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Old 07-01-2015, 12:00   #91
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Re: Another foundering how do chart plotters get it wrong

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Originally Posted by stillbuilding View Post
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.
Indeed , no such thing as a new stuff up. I have seen a photo taken in the 1920's of an american timber schooner scooting past Cowes 'inwards to Melbourne'.
Major difference with TC was that she was a Melbourne boat going 'home' from Hobart on a clear dark night... no long DR passages involved.

Back on topic... the main thing required when using any 'aids to navigation' -including the Mk 1 eyeball - is common sense which is a rather uncommon item.
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Old 07-01-2015, 20:28   #92
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Re: Another foundering how do chart plotters get it wrong

It seems to me that a very basic addition to chart plotter software would be to issue a warning when the autopilot plot crosses over a hazard. The data is there even if you cannot see it on the chartplotter.
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Old 07-01-2015, 21:29   #93
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Re: Another foundering how do chart plotters get it wrong

Mark J I think your comment about measuring the movement of the San Andreas Fault was spot beginning just after the word "sextant "

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Old 07-01-2015, 21:32   #94
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Re: Another foundering how do chart plotters get it wrong

Such a feature is not nearly as simple to implement as it would seem. And it would invite even more reliance on machinery in lieu of seamanship which seems to me the wrong direction we should be heading.
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Old 07-01-2015, 21:57   #95
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Re: Another foundering how do chart plotters get it wrong

AND still rely on the chart data. Stay out at night, or use a RADAR to confirm what the chart says is correct. Sonar as well is good!
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Old 07-01-2015, 22:00   #96
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Re: Another foundering how do chart plotters get it wrong

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Originally Posted by hellosailor View Post
"your GPS can have up to a 10% variation on your location at any given time."
Your friend may have los something in translation. While the civilian signal MAY be degraded, IIRC it was Jimmy Carter who ordered that the intentional signal degradation be stopped, and it would take a similar presidential order or other directive "from the top" before any degradation was introduced to the system as a whole.


Extremely unlikely that the US government would degrade the signal over some godforsaken rock halfway around the world.

And "10%" ?? is definitely a translation error. The folks who run it, just don't talk about it that way. 10% without saying 10% of WHAT, is gibberish.

I was tied up to the dock on a day when war games were going on, that moved my GPS location into a farmers field 6 miles away.

Albeit there was a radio broadcast on channel 16 that it was going on, but it was quite interesting to see.

I say, trust it... but verify there are no geo-political upheavals in your area, if you are close to anything hard that you can't see...
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Old 07-01-2015, 22:03   #97
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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..

Pirate method: put yer eye patch over one eye on deck, then switch eyes when below deck and exposed to light. It takes over 30 minutes for your eyes to fully adjust to darkness; a proper pirate eyepatch allows you to preserve perfect night vision in one eye while below decks to prick the chart. Arrr! Lol...
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Old 07-01-2015, 22:28   #98
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Re: Another foundering how do chart plotters get it wrong

I had that problem when sailing into Tonga. Upon radioing for some help, I was told the same thing happens quite often as charts have errors for that particular entrance. When sailing into new port or island I always planned my trip to allow me to arrive after sunrise. Not to make the fastest time but the needed speed to arrive safely and not in the dark. Iceman
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Old 08-01-2015, 00:05   #99
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Re: Another foundering how do chart plotters get it wrong

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I was tied up to the dock on a day when war games were going on, that moved my GPS location into a farmers field 6 miles away.

Albeit there was a radio broadcast on channel 16 that it was going on, but it was quite interesting to see.

I say, trust it... but verify there are no geo-political upheavals in your area, if you are close to anything hard that you can't see...
A non issue with radar, compass, sounder and chart.

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Old 08-01-2015, 00:08   #100
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Re: Another foundering how do chart plotters get it wrong

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I had that problem when sailing into Tonga. Upon radioing for some help, I was told the same thing happens quite often as charts have errors for that particular entrance. When sailing into new port or island I always planned my trip to allow me to arrive after sunrise. Not to make the fastest time but the needed speed to arrive safely and not in the dark. Iceman
Never rely on chartplotters alone in coral particularly in the Pacific.
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Old 08-01-2015, 00:15   #101
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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"
Not sure how many paper charts you carry on board or when they were last updated for changes to navigation information.
The beauty of electronic charts, assuming they are current, is that a single CMAP SD Card (or Navionics etc) contains copies of hundreds and sometimes thousands of paper charts. One SD Card covers all of the Mediterranean for example.... the equivalent of about 1500 paper charts from memory.
The plotter seamlessly selects the electronic chart for the area you are in. (Remembering the Paper/Electronic charts still contain errors from Captain Cook) As you zoom in on the chart plotter the plotter software seamlessly moves to the more/most detailed chart. The one that shows the small hard objects...Unless you are carrying the full range of paper charts ( read dozens or hundreds and they are up to date) you will likely not have all the information required to safely navigate your route. Now lets all fess up. Is there any one that carried a full set of paper charts, that are up to date and don't have stains or creases, for their last passage? Using a good quality plotter and an up to date SD card you do have all the up to date information for your passage.
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Old 08-01-2015, 01:09   #102
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Re: Another foundering how do chart plotters get it wrong

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Not sure how many paper charts you carry on board or when they were last updated for changes to navigation information.
The beauty of electronic charts, assuming they are current, is that a single CMAP SD Card (or Navionics etc) contains copies of hundreds and sometimes thousands of paper charts. One SD Card covers all of the Mediterranean for example.... the equivalent of about 1500 paper charts from memory.
The plotter seamlessly selects the electronic chart for the area you are in. (Remembering the Paper/Electronic charts still contain errors from Captain Cook) As you zoom in on the chart plotter the plotter software seamlessly moves to the more/most detailed chart. The one that shows the small hard objects...Unless you are carrying the full range of paper charts ( read dozens or hundreds and they are up to date) you will likely not have all the information required to safely navigate your route. Now lets all fess up. Is there any one that carried a full set of paper charts, that are up to date and don't have stains or creases, for their last passage? Using a good quality plotter and an up to date SD card you do have all the up to date information for your passage.
I confess - I'm a nerd (and hopelessly old). My charts are updated every time the "Notices to Mariners" comes out. But I do realize I'm one of very very few.
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Old 08-01-2015, 02:20   #103
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Re: Another foundering how do chart plotters get it wrong

To those who harp on about GPS is so accurate (to say a couple of metres), please don't forget they are only accurate to a particular datum.

Any one location on the earth's surface can have more than one set lat/lon co-ordinates; it all depends on the survey datum being used.

While we have got used to thinking there is only one datum in use (WGS84), there are some (many?) charts still around that don't use this datum. In fact, I can chose a couple of dozen different datum's within my GPS so the GPS will provide a different lat/lon for the same spot depending solely on the datum selected.

Does anyone know what datum google earth uses?????
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Old 08-01-2015, 06:04   #104
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Re: Another foundering how do chart plotters get it wrong

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Originally Posted by colemj View Post
To be fair, consumer GPS's (even your 64 channel) are only accurate to a few feet.
Considering that we are at the helm piloting the boat, that would seem to be accurate enough. The GPS is a tool to help navigate. What you see in front of you with your own eyes is the most important.
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Old 08-01-2015, 06:57   #105
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Re: Another foundering how do chart plotters get it wrong

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Originally Posted by zboss View Post
It seems to me that a very basic addition to chart plotter software would be to issue a warning when the autopilot plot crosses over a hazard. The data is there even if you cannot see it on the chartplotter.
What good would a warning do if you are already passing over a hazard? Why just when on autopilot?

There are charting programs that do list hazards under any charted route, as well as chart plotters that with "fly" routes for you to look for hazards. Some of these even can ring alarms when you approach hazards.

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