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Old 05-01-2015, 08:21   #16
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Re: Another foundering how do chart plotters get it wrong

were motoring in the islands of the perlas islands with chart plotter, depth sounder and paper charts up graded by a local ,,we were hundreds of yards from any structure above the water but was mid tide and hit a rock hard ,, tore a chunk out of one of my keels " solaris 42 " Jared my teeth .. no other damage .. we plotted it but the captain i was sailing with did not pass the coordinates on to me .. " **** happens " if i had been a power boat with a thin bottom and a 3 foot draft i would have had a major hole ..
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Old 05-01-2015, 08:26   #17
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Re: Another foundering how do chart plotters get it wrong

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Originally Posted by colemj View Post
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
That has been my experience. Both with marine and auto GPS.
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Old 05-01-2015, 08:31   #18
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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.
Could you suggest to your neighbor that they check out the Pacific coast of Baja? Some major harbors are off by as much as 3/4 nm. While you are at it, you might suggest that NOAA figure out some way to crowd source its data. But that suggestion should not be made to your friend or his boss (or maybe even anyone in their division) -- they may be talking themsevels out of a pretty sweet gig.
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Old 05-01-2015, 09:07   #19
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Re: Another foundering how do chart plotters get it wrong

Not a "all you need is a chartplotter" vs why did this happen? thread. I think I will go back to navigating in my trusty bluewater boat.
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Old 05-01-2015, 09:23   #20
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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
...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?
No. Their navigation skills sucked.

Many navigators nowadays are too lazy to bother looking at anything but their GPS or chartplotter, when they really need to use ALL the data available to them, like their depthfinder, which might have alerted them to the bottom coming up to meet them.
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Old 05-01-2015, 09:40   #21
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Re: Another foundering how do chart plotters get it wrong

Have had the same problem with GPS reading North and east. Sailed right down the main road on Grand Isle, Louisiana about a year ago. Also ran aground in the Chandeliers as the high spots were "East" of where the GPS and charts said they were. I was told that NOAA had not updated US charts since 2005. In the gulf we have had how many hurricane induced restructurings since then? Bottom line (pun intended) is sonar should be set to the keel and believed. CPs are ballpark and plus or minus 30 feet misses a lot of channels.
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Old 05-01-2015, 09:57   #22
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Re: Another foundering how do chart plotters get it wrong

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Originally Posted by boatman61 View Post
Must have been a singlehander..
I hear they are prone to this...
Maybe.... But they usually stop doing it after a time or two...

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Tasmania only has 1 coast. It goes all around the island.
HAAAAAAA!!!
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Old 05-01-2015, 09:57   #23
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Re: Another foundering how do chart plotters get it wrong

If there is a good antenna installation (good antenna siting), you will easily get better than 30' accuracy in position with GPS. Velocity accuracy is less than 1 knot. Velocity provides an accurate heading and is much less than 4 knots (less than 1 knot).

Satellites located at elevation angles less than 5 degrees are not used in the Position/Velocity calculation (unless overridden by the user). These signals cause large errors in position/velocity, hence they are not used. Error characteristics from the other satellite signals (errors from the troposphere, ionosphere, satellite, etc) are included in the predicted error. On a typical day with good antenna siting, you should get 3 to 4 meters of position error in the horizontal. You get better than this with WAAS, but WAAS isn't available for offshore cruising.

There are events such as major sunspot eruptions that can add errors not used in GPS predicted error. These events are atypical, but they do occur.
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Old 05-01-2015, 10:40   #24
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Re: Another foundering how do chart plotters get it wrong

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Originally Posted by Group9 View Post
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 believe I read somewhere that, due to low funding, it will be 2040 by the time the charts are all updated. With the widespread use of GPS for many different things, you might think it would be a higher priority!
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Old 05-01-2015, 11:22   #25
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Re: Another foundering how do chart plotters get it wrong

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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..
If you have one of the older GPS that ONLY update at 1Hz (very old) then, traveling at 10 knots, one would traverse about 5.144 meters from your previous fix. Again, assuming an old GPS you should have had a fix that is accurate to 15 meters. So, you are off by 21 meters from your true position. I would not be blaming the time lags for errors. More likely, if there are errors, it is due to bad chart data.

There is another problem that few people mention. A chart plotter at the moment shows pretty pictures. It should also help the sailor in UNDERSTANDING the pretty pictures. By processing the chart data and understanding what are depth soundings, rocks, reefs as well as islands... a chart plotter ought to scream BLOODY MURDER as it realizes it is on a doomed course. The same way we have AIS collision avoidance information and alarms working on any modern AIS receiver.

A common problem with Chart Plotters is as they zoom out, details are hidden. (Otherwise they would be unreadable) Doing this by the Coronado Islands (on the North Pacific Coast of Mexico) makes a small island in between the two larger ones vanish... There was speculation that this may have led to the tragedy of the Aegean during the 2012 Newport to Ensenada Yacht Race

Of course, there is the elephant in the room of bad data. Unfortunately, we (as humans) put a higher value on knowing the surface of Mars and the moon... and miss out on the ocean on our little planet. Even though it compromises 71% of the surface of the planet. To be fair, strategically, it makes sense for each country to not let a potential enemy know how best to approach their shores safely. Hopefully, this too shall change, as the current practice of inaccurate and outdated maps is insane. (in my opinion of course )

Some countries are becoming smart and giving away the data freely. Hopefully more countries shall realize that this is the moral, mutually beneficial, and proper way to do this as well.

ok, enough of ranting.. Safe sailing... watch that chartplotter, and learn why it is showing you what it thinks is reality.... and how you may be misinformed due to zoom or ... bad data or ....
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Old 05-01-2015, 12:29   #26
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Re: Another foundering how do chart plotters get it wrong

This is interesting as the logic and flows. When the Korean Air liner hit short of the runway in San Fran a while ago, the investigation showed that the pilots were warned that they were too low. A study shows we ASSUME the interments are right, as they did. Rather than believe the alarms, they took 10 seconds...a long time in a plane...to check the instruments, rather than believing what they were telling them...and crashed. This thread starts off "How do the instruments get it wrong?" It is safer to work under the question, "How do they get it right?" I have my radar on when anywhere close to land, and never advance waypoints automatically. I have been in places where the channel markers are clearly out of place (Cuba) At least one govt agency in the USA works well...It is the USCG. Because the markers are in the right place, I made an assumption they always were and grounded my boat in Florida. Luckily, it was sand.
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Old 05-01-2015, 12:48   #27
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Re: Another foundering how do chart plotters get it wrong

The skipper was perfectly competent. The software flawless, and the charts perfectly accurate.


Jihadis moved the island in the dead of the night, in order to wreak havoc on Australian recreational night sailing.


Next.
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Old 05-01-2015, 12:52   #28
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Re: Another foundering how do chart plotters get it wrong

RusticCharm,

In our experience, many of the charts themselves are inaccurate (as carsten mentioned), which is one reason people's tracks go over the land, when they know darn well they were in the water. On the west coast of Tassie, there are huge areas marked as unsurveyed, and many outlying rocks. It seems like the charted ones are where they are shown.

If the grounding you mention occurred during the most recent frontal passage, well there were some embedded thunderstorm cells with high winds (55 knots gusts in Port Cygnet with a downpour of hail, as well as torrential rain), the skipper might have had too much sail up or lost visibility in a local wx situation.

In other cases, you have to zoom in closely to see where to go, and people avoid doing that because they lose the larger picture (this might be what happened to put Team Vestas on Caraja Cargados Shoals.)

Also, modern people with good quality electronics tend to trust them more than old fashioned methods.

Of course, i don't know what happened in the case you mentioned, but I do know of one incident where the skipper had gone to check the boat's position, when it ran into the island of Lifou in New Caledonia. A current swirly may have been setting him unbeknownst). So you can have a case of what ultimately comes down to insufficient attention, and, as Zeehag suggests, not allowing quite enough clearance off the hard bits.

As one of the antiques who started out with Jim using the sextant, and paper charts, one of the things you learn is to give everything that might sink you a wide berth at night, and to keep your DR (dead reckoning) going regularly. We tend to think our GPS track represents the one true reality, and following it as if it were the yellow brick road can get you in trouble when the chart is inaccurate.

Boatie wrote of heaving to for a rest and better visibility, a tried and true practice, but in the incident you mention, perhaps that was not an option.

Ann
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Old 05-01-2015, 13:35   #29
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Re: Another foundering how do chart plotters get it wrong

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.....one of the things you learn is to give everything that might sink you a wide berth at night, and to keep your DR (dead reckoning) going regularly.....
The word.
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Old 05-01-2015, 13:54   #30
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Re: Another foundering how do chart plotters get it wrong

Cartographers make mistakes such as found in the generally accurate Lowrance Insight mapping software which fails to note the nearly half mile long east-west breakwater to the S of Bar Harbour, ME. At low tide the breakwater is as dry as the gravel bar at Bar Harbour, but submersed by at least 5 feet at high tide. This electronic chart shows the passage as navigable with minimnum water of 13 feet at low tide. If a mistake of this magnitude can be made and left uncorrected on mapping software in one of the most commonly mapped and traversed passages on the Maine coast, then I would suggest that use of any of this company's cartograpy and perhaps others should be condidered suspect until personal navigational experience of the area proves otherwise.
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