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Old 03-03-2014, 08:17 PM   #41
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Greetings,
Mr. m. Suuure it is. I refer you to post #38.
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Old 03-03-2014, 08:33 PM   #42
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GPS is more accurate than charts.
You are right about that ... I swear I have not grounded the boat there (red arrow).
The Garmin GPS/chart was showing same ... I went by visual clues ... never fails ...
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Old 03-03-2014, 08:54 PM   #43
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Greetings,
Mr. RW. Well, if the yellow line is your track and you did NOT ground, how accurate is your GPS? I have experienced the same phenomenon (post #'s 35 and 38) with both my nautical AND automotive systems.
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Old 03-03-2014, 09:38 PM   #44
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Yes, the yellow line is the track captured on my iPad that is my secondary GPS plotting device I use to plan and track the cruises.

As I mentioned above, my primary GPS (Garmin with BlueChart) had projected the same situation. No, I have not grounded ... the boat sailed cleanly between the islands (sticking out rocks really).

Either both GPS units failed at the same time or were affected by signal accuracy degradation in very uniform way ... or both charts are inaccurate. I suspect the later as both charts are based on the same source and only source of data.
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Old 03-03-2014, 10:58 PM   #45
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I coastal Louisiana we often cruise over "land" shown on charts without going aground these are sections of marsh or barrier islands, sometimes large, which have eroded away over the years.
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Old 03-03-2014, 11:14 PM   #46
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Greetings,
Mr. RW. "I suspect the later as both charts are based on the same source and only source of data." Yup, probably British Admiralty charts with the data acquired in 1762! Another thing to consider regarding signal accuracy. It could be BOTH units were picking up information from the same satellites. So not necessarily a fault of the units, a fault with the input information from "up there".
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Old 03-03-2014, 11:38 PM   #47
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You are right about that ... I swear I have not grounded the boat there (red arrow).
The Garmin GPS/chart was showing same ... I went by visual clues ... never fails ...
Without some serious triangulation how can you say definitely that the chart was off and not the GPS. There is a stretch near Venice, FL that the plotter shows me running on shore, but I'm actually in the center of the marked channel. I'm not certain that there is not something throwing the GPS off in that area. It is between the Albee Rd. Bridge and the Venice Inlet.

In zero visibility that could cause a real problem as in your case. Then visuals don't work so well. Caution is the watch word.

We run with paper charts, compass, and chart plotter. If the plotter/Gps goes out we can pick right up and continue.
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Old 03-04-2014, 12:47 AM   #48
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1. Without some serious triangulation how can you say definitely that the chart was off and not the GPS. There is a stretch near Venice, FL that the plotter shows me running on shore, but I'm actually in the center of the marked channel. I'm not certain that there is not something throwing the GPS off in that area. It is between the Albee Rd. Bridge and the Venice Inlet.

2. In zero visibility that could cause a real problem as in your case. Then visuals don't work so well. Caution is the watch word.

3. We run with paper charts, compass, and chart plotter. If the plotter/Gps goes out we can pick right up and continue.
You are right on all three points ...

1. I cannot be sure. Could be a fluke, one off, that affected both GPS devices in that very moment. I will go out there this season again and report back.

2. No visual clues in zero visibility ... visual method can't be used. I would not be there, rock shelf 2-6' depth, in zero visibility in the first place ...

3. Same here ... magnetic compass and paper charts on board at all times.

BTW, I think I would feel much safer with redundant GPS/chartplotter (and a radar if I had one) in zero visibility than with the best compass and the best charts money can buy.
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Old 03-04-2014, 12:56 AM   #49
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Greetings,
Mr. RW. Any unusual, ahem... "sightings" while you were in the area?



They're here you know.....
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Old 03-04-2014, 01:08 AM   #50
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Okay, I will watch the sky too next time ...
They are there ... the sky is full of shiny objects.
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Old 03-04-2014, 06:34 AM   #51
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BTW, I think I would feel much safer with redundant GPS/chartplotter (and a radar if I had one) in zero visibility than with the best compass and the best charts money can buy.
LOL. Richard, there are just some times when it is better to stay on the porch. On Moonstruck I have radar and backups for radio, plotter/GPS, and depth finder. So, I guess maybe I'm a belt, suspenders, and thumb tack kind of guy.
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Old 03-04-2014, 06:40 AM   #52
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It's almost always the charts that are incorrect....just old chart info. If you have google earth...and a GPS (not phone based) tablet/computer/smart phone...see where the GPS puts you on a satellite image.

here's more to the story...

Chartplotter Accuracy - Marine Electronics Installation Inc.

This article will hopefully clear up some of the confusion about the accuracy of Chartplotters. I often hear someone say that their chartplotter shows the boat on land as they are heading down a narrow canal, or the channel markers are not in the right location on the Chartplotter. This is NOT a problem with the Chartplotter's accuracy. It is a problem with the Charts that are loaded into the Chartplotter's memory or Chart chip.
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Old 03-04-2014, 07:13 AM   #53
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Leave your GPS and plotter on overnight and logging the track while tied to the dock.

In the morning have a look at where the boat went while you were sleeping.
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Old 03-04-2014, 07:44 AM   #54
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GPS is more accurate than charts.
The GPS is only as accurate as the charts it is based on. The chart chip that you put into your GPS (or the chart installed at the factory) is based on paper charts that you might buy at your local chart store. In some areas these paper charts haven't been updated for several years.

You may find that purchasing an update leaves you with some of the same dated charts you already had.
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Old 03-04-2014, 09:16 AM   #55
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Leave your GPS and plotter on overnight and logging the track while tied to the dock.

In the morning have a look at where the boat went while you were sleeping.
And how fast you traveled!
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Old 03-04-2014, 10:03 AM   #56
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If your autopilot is running off your GPS (as most do), the anomaly near Kingston, ON will have no effect.
Are you thinking of the new GPS compass's? Not so sure saying "most do" would be correct, it may give some a false sense of security. The new GPS compass's such as the Comnav Vector are indeed immune to magnetic interference, as are the ultra expensive "true" gyro compass's but in my opinion I would say the majority still get their heading info from a fluxgate. These, in all the many forms and trade names would be sensitive to magnetic interference.

"Also" electrical interference, a friend had an older Raymarine pilot and whenever he keyed the VHF mike on the bridge near his pilot, it scrambled the heading and caused it to veer every which direction. Pretty funny actually, was a case of a bad installation but it demonstrates electrical interference could be a valid concern.
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Old 03-04-2014, 10:06 AM   #57
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Just to add to the gps in accuracy, according to my Garmin plotter I'm I'm facing toward the inlet of the marina. But then my simrad plotter shows me facing in the right direction, they both use the same gps antenna via N2K/Simnet.
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Old 03-04-2014, 10:24 AM   #58
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1st photo is my boat track while tied at the dock. The "yellow" boat I photo shopped is where I actually was.

Second photo is an example of the radar jamming going on in Norfolk just after 9/11. I found GPS and even the depth sounder highly erratic going through there 2 weeks after the towers fell.

Just shows that all available navigation means should be used.
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Old 03-04-2014, 11:24 AM   #59
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Just to add to the gps in accuracy, according to my Garmin plotter I'm I'm facing toward the inlet of the marina. But then my simrad plotter shows me facing in the right direction, they both use the same gps antenna via N2K/Simnet.
Your Simrad is likely connected to a heading sensor, which it is either set to or defaults to. The Garmin is pointed the last direction the boat traveled (NOT where it was pointed). Just like when you back up, the plotter shows the boat "pointed" in the direction of your stern, or when swinging at anchor, pointed in the direction the boat is moving. The GPS tells you what direction the boat has been moving in, not necessarily the direction it is pointed in.

I regard plotters as representing a theory of where you are. The GPS coordinates on modern systems is very accurate (unless the gov is doing something with it, typically announced in the LNMs), but the match to the plotter's cartography, as shown in the examples above, may be poor. Sometimes it is the cartography, and apparently sometimes not (I have on occasion plotted a GPS fix on a paper chart that was more accurate than the the plotter, but usually they are both identical).

That, to me, is why a pair of good binoculars, good depth sounders and a radar are the most important tools; they convey the reality of where you are, not a theory. I still think, at least for us, an accurate compass and a paper chart are very important back ups and adjuncts, fading perhaps in relevance but far from gone. On long trips I still like to use a big chart as a reference and as a visual log book. If the electronics go down you can quickly DR from your last plotted point. I still often take compass bearings after setting the anchor for the same reason... you never know! Another matter of personal style and ergonomics, but we, especially she, still finds it easier to steer to a given compass course, the compass being straight ahead in line of sight, rather than off the plotter, when "Otto" is not at the wheel.
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Old 03-04-2014, 11:33 AM   #60
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Your Simrad is likely connected to a heading sensor, which it is either set to or defaults to. The Garmin is pointed the last direction the boat traveled (NOT where it was pointed). Just like when you back up, the plotter shows the boat "pointed" in the direction of your stern, or when swinging at anchor, pointed in the direction the boat is moving. The GPS tells you what direction the boat has been moving in, not necessarily the direction it is pointed in. I regard plotters as representing a theory of where you are. The GPS coordinates on modern systems is very accurate (unless the gov is doing something with it, typically announced in the LNMs), but the match to the plotter's cartography, as shown in the examples above, may be poor. Sometimes it is the cartography, and apparently sometimes not (I have on occasion plotted a GPS fix on a paper chart that was more accurate than the the plotter, but usually they are both identical). That, to me, is why a pair of good binoculars, good depth sounders and a radar are the most important tools; they convey the reality of where you are, not a theory. I still think, at least for us, an accurate compass and a paper chart are very important back ups and adjuncts, fading perhaps in relevance but far from gone. On long trips I still like to use a big chart as a reference and as a visual log book. If the electronics go down you can quickly DR from your last plotted point. I still often take compass bearings after setting the anchor for the same reason... you never know! Another matter of personal style and ergonomics, but we, especially she, still finds it easier to steer to a given compass course, the compass being straight ahead in line of sight, rather than off the plotter, when "Otto" is not at the wheel.
The Simrad is not connected to a heading sensor, nor auto pilot. The picture of the garmin at that present time the AP was not hooked up to it, but it now is. So I'll have to check and see if it's right. That's one of the additions we have to do the network is add a heading sensor. Ultimately we want a full maretron monitoring package but were going wait a while. Also a N2K weather station is next on the list.
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