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Old 06-01-2015, 16:00   #76
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Re: Another foundering how do chart plotters get it wrong

Erm... Cook didn't do any Tas/Bass Strait survey work.
And not so much errors in original surveys as ommissions.... if one excludes the longitude issue. Nothing wrong with a good old survey using trig beacons and station pointers.

Back whence we came ... two words .. 'over reliance ' and 'lack of understanding' ... ok thats 5....
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Old 06-01-2015, 16:41   #77
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Re: Another foundering how do chart plotters get it wrong

Sure glad I tied knots in my depth sounder...ooops...I meant Lead Line...I need it when I can't find my "near glasses" for my plotter reading.....😆

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Old 06-01-2015, 17:08   #78
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Re: Another foundering how do chart plotters get it wrong

Two reasons. Tolerance and deviation. Nothing on a chart is absolutely accurate. All data is relative to a datum and the original survey.

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Old 07-01-2015, 01:06   #79
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Re: Another foundering how do chart plotters get it wrong

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Originally Posted by Pauline SA View Post
Agree. GPS Accurately places your boat on the earths surface. Charts were created with vastly inferior plotting devices I.e. Sextants. And DR
Both GPS and Sextants have the same intent, correctly placing a boat on the earths surface. Sextants and hence charts created by Sextants can be wrong by an amount. Pick a number 1,2,3,10 NM? Rely on paper charts at your peril.
If there is a dangerous object shown on a chart give it a wide wide berth.
Why do you say this? Paper charts are NOT more inaccurate than electronic charts. Virtually all electronic charts used on chartplotters are simply digitalized versions of paper.

Perhaps our statement shoudl be "Rely on charts, electronic or paper, at your peril"
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Old 07-01-2015, 05:19   #80
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Re: Another foundering how do chart plotters get it wrong

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Perhaps our statement shoudl be "Rely on charts, electronic or paper, at your peril"
Exactly. Paper and electronic charts are both derived from the same data. Neither is more (nor less) accurate than the other.

The disconnect here, as mentioned already, is that the GPS signal is derived from a satellite calculation completely unrelated to any particular formations, land masses, coastlines, or any thing else that is actually on the Earth. It is a nearly perfect calculation that is based on a theoretical "datum," which represents the Earth.

The charts we use, on the other hand, are not created by satellites in space. They were created--sometimes a very long time ago--by people going out and looking at formations, land masses, coastlines, and all that kind of stuff that actually exists on the Earth. The way the charts were created is fundamentally and dramatically different from the way the GPS calculates your location. That is the underlying cause of the differences.

Then we have the basic I-D-10-T user error, where a person looks at a chartplotter and assumes it must be always and perfectly correct, because... Well, after all, it's a technological wonder and it says right there that the GPS fix is accurate to 20 feet. The fact that the GPS fix is being plotted on a chart that was created in a way that has no relationship to the way the GPS fix is determined, is overlooked. The fact that the beautiful, detailed, digital representation being displayed on the high-definition screen is still just a 100-year-old chart, is overlooked.

And so we have "technology induced groundings."
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Old 07-01-2015, 05:39   #81
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Re: Another foundering how do chart plotters get it wrong

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Why do you say this? Paper charts are NOT more inaccurate than electronic charts. Virtually all electronic charts used on chartplotters are simply digitalized versions of paper.

Perhaps our statement shoudl be "Rely on charts, electronic or paper, at your peril"

Google earth is exact when displayed in a plotter whilst cached.



Also ECN is often updated more than paper so is more correct. Plus its ease of use, except for the computer challenged, makes them more accurate in real use.
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Old 07-01-2015, 05:48   #82
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Re: Another foundering how do chart plotters get it wrong

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Exactly. Paper and electronic charts are both derived from the same data. Neither is more (nor less) accurate than the other.

The disconnect here, as mentioned already, is that the GPS signal is derived from a satellite calculation completely unrelated to any particular formations, land masses, coastlines, or any thing else that is actually on the Earth. It is a nearly perfect calculation that is based on a theoretical "datum," which represents the Earth.

The charts we use, on the other hand, are not created by satellites in space. They were created--sometimes a very long time ago--by people going out and looking at formations, land masses, coastlines, and all that kind of stuff that actually exists on the Earth. The way the charts were created is fundamentally and dramatically different from the way the GPS calculates your location. That is the underlying cause of the differences.

Then we have the basic I-D-10-T user error, where a person looks at a chartplotter and assumes it must be always and perfectly correct, because... Well, after all, it's a technological wonder and it says right there that the GPS fix is accurate to 20 feet. The fact that the GPS fix is being plotted on a chart that was created in a way that has no relationship to the way the GPS fix is determined, is overlooked. The fact that the beautiful, detailed, digital representation being displayed on the high-definition screen is still just a 100-year-old chart, is overlooked.

And so we have "technology induced groundings."
Exactly. You are going to be pretty darn close to the lat-lons your GPS gives you as your position.

The problem is that things on the chart, including land and shoals, are often not exactly in the position the charts put them at. I think in this day and age of instant gratification, people think that with the advent of GPS, somebody ran out and immediately corrected all of the charts for accuracy.
In actuality, there is a very large federal agency that has been attempting to do that for some time, but has a very long way to go. A real eye opener for me, is that they are not even finished doing that for the US Coast yet, according to my neighbor who does this for a living.
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Old 07-01-2015, 05:57   #83
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Re: Another foundering how do chart plotters get it wrong

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The charts we use, on the other hand, are not created by satellites in space.
Not quite true. As MarkJ points out, many of us use Google Earth generated charts and overlays, which indeed are created by satellites in space!

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Old 07-01-2015, 05:58   #84
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Re: Another foundering how do chart plotters get it wrong

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Nothing new to any of this.... they may be experienced 'racers'.... experienced navigators? Probably not....



Back before GPS.... late 80's or v early 90's .... the day job...poking along in northern Bass Strait at about 02dark and keeping a listening watch on 16/12 ...paraphrased a bit but pretty close to what occured....this is what I heard...

' Lonsdale light this is yacht 'Trump Card'

'Go ahead'

'We are in the West Channel and have anchored next an oil rig...where are we'

'I don't know where you are but there aren't any oil rigs in the West Channel'



further chatter ensues..



Then Melbourne Radio VIM buys into the act...



'Trump Card... what does this oil rig look like?'

'Oh... lots of bright lights and three big towers sticking up in the air'



' You are next to a Maersk jack up which is anchored off Cowes, (Phillip Island, Westernport)'



a pause...



'Oh ... I think we must have hung a right a bit early... we will go out and have another shot at it when the sun comes up....' ( that bit is close to verbatum)



Anyone knowing this area will understand how truly remarkable that feat was... the rest of you can crack open OpenCPN and go have a looksee.



Nothing new under the sun........

Well, I quess this was a variation on an event quite a few years before when a full rigged sailing ship accidentally sailed past The entrance to Port Phillip, hung a left into Westernport Bay and sailed up the eastern side of French Is until the water ran out. Several days of confused messages before steam tugs from Melbourne finally located it and tried to salvage.
Actually if you check out the charts and sail the area you will understand it is an easy error after a long DR passage.


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Old 07-01-2015, 06:08   #85
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Re: Another foundering how do chart plotters get it wrong

The GPS is normally accurate to within several feet. The chart, not so accurate. My GPS chart shows me on the other side of my marina fairway, about 60 feet from where the chart shows my slip. A slip neighbor had the same GPS but an earlier version of the chart and his showed him in the correct position. He updated his chart chip and it showed his boat on the other side, same as mine.

The only thing that changed was the version of the chart chip.

I can say from personal experience that there are sections of the AICW between Charleston, SC and Georgetown, SC where the plotter will show the boat traveling on land when it is actually in the narrow land cut. It's not the GPS, it is the chart that is inaccurate.
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Old 07-01-2015, 06:09   #86
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Re: Another foundering how do chart plotters get it wrong

Interesting about how many inadequately surveyed areas there are around there.
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Old 07-01-2015, 06:18   #87
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Re: Another foundering how do chart plotters get it wrong

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Interesting about how many inadequately surveyed areas there are around there.
yep, and if you really study the chart history - you'll find lots where although they are "adeuately surveyed" - they were surveyed with a casting lead and a row boat at 100 meter intervals.

Looked at in this light - you can imagine what "inadequately surveyed" means
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Old 07-01-2015, 10:37   #88
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Re: Another foundering how do chart plotters get it wrong

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The GPS is normally accurate to within several feet.
In the early 1990's I visited California (a town in Northern America, I thnk ) and they were measuring the daily movement of the San Andreas fault using..... What? sextant? Piece of bra string? 2 beer bottles and broomstick?

GPS.

My latest GPS looks at 64 satelites simultaneously. Its so accurate you could fly a missile through a window in Iraq. Or has someone already thought of that??
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Old 07-01-2015, 10:47   #89
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Re: Another foundering how do chart plotters get it wrong

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In the early 1990's I visited California (a town in Northern America, I thnk ) and they were measuring the daily movement of the San Andreas fault using..... What? sextant? Piece of bra string? 2 beer bottles and broomstick?

GPS.

My latest GPS looks at 64 satelites simultaneously. Its so accurate you could fly a missile through a window in Iraq. Or has someone already thought of that??
Only if the window is where it is supposed to be. And, even then, there is still the human factor to mess things up.

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Old 07-01-2015, 10:47   #90
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Re: Another foundering how do chart plotters get it wrong

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In the early 1990's I visited California (a town in Northern America, I thnk ) and they were measuring the daily movement of the San Andreas fault using..... What? sextant? Piece of bra string? 2 beer bottles and broomstick?

GPS.

My latest GPS looks at 64 satelites simultaneously. Its so accurate you could fly a missile through a window in Iraq. Or has someone already thought of that??
To be fair, consumer GPS's (even your 64 channel) are only accurate to a few feet. Surveying and other high-resolution work uses specialized differential GPS's that are accurate to a centimeter (maybe less?).

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