Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 06-01-2015, 17:00   #76
Registered User
 
El Pinguino's Avatar

Join Date: Aug 2013
Location: Punta Arenas ahorra
Boat: 39' Westerly Sealord
Posts: 3,957
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....
__________________

El Pinguino is offline   Reply With Quote
Old 06-01-2015, 17:41   #77
Senior Cruiser
 
fjwiley1's Avatar

Cruisers Forum Supporter

Join Date: Feb 2009
Location: Jacksonville, FL
Boat: Lindsey Center Cockpit 39' Ketch
Posts: 463
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.....😆

Life is good if you Keepa Smilin....😄


Sent from my iPhone using Cruisers Sailing Forum
__________________

__________________
enjoy the winds for they are free

S/V Water Wings
fjwiley1 is offline   Reply With Quote
Old 06-01-2015, 18:08   #78
Registered User
 
leftbrainstuff's Avatar

Join Date: May 2011
Location: San Francisco and Australia
Boat: Liberty 458
Posts: 1,979
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.

Sent from my SM-N900T using Cruisers Sailing Forum mobile app
__________________
leftbrainstuff is offline   Reply With Quote
Old 07-01-2015, 02:06   #79
Moderator
 
carstenb's Avatar

Cruisers Forum Supporter

Join Date: May 2012
Location: Copenhagen
Boat: Jeanneau Sun Fast 40.3
Posts: 4,941
Images: 1
Re: Another foundering how do chart plotters get it wrong

Quote:
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"
__________________
I spent most of my money on Booze, Broads and Boats. The rest I wasted - Elmore Leonard
carstenb is offline   Reply With Quote
Old 07-01-2015, 06:19   #80
Registered User
 
denverd0n's Avatar

Join Date: Jun 2008
Location: Tampa, FL
Posts: 3,953
Images: 6
Re: Another foundering how do chart plotters get it wrong

Quote:
Originally Posted by carstenb View Post
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."
__________________
denverd0n is offline   Reply With Quote
Old 07-01-2015, 06:39   #81
Long Range Cruiser
 
MarkJ's Avatar

Cruisers Forum Supporter

Join Date: Dec 2007
Location: Australian living on "Sea Life" currently in England.
Boat: Beneteau 393 "Sea Life"
Posts: 12,828
Images: 25
Re: Another foundering how do chart plotters get it wrong

Quote:
Originally Posted by carstenb View Post
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.
__________________
Notes on a Circumnavigation.
OurLifeAtSea.com

Somalia Pirates and our Convoy
MarkJ is offline   Reply With Quote
Old 07-01-2015, 06:48   #82
Registered User
 
Group9's Avatar

Join Date: Sep 2010
Posts: 1,599
Images: 7
Re: Another foundering how do chart plotters get it wrong

Quote:
Originally Posted by denverd0n View Post
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.
__________________
Cruising again!
Group9 is offline   Reply With Quote
Old 07-01-2015, 06:57   #83
Senior Cruiser
 
colemj's Avatar

Cruisers Forum Supporter

Join Date: Oct 2005
Location: Presently on US East Coast
Boat: Manta 40 "Reach"
Posts: 10,049
Images: 12
Re: Another foundering how do chart plotters get it wrong

Quote:
Originally Posted by denverd0n View Post
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!

Mark
__________________
www.svreach.com

You do not need a parachute to skydive. You only need a parachute to skydive twice.
colemj is offline   Reply With Quote
Old 07-01-2015, 06:58   #84
Registered User
 
stillbuilding's Avatar

Join Date: Jan 2008
Location: Hong Kong
Boat: Custom Freya 20m
Posts: 961
Re: Another foundering how do chart plotters get it wrong

Quote:
Originally Posted by El Pinguino View Post
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.


Sent from my iPhone using Cruisers Sailing Forum
__________________
stillbuilding is offline   Reply With Quote
Old 07-01-2015, 07:08   #85
cruiser

Join Date: Feb 2008
Location: North Charleston, SC
Boat: Camano Troll
Posts: 4,669
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.
__________________
rwidman is offline   Reply With Quote
Old 07-01-2015, 07:09   #86
Registered User
 
hoppy's Avatar

Join Date: Sep 2009
Location: Melbourne, Australia
Boat: Jeanneau Sun Odyssey 40
Posts: 2,842
Re: Another foundering how do chart plotters get it wrong

Interesting about how many inadequately surveyed areas there are around there.
Attached Thumbnails
Click image for larger version

Name:	unsurveyed.jpg
Views:	61
Size:	17.1 KB
ID:	95109   Click image for larger version

Name:	gull.jpg
Views:	86
Size:	279.0 KB
ID:	95110  

__________________
hoppy is offline   Reply With Quote
Old 07-01-2015, 07:18   #87
Moderator
 
carstenb's Avatar

Cruisers Forum Supporter

Join Date: May 2012
Location: Copenhagen
Boat: Jeanneau Sun Fast 40.3
Posts: 4,941
Images: 1
Re: Another foundering how do chart plotters get it wrong

Quote:
Originally Posted by hoppy View Post
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
__________________
I spent most of my money on Booze, Broads and Boats. The rest I wasted - Elmore Leonard
carstenb is offline   Reply With Quote
Old 07-01-2015, 11:37   #88
Long Range Cruiser
 
MarkJ's Avatar

Cruisers Forum Supporter

Join Date: Dec 2007
Location: Australian living on "Sea Life" currently in England.
Boat: Beneteau 393 "Sea Life"
Posts: 12,828
Images: 25
Re: Another foundering how do chart plotters get it wrong

Quote:
Originally Posted by rwidman View Post
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??
__________________
Notes on a Circumnavigation.
OurLifeAtSea.com

Somalia Pirates and our Convoy
MarkJ is offline   Reply With Quote
Old 07-01-2015, 11:47   #89
Registered User
 
Group9's Avatar

Join Date: Sep 2010
Posts: 1,599
Images: 7
Re: Another foundering how do chart plotters get it wrong

Quote:
Originally Posted by MarkJ View Post
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.

http://wwwhomes.uni-bielefeld.de/cgo...fghanistan.pdf
__________________
Cruising again!
Group9 is offline   Reply With Quote
Old 07-01-2015, 11:47   #90
Senior Cruiser
 
colemj's Avatar

Cruisers Forum Supporter

Join Date: Oct 2005
Location: Presently on US East Coast
Boat: Manta 40 "Reach"
Posts: 10,049
Images: 12
Re: Another foundering how do chart plotters get it wrong

Quote:
Originally Posted by MarkJ View Post
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?).

Mark
__________________

__________________
www.svreach.com

You do not need a parachute to skydive. You only need a parachute to skydive twice.
colemj is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

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


Similar Threads
Thread Thread Starter Forum Replies Last Post
For Sale: Brand New top Dollar Radars, Chart Plotters,Displays 911jimmy Classifieds Archive 2 06-06-2013 23:02
Chart Plotters Exmoor Navigation 12 21-02-2011 20:25
GPS - The Venus Fly Trap - Chart Plotters maxingout General Sailing Forum 5 20-12-2010 13:41
Cockpit Chart Plotters drh1965 Marine Electronics 16 14-04-2008 17:30
Chart plotters?? Chrisc Navigation 7 25-09-2006 17:56



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 08:54.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.