hazards of GPS chart plotters - SailNet Community

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Old 02-28-2010
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hazards of GPS chart plotters

The Cruising Yacht Club of Australia has just released its inquiry into the deaths of two sailors (including owner-skipper Andrew Short) in a catastrophic grounding during the Flinders Islet race last October. The inquiry is a crucial exposure of the hazards of relying on GPS chart plotters for safe navigation. I've posted at greater length on my cruising blog (SWEETWATER CRUISING Great Lakes wilderness cruising Recreational boating on Georgian Bay • North Channel • Lake Huron • Michigan UP • Lake Superior • and more) about the inquiry, with a link to the report, but the main takeaway points are that this maxi managed to run smack into Flinders Islet in the night despite having two GPS chart plotters. The owner-skipper was navigating almost exclusively by plotter, and as the inquiry outlined, GPS is prone to position degradation linked to the particular configuration of the satellite constellation. A the time of the grounding, there was an estimated degradation of at least 100 meters. The inquiry is critical of the growing trend of sailors relying exclusively on chart plotters, which can have other problems related to internal software. An eye-opener of an inquiry.
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Old 02-28-2010
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I'd point out that you really shouldn't rely solely on any one navigation tool, especially in bad weather or at night. This is particularly true if you've previously noticed a serious error in navigation information being provided by a particular instrument or instruments as was the case of the maxi-yacht in your article in question.
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Old 02-28-2010
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The big problem with the chart plotters is you can get a coruse/distance to a mark without seeing the territory that course will take you across, typically you are only looking at the destination mark.

You should never navigate by GPS, use the GPS only as a refernce to support/confirm your paper chart plotting.
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Old 02-28-2010
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curious,

this was at night, and in weather.. what other system could they have used...? that would have been less vulnerable to error...

sounds like there should have been a light house type marker...
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Old 02-28-2010
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I've given a lot of thought to the problem of out-of-date and/or inaccurate charts used in chartplotters, but had not thought much about position degradation since, perhaps we're spoiled here in the US with WAAS capability. What is the status with respect to WAAS-like capabilities in that part of the world?

A quote from the Garmin web site:

"You've heard the term WAAS, seen it on packaging and ads for Garmin® products, and maybe even know it stands for Wide Area Augmentation System. Okay, so what the heck is it? Basically, it's a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. How much better? Try an average of up to five times better. A WAAS-capable receiver can give you a position accuracy of better than three meters 95 percent of the time. And you don't have to purchase additional receiving equipment or pay service fees to utilize WAAS."
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Quote:
Originally Posted by sarafinadh View Post
curious,

this was at night, and in weather.. what other system could they have used...? that would have been less vulnerable to error...

sounds like there should have been a light house type marker...
One thing the inquiry criticized about the deceased owner-skipper was that the evidence indicated he was navigating entirely by chart plotter, and had no one on alert at the bow or on the rail to keep a lookout. He seems to have thought he was overstanding the island when he altered course and ran right into it. It was only when a crewmember detected surf that they went to panic stations. Whether or not a lighthouse would have helped, I don't know.
I've also heard from a sailor I know in Texas who told me there was a guy a couple years ago who went aground on Nekker Island by plotting waypoints and then letting his autohelm take over.
Wish I could answer the other query about WAAS in Australia. The main point, as others have pointed out, is not to blindly rely only on a GPS plotter and to do your navigating old school: have paper charts on hand, get out the binos, and keep a lookout. Even with WAAS, a GPS unit can have internal system errors. I'm disappointed the manufacturer in question declined to cooperate with the inquiry. They probably felt the standard disclaimer when you start up the plotter was enough.
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I have seen problems with these on my boat. I will plot in a rough course because were I actually sail will depend on whats there when I get there. Yet I've noticed other crew members (better sailors than me) follow the charted course coming way too close to stuff. At the same time they never tried to follow the course when it is drawn overland, LOL.

Ours often loses signal the GPS signal so it is easy for us to not fall into the trap of thinking it is right, though we do use the charts to determine markers and other information. I'd be pissed if the charts were wrong but then regular charts often are so you have to keep that in mind.
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I have run into several areas where the chart was accurate to itself but the coordinates were wrong. Two summers ago, we anchored in New Brunswick in dense fog using lookouts, radar, and depth sounder and the gps said that we were inland by over 1/4NM when we were actually a few hundred feet offshore in 30 feet of water.

My experience has been that charts produced recently are pretty good on the chartplotter but the older ones are sometimes shifted.
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" A the time of the grounding, there was an estimated degradation of at least 100 meters."
And what kind of GPS didn't throw up a red flag about the inaccuracy? A 2d/vs/3d indicator, or an EPE reading, something that should alert the user that there's a gross inaccuracy expectable?
I know I'd seen 1000 mile errors (literally) with LORAN-C, but I've never known a GPS to be that far off without some ability to let the user know about it. Or user error, like using DDMMSS versus DDdddddd formats.

MC1-
WAAS relies on a ground refernce station in a fixed known position, which reads the GPS signal, compares the position, and sends back the actual error information, IF your GPS can receive that information, and IF there's a WAAS ground station locally, your GPS then learns "Oh, there are atmospheric problems today, add a hundred feet north-northwest to whatever the satellites are telling you." WAAS augmentation is nowhere near universal.
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"A WAAS-capable receiver can give you a position accuracy of better than three meters 95 percent of the time."

that means one time out of twenty it is worse than that. They dont say how much worse.
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