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Old 28-09-2009, 07:36   #31
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- - Do you know any names of systems/brands/programs that do this?
Not off the top of my head, no, but I suspect Furuno NavNet 3D does it, and I also suspect it's called "chart offset": you nudge the chartplotter info to match the radar display, and the lat/lon values for all that screen's information are corrected to reflect reality. It's a sort of "cook your own datum" function, and, like pasting AIS info over a plot, it's one of the few reasons I would like to mix and match data on one screen.
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Old 28-09-2009, 08:06   #32
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Osiris,

I could do that even with my old Raymarine C80 set. It's not automatic though, you create an offset in the chart-setup.
I can only guess and hope that the Furuno 3D set I'm buying can do it too, I just don't know.

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Old 28-09-2009, 08:10   #33
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One reason to have a compass is that the heading from the GPS is not real time. There was an article in one of the sailing magazines where someone was trying to navigate in fog with current attempting to enter a narrow channel. Trying to steer by GPS was hopeless, he kept making wild oscillations. When he finally used his compass to hold a constant heading, and then use the GPS to determine which heading he wanted, he managed to go where he wanted.

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Old 28-09-2009, 16:35   #34
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One reason to have a compass is that the heading from the GPS is not real time. There was an article in one of the sailing magazines where someone was trying to navigate in fog with current attempting to enter a narrow channel. Trying to steer by GPS was hopeless, he kept making wild oscillations. When he finally used his compass to hold a constant heading, and then use the GPS to determine which heading he wanted, he managed to go where he wanted.
John
- - Were they using a new 12 to 20 channel GPS hooked into a digital chart navigation program? Or just trying to use a GPS such as a Garmin 76 which displays a little triangle and a "leader line" off it? Both the modern GPS's and Electronic Charts display/navigation systems contain an "averaging" system to dampen the swings in heading put out by typical GPS's.
- - If you watch the "leader line" or "track prediction" line on GPS's and even if displayed on chart navigation systems you will see these things swing left and right as your vessel rolls and twists in the seaway. The GPS is simply recording the fact that its antenna is sensing that you are not going straight but "zig-zagging" due to the waves, etc.
- - The track heading readouts of the navigation and GPS's contain the "averaging" system which can be set to different degrees of averaging in the most set-up menus.
- - If you are doing it "manually" you have to provide the "averaging" in your own head and use the "general direction" to set your course. In tricky currents or tight seaways or channels I always operate in compass mode or heading mode and watch the "little boat" to see if it is drifting port or starboard from the plotted line I want to follow. I average in my own mind how much time the little "leader/predictor" line is spending either port or starboard to judge how much correction is need to keep on course.
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Old 28-09-2009, 17:06   #35
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Another way to avoid $20 charts is to buy the area chartbooks. W florida and keys is around $100. Has coastal offshore and piloting charts that are handy turn the page to next chart as you go down coast.

Gotta love those gps jumps. I always get a reading like 250 kts top speed reached when making a trip.
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Old 29-09-2009, 15:47   #36
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The zig-zagging is most often caused by the rolling of the boat, which makes the GPS antenna swing sideways. This is why GPS antenna's should be mounted as low as possible and not on top of the spreaders like many do.

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Old 29-09-2009, 21:47   #37
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Gotta love those gps jumps. I always get a reading like 250 kts top speed reached when making a trip.
I can't tell you how refreshing it is to hear that others have experienced this. Some sailors have more faith in the infallibility of GPS to work 100% than some creationists have in the first ten chapters of Genesis.
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Old 05-10-2009, 15:17   #38
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This rescue boat skipper lost his eye and he was criticised for relying on GPS as his only means of navigation.

Lookout! – issue 14, September 2009 - Maritime New Zealand
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Old 05-10-2009, 15:27   #39
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I hate to break the news to you, but most nautical charts are not correct. Add in the 140 different Datums used to make them around the world and it is rare indeed that the chart and the GPS position will ever agree.
Failing to set the GPS to the correct chart datum figures prominently in coast guard accident reports.
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Old 06-10-2009, 09:56   #40
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This rescue boat skipper lost his eye and he was criticised for relying on GPS as his only means of navigation.

Lookout! – issue 14, September 2009 - Maritime New Zealand
Showing that even those who know better frequently don't.

Thanks for this. It supports my instinctual response to watching people peer at displays instead of looking beyond the bow.

I also dislike the habit of radar overlays on chart plotter displays. I think it's useful for determining the offset of a plotted chart, but it's best to use radar alone to avoid exactly this sort of confusion.
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Old 06-10-2009, 10:33   #41
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The question I would ask is do you know how to navigate?

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So I'm doing the electronics on my boat,a 63 ft steel shrimp boat conversion, and I'm looking at compasses. Some books stress how important they are and say to get the best you can afford, but with the redundancy of electronic compasses and GPS systems, how often do you really rely on a free-standing compass? I've got a Raymarine E120 with a Coursemaster autopilot.
What do you guys use and recommend?
Also, how detailed are your paper charts And how often do you pull them out?
Sorry about these dumb questions, but I'm a few days from pushing off and know nothing.
Don't worry, she's real slow and loud, a DD with straight exhaust, so I won't be sneaking up on you. You'll hear me for a good hour before I come over the horizon.
Not to insult you, but not knowing your history... do you know how to navigate? Many of the answers her are from people that know navigation. I think its absolutly imperative to have a decent compass and paper charts! I use mine often if for nothing else for practice. I am teaching my kids as well.
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Old 06-10-2009, 18:36   #42
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Aside from all the other reasons to have a compass...

A study was done (I think about 10 years ago?) where seasoned skippers and novices were told go from point A to point B using only a compass. They were tracked on GPS. The results were a little surprising. The novices traveled the same distance and time as the seasoned skippers, but the skippers made far fewer course changes. They went further from the course. In other words, they were more relaxed and didn't worry about holding it exactly on course.

I find that it is easier to steer smoothly and with less hassle by compass. I'll let it swing back and forth a few degrees, just keeping in mind to try to make sure the average isn't favoring one side. Then I check the GPS every once in a while to make sure I haven't strayed too much. BTW: if you don't practice, it IS easy to find you've been erring more to one side than the other.

Basically, steering by compass is more relaxing to me. And I figure the practice can't hurt in case I have to some day.

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Old 06-10-2009, 18:52   #43
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I find that it is easier to steer smoothly and with less hassle by compass.
Hi Dan,

The only time we steer compared to using the auto pilot is miniscule in our type of passage cruising. Using an auto pilot one sets it up (not so its conected to the plotter) so that the boat is going along the course line (or within our allowed cross track error) on the GPS plotter. The compass is superfluous. The compass is used occasionally.... verrrrrry occasionally

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Old 11-11-2009, 08:54   #44
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on a steal boat u have to make a deviation card, that is a must, there is a lot of electrical interference and know how to convert magnetic to true, c-d-m-v-t. Look to have a compass or charts anything more is always good to have then not to have nothing at all. so learn more about it. I use all the instruments, and be confident to what you know will not fail.
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Old 11-11-2009, 09:21   #45
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NOAA these correct charts every time, we the navigators find errors under way, and fill the corresponding form in NOAA web site to let them know about a error. So it is not if there is a error. Just have the time to fill the information in there web site to let them correct a unknown change to the cartographic prints, and electronic charts they have, and the problem will be solved. So other navigators will have a accurate chart's and electronic charts as well. We have to help too.
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