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Old 27-01-2012, 11:52   #31
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Re: navigation trouble

The GPS is likely correct. Fire up the other unit ans see if they agree.

The land must be wrong then.

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Old 27-01-2012, 11:59   #32
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Re: navigation trouble

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Originally Posted by barnakiel View Post
......
The land must be wrong then.

b.
It usually is
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Old 27-01-2012, 12:02   #33
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Re: navigation trouble

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Originally Posted by SteveT View Post
IslandHopper - It was not your post i was referring to. I absolutely agree that before you venture very far in any kind of boat some kind of formal navigational training is a big help and particularly if you go offshore. FWIW I am definitely not a new age type - not even sure what one is. I am advised that I am a Grumpy Old Man, although I prefer to think I am Master under God (and my Wife) and not necessarily in that order.
Roger Roger, i just read back and realised....

By the way, i always make myself available should God seek counsel or advice, I'm just that kind of guy.......(and i have my wifes permission to say so)...
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Old 27-01-2012, 12:04   #34
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Re: navigation trouble

To Becaldoc:
Just to understand you better and have the real situation: How do you know the GPS is giving you a position 200' N from the real one? What reference are you using: chart, dead reckoning, or... ?
Thanks
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Old 27-01-2012, 12:17   #35
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Re: navigation trouble

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Originally Posted by Wotname View Post
And perhaps I also haven't explained myself well (or perhaps I am just grumpy tonight )

I am supposing that the OP's stated NEW GPS is exciting and what better way to get to know all its functions (and accuracy) than playing with it on a fine sunny clear day when good pilotage is easy and its safe to discover the benefits and shortcomings of the new toy.
I agree with this summation but I have just re-read the OP's original post and it reads to me as they were actually navigating with the GPS and through some accident of good fate found it to be inaccurate. If they had been testing it why not post that is what they were doing. Then reading further the second post re chart datum confirmed to me they were relying heavily on the GPS.

It is great they asked for help as they are at the beginning of the learning curve with GPS and are seeking knowledge. We are all in that curve and none of us should be afraid of asking for help in new areas but we should also ensure we read from whatever source as much as we can on what we want to know....then go and practice.
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Old 27-01-2012, 12:31   #36
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Re: navigation trouble

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Thanks again Wotname. I appreciate your helpful attitude. Too bad it is not universal.
If I understand you correctly, the question is whether the chartplotter GPS is using a different datum from the chart that came preloaded into the chartplotter. Seems unlikely if not negligent. The charts are up-to-date NOAA charts using WGS 1984. Now that I know what a datum is, my initial research on the internet tells me that this is also the standard for all commerical and recreational GPS units, and has been since 1997. So it seems that conflicts between charts and GPS data are a thing of the past.

I will check the GPS co-ordinates with the paper chart next time I experience a discrepancy.
I wouldn't assume the entire world is completely up to date so fast. RNC charts can still be in older datums. I believe that most of the west coast of Vancouver Island's charts are still NAD 27. About 5 years ago a friend was going to use his chartplotter to enter a harbor near the north end of the western side that he had never been in before in poor visibility. The crew basically mutinied. Later on after they entered under conditions where they could see, the crew found that the position on the GPS was off enough to be on the rocks entering. Charts in GPS were NAD27, GPS set for WGS84.



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Old 27-01-2012, 18:11   #37
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Re: Navigation Trouble

Yep. The datum is one thing. In any chart, no matter what datum, an object may be plotted in a place that is off its satellite derived position.

Not common in well frequented areas but a commonplace elsewhere.

b.
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Old 27-01-2012, 18:44   #38
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Re: Navigation Trouble

Given the questions the OP is asking I suspect that he has little understanding of charts and strongly recommend a piloting course before even turning on the GPS. A thorough understanding of piloting and charting skills is the basis for an understanding of GPS, its flaws and limitations.

GPS occasionally tells fibs
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Old 27-01-2012, 18:57   #39
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Re: Navigation Trouble

Do we as yet know how he knew the gps was 200 ft in error??? I like to start at the beginning.
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Old 27-01-2012, 19:00   #40
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Re: Navigation Trouble

bcaldoc, to reiterate what some helpful people up thread wrote, I believe all current NOAA charts are WGS84 datum, but it is possible that for some reason your GPS unit has been reset to another datum, which could cause position errors. Somewhere in your instruction book or online you can find information on how to check what datum your GPS unit is set to, and you should readjust it to WGS84. Not every chart in the world is WGS84--once you leave the USA you may have to reset your GPS to match some charts, particularly very old charts you might find in places like the South Pacific.
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Old 27-01-2012, 19:09   #41
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Re: Navigation Trouble

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bcaldoc, to reiterate what some helpful people up thread wrote, I believe all current NOAA charts are WGS84 datum, but it is possible that for some reason your GPS unit has been reset to another datum, which could cause position errors. Somewhere in your instruction book or online you can find information on how to check what datum your GPS unit is set to, and you should readjust it to WGS84. Not every chart in the world is WGS84--once you leave the USA you may have to reset your GPS to match some charts, particularly very old charts you might find in places like the South Pacific.
Correct. My garmin541s has a chart setup page wherein one can select chart datum.
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Old 27-01-2012, 19:10   #42
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Re: Navigation Trouble

BTW, the Okeechobee chart from Maptech is using wgs84.
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Old 27-01-2012, 19:23   #43
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Re: Navigation Trouble

A little off topic but I have seen places down in the Caribbean where the charts were a mile or two off compared to the GPS even if you had the right datum plugged in. I've seen stuff in the Bahamas that was a half mile off or so. Even in the USA you will find areas where your charts do not agree exactly with the GPS.
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Old 27-01-2012, 20:03   #44
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Re: Navigation Trouble

I am a Nav Dummy, I learnt mine in a day and a half, and thats after learning how to turn it on, Then I was on my Own,

A new Garmin needs to be sailed around in two complete circles so it knows where it is, Lined up with 3 satellites,

My GPS, I found was dead accurate in Fiji, Vanuatu, and down the coast of Australia, even going up rivers, I found it to be dead accurate,

But Coming into Bundaberg, on the south side of the river entrance, I found that I was sailing along the beach, about 200 yards away.

But once in the Burnett River, It was dead accurate again,

I was sailing north, So the error was to the west of me,

I was in Vanuatu when I learnt how to find my position on it,

Also thanks to another sailer When my maps and GPS failed me coming through the reefs at night,

And being a noob, I didnt know how to fix it,

It took him a whole ten minutes to fix the problem, And his comment to me was, I had better maps and GPS than the four boats I was anchored near,

But they do have mistakes in them,

Cheers,
Brian,
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Old 27-01-2012, 20:16   #45
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Re: Navigation Trouble

Just for the record, a new Garmin GPS doesn't ned to be sailed around in two complete circles so it knows where it is.

A Garmin unit with a flux gate compass inside it (if they make one) might.
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