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Old 12-03-2009, 10:19   #76
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Datum errors are certainly problematic. I witnessed/participated in the rescue of a large yacht in the BVI some years ago which had attempted to run a narrow channel at night with his GPS, not noting that the chart datum was not WGS84! Gotta watch that.

But there are other errors as well, quite apart from datum errors. In some parts of the world, islands and coastlines are improperly georeferenced in the first place, so even if your gps is set to use the chart datum there can still be significant errors.

There are other non-georeferencing errors also which can occur with GPS, though occasional.

Anywhere you go, in my experience you've gotta be watchful for charting errors, no matter the maker.

That said, I've found the official U.S. charts of the East Coast -- in general -- to be balls-on dead accurate (that's an industry term :-). The NOAA and NIMA folks are extremely diligent in their work, and it shows.

Charts of the Caribbean, west coast of Mexico, and elsewhere sometimes contain significant errors. The prudent mariner will be .... er, prudent!

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Old 13-03-2009, 02:15   #77
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There are other non-georeferencing errors also which can occur with GPS, though occasional.

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Hey Bill,

I was in the USA in 1993 at the San Andreas fault. There was some jerk there with a machine and I asked him what he was doing. It was a GPS and he was measuring the distance the San Andreas Fault was moving.

So GPS is accurate enough for me

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Old 13-03-2009, 03:30   #78
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Interesting as the GPS/plotter charts are taken from paper chart data. Recently a number of boats have hit reefs off northern Queensland because the reef was not on the electronic chart (error since admitted) but the same reefs are clearly shown on paper charts. My suggestion is always carry paper charts even if you use GPS/plotter most of the time.
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Old 13-03-2009, 04:30   #79
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Recently a number of boats have hit reefs off northern Queensland because the reef was not on the electronic chart (error since admitted).
Reference please Sounds like porkies
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Old 13-03-2009, 05:17   #80
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I was in the USA in 1993 at the San Andreas fault. There was some jerk there with a machine and I asked him what he was doing. It was a GPS and he was measuring the distance the San Andreas Fault was moving.
It is true. They do require taking readings over a very long period of time. THE USGS establishes datum benchmarks using this approach too. This is close enough that you need a low power microscope to see the smallest measurement that can be taken. This is not what you can do in your boat nor ever hope to. It takes weeks of data collection to get this close.

I was involved in a project where we did what we call video surveying using triangulation from digital images taken from a moving vehicle or airplane. The part of the process that made it work required the location of the camera focal plane in 6 dimensions X,Y,Z, Roll, pitch and Yaw. To do that we also needed a very accurate measurement for time as well. To get high accuracy GPS you need two things you can't do on board. You need detailed base recording equipment to record all the GPS data at the same time you are moving about measuring. WAAS is only sort of like that but not as good. The same is true for measuring the San Adrea's fault. You then need to post process the data you collected on both sets and use the base station to correct the field data. You can not do this in real time. WAAS is actually doing this but the delay from when it's computed to when it's broadcasted and received is too long to make it work better than it is now but that still is pretty good. It would be the basis for an automatic landing system for aircraft.

We could locate the camera focal plane at 60 miles per hour from a ground vehicle after we post processed it. I didn't work on the airplane version but it used the similar process. We also had an insurance policy - a ring laser gyro. Back then I think we paid $175,000 USD for them. The GPS signal could be lost for about 45 minutes without losing accuracy. It's also what they put inside the smart missles. From the images we could locate a point in space to within 4 inches absolutely on the planet 100 yards away from the vehicle. That was before they upgraded to HDTV images so the range and accuracy are probably at least double now.

The biggest problem we had was the same one you have with charts. The maps are not good enough. When the map is wrong it does not matter if you locate yourself accurately.
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Old 13-03-2009, 05:25   #81
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Hey Bill,

I was in the USA in 1993 at the San Andreas fault. There was some jerk there with a machine and I asked him what he was doing. It was a GPS and he was measuring the distance the San Andreas Fault was moving.

So GPS is accurate enough for me

Mark
Mark,

In my experience, the issue is not the accuracy of GPS, it's the accuracy of the charts. My WAAS enabled GPS can tell me my lat/lon correctly within less than a boat length, but the position of the boat shown on the electronic chart can still be 1,000' from the physical location of the boat. That is the most extreme case of inaccuracy I've personally seen, from my actual experience in Antigua. Many of the eastern Caribbean charts are notoriously inaccurate for close-in navigation, even when you make absolutely certain that you're using the correct datum.

The problem here is that these charts in many instances are based on surveys done over a century ago. But, as Bill said, I found the charts for the east coast of the U.S. (and the USVI) to be extremely accurate. So, my practice is to assume a chart is wrong until proven right!
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Old 13-03-2009, 07:07   #82
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Mark,
So, my practice is to assume a chart is wrong until proven right!

Never really took that attiude (chart wrong until proven right), but can see the prudence in viewing in that way. My attitude has been to trust the chart but keeping an eye open for descrepancies (which in a sense I guess that says I don't trust the chart). Anyway, this small attiude change in viewing the chart sounds a little safer than I'll trust you until you burn me, now I don't trust you, and I'm not gonna let you burn me.

Papper chart or electronic, I'll take the papper, I still have a hard time reading a book or newspapper online, something about being able to hold it.
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Old 13-03-2009, 08:25   #83
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My thoughts are the prudent navigator should NOT be using paper charts at all.

They are a historical thing used in the past. They are neither updated, nor correct. They are stupidly slow to use even by an expert navigator compared to electronic charts and a chart plotter.

I think they will be illegal within a few years in any area that has modern electronic charting.

If someone ran over my boat killing crew and said they were using paper charts I would sue their butt off.


Mark
I work with theses things.. For a living. You are plain out wrong here. Paper charts will not be illegal. Paper charts are constantly updated, just the same as electronic charts are. All electronic charts are based on exactly the same information as the paper charts, same readings and measurments. If the paperchart is wrong, so is the electronic chart and vice versa. They are not slow to use for someone who knows what he's doing. Paper charts and those funky hand operated instruments have been used for centuries to get in and out of places, where it apparantly has become "impossible" to navigate without a plotter over the past ten years. Strange. If someone ran over your boat, it wouldn't matter if he was using paper charts or electronic charts, as your boat isn't marked on either one of them.

Just using electronics is fine, but you should know how to use paper and keep them on board. Anything else is plain out irresponsible.
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Old 13-03-2009, 08:49   #84
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We're where?

At this point I cannot count the number of times our WASS enabled, accurate to 3 meters, top of the line GPS's, have shown us to be ashore--most recently on the approach to Sarasota Yacht Club where we had to negotiate a very shoaled in channel across Big Pass. The GPS added so much confusion to the cockpit (my wife rushed up from the nav table to warn me we were going aground only to discover open water ahead of us) we ended putting the covers on the damed things and using our Chart, on a plotting board, and a hocky-puck hand bearing compass in the cockpit.

The merit of a chart is that generally, even if the lat-lon positions are "wrong", the relationships of the land masses and land-marks are usually correct. With cross bearings, which are easily taken, one can locate oneself in relation to the shores very quickly and determine required headings. With forward or back-bearings, one can also tell very quickly if one is "on track" and if not whether off to port or starboard.

Different ships--Different long splices eh?
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Old 13-03-2009, 09:50   #85
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The merit of a chart is that generally, even if the lat-lon positions are "wrong", the relationships of the land masses and land-marks are usually correct. With cross bearings, which are easily taken, one can locate oneself in relation to the shores very quickly and determine required headings. With forward or back-bearings, one can also tell very quickly if one is "on track" and if not whether off to port or starboard.

Different ships--Different long splices eh?
This is a direct answer to the question I started this thread with. I like the idea of using the plotter in American waters. When I go to Mexico this summer I wil be more inclined to use the paper charts and the radar. It is the acuracy of the original survey I am worried about.

There are times when a plotter can really save your butt. For instance last week I waw sailing in San Diego Bay. I debated putting the plotter but in the end, even after some teasing, decided to put it at the helm station. I had given the helm to a local. We were coming out of the South Bay and were going over to the anchorage North of the Coronado Bridge. I hadn't studied the charts very much and mostly stayed in the channel but with a local aboard I figured I'd search around a little. Well you know that feeling when your keel goes thru some mud. The boat slows down and then stops. His boat draws 4' mine 6' I zoomed in the plotter and sure enough there is a small shoal right where we were aground. It was a pretty low tide and the water was coming in so we had a couple of choices 1) sail off, 2) wait for the tide to come in, or 3) motor off. We tried one and managed to get the boat turned aroundand headed back to the deeper water. Hung the crew out as far as we could get, set the sails for maximum heel and managed to move a few feet. I thought about waiting but then turned the engine on and managed to get the boat off the shoal w/o more than a minute of engine power and not redlining the engine. Soft bottom, strong boat. No harm no foul.

I try to learn from my mistakes and I can see that studying the charts is very important. If I was at the helm I don't know that I would have found that shoal or not. It looked like deep water. If I had been watching the depth sounder and had had the plotter zoomed in I would have had no problem seeing it. I think to myself what about those times when you set up a bunch of waypoints and don't zoom in to see the hazards along the route. I am pretty good about looking over the waypoints but one little slip up can cause a boat to go aground. Thank God for soft bottoms.
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Old 13-03-2009, 09:59   #86
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Navigating vs Piloting

There's no one answer here. For me, when I'm in and around the crinkly edges, I have everything out and scattered from GPS to charts to cruising guides. When I'm heading away from the edges, well, I know where I *put* the GPS...

Piloting is more of a paranoia than a practice.
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Old 13-03-2009, 10:09   #87
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It helps to read the chartlet showing when hydrographic surveys, supporting the chart, were done. In the Chesapeake, outside of major shipping channels, the data is often twenty to thirty years old (long enough for Grog Island, north of the Rappahannock, to disappear, for example), some dates back to the 1940's (80 years old), and some goes back to the 19th Century. If the data isn't right, no chart, paper or digital, has much hope of being right. A healthy degree of skepticism never hurts...
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Old 13-03-2009, 14:48   #88
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Mark - C-Map admitted there was an error and yes paper charts can also be incorrect but this is why you should have both on board.
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Old 13-03-2009, 15:59   #89
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Jeppsen has a recall web app specifically for a software problem between certain MAX cartridges and their displays. Since they are only checking the c-map cartridges, I think we can safely assume this is a chart error: Recall

Interestingly, the description of how to update cartridges makes it sound pretty much like updating old bios proms...
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Old 13-03-2009, 17:04   #90
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Mark - C-Map admitted there was an error and yes paper charts can also be incorrect but this is why you should have both on board.
Where do you think the C-Map data comes from? It's the same data sources that drive the charts, if not the same basic artwork. Couple that with C-Map's slow update cycle (vs weekly updates for NOAA, and even those lag reality) and "having both on board" is, if anything, worse than nothing if you think C-Map is somehow more right than NOAA. Just because it's on an LCD display doesn't make it right.

And, of course, just because it's on a NOAA chart doesn't ensure 100% accuracy, either. There is room for healthy skepticism when it comes to charts, and navigation in general...
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