (I think) in the dark.

My question is, how does this happen? In relation to chart plotters.

Assuming they were following a chart plotter and not recording where they were on a map, how do experienced sailers like Vetus for example get it so wrong?

Are the c-ordinates coming from satalites wrong or are the chart plotters themselves misinterpreting the data?
I can't speculate how this captain grounded his boat but chart plotters contain electronic copies of paper charts. If the paper chart is wrong, the electronic copy will be wrong. Garmin and others don't go out and re-chart the world.
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Old 05-01-2015, 08:21   #15
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Re: Another foundering how do chart plotters get it wrong

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Originally Posted by zeehag View Post
novel concept---mark your place with gps/spot/wtf you are using for navigation. see how far OFF your coordinates you are placed....you willl be impressed with the inaccuracy you see.
then go to sea KNOWING you will go aground in rocks and be wary of your surroundings near shore.
here i have found i am 500 yards from my gps position marked. (to the north and east...)
use these tools wisely as they are only tools and are inaccurate in many locales far from us navy bases.....
You will be exactly on your GPS position to within the accuracy of the GPS (generally 30' or less). Your charts may not be accurate, however.

Mark
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