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post #1 of 43 Old 03-04-2014 Thread Starter
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GPS Accuracy - how much is actually needed

How accurate is your GPS. I have a new Raymarine A67 chart plotter which I love. When mounted on deck without an external gps sensor has an accuracy of about 4-10 feet. When mounted below deck it has an accuracy between 8-24 feet without an external gps. I plan to mount it below deck without a external gps sensor. With an accuracy of +/- 24 feet on a 40 foot is it worth the additional money to get another few feet of accuracy?

Schock - New York 36
Portsmouth, RI
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post #2 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

Yes, mount an external sensor.

Here's the deal. I'd never navigate per the charts. So accuracy is not that big of a deal. The charts don't match reallty.

But; once you establish way points to all the places you go. It pays to hit them accurately. 40' could be HUGE. Could be BIG rock or open channel. That's why you want accuracy.
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post #3 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

Why do you plan on mounting it below decks? It is well sealed and designed for outdoor mounting.

For about the same price as the external NMEA 2000 GPS sensor you could buy a tablet that would give you a remote display from the A67 that is usable below decks. Any Android or iOS tablet will do it.

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post #4 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

Yea; why do you want to mount it below deck? It won't do you much good down there. Are you talking about putting it on a swing out bracket in the campanion way?
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post #5 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

NO,
GPS (chartplotter) 'accuracy' is totally dependent on 'resolution accuracy' of the original charting from which it is based. Problem is that for most areas that do NOT have large scale commercial marine traffic, the base chart 'may' still be based on 1927 or earlier surveys. Plus and with respect to 'resolution errors', using GPS charting based on 'old' lead-line surveys may also induce EXTREME errors especially when the GPS chartplotter MAGNIFICATION greatly exceeds the 'magnification' of the original hand surveyed charting.

FWIW .... most USA marine charts are only upgraded if and only if there is large commercial boats constantly using the area of the chart.

OEM Charting error trumps GPS accuracy; 'over-magnification' by chartplotters MAGNIFIES the original errors.

;-)
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post #6 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

"Repeatability" of GPS postitions is not as good as Loran was. You may get the same reading from your GPS unit the next time you get near your favorite fishing spot (or wherever) but you may not be exactly in the same place. There may be several more (or many fewer) satellites over the horizon providing data, or the tide may be at a different level, or the military may have degraded the signal for some reason they didn't tell you. Tie up in a slip and watch how your GPS position changes because of these (and other?) variables. Mix that in with charts that may have last been sounded with a lead and line in 1840, and it becomes obvious that GPS is nice, but not necessarily definitive. I consider it pretty accurate, and then I look around and watch my depthsounder.
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post #7 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

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"Repeatability" of GPS postitions is not as good as Loran was. You may get the same reading from your GPS unit the next time you get near your favorite fishing spot (or wherever) but you may not be exactly in the same place.
That was indeed the case in the early days of consumer GPS and Selective Availability, but I believe today that WAAS addresses the repeatability issue.

Also, more accuracy in heading and speed readings is valuable to relatively slow moving sailboats.
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post #8 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

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NO,
GPS (chartplotter) 'accuracy' is totally dependent on 'resolution accuracy' of the original charting from which it is based. Problem is that for most areas that do NOT have large scale commercial marine traffic, the base chart 'may' still be based on 1927 or earlier surveys. Plus and with respect to 'resolution errors', using GPS charting based on 'old' lead-line surveys may also induce EXTREME errors especially when the GPS chartplotter MAGNIFICATION greatly exceeds the 'magnification' of the original hand surveyed charting.

FWIW .... most USA marine charts are only upgraded if and only if there is large commercial boats constantly using the area of the chart.

OEM Charting error trumps GPS accuracy; 'over-magnification' by chartplotters MAGNIFIES the original errors.

;-)
Exactly...

How accurate are our charts? - Ocean Navigator - January/February 2003

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With an accuracy of +/- 24 feet on a 40 foot is it worth the additional money to get another few feet of accuracy?
Such accuracy within a dimension smaller than the size of your boat is really only important if you care which PORTION of your boat is at a particular geographical position on the earth's surface... in other words, with such precision on a 40-footer, you really have to decide: Do i want to know where my KEEL is, or the NAV STATION, or the BOW, or the HELM ? And then, position the antenna accordingly...

When you think about it, such accuracy really is more valuable for your 8' tender, than for the 40' mother ship... Not to mention, far less important on a $80 million 200' megayacht...

Most of which will have multiple GPS receivers both fore and aft, anyway, and at other critical locations like helo pads, and so on... :-)

As for the 'repeatable accuracy' of GPS for waypoints, LORAN was probably superior in that regard... And, if I need to rely on GPS accuracy within a few feet to keep me from hitting a submerged rock, I'm putting WAY too much faith in my gizmos, and it's probably time for me to think about taking up another hobby... :-)
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Last edited by JonEisberg; 03-04-2014 at 09:34 PM.
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Re: GPS Accuracy - how much is actually needed

Good God! If you need your GPS to be more accurate than 1000' you are doing things wrong. If you are in a situation needing better than that you better be using sights on objects using a hand compass cuz GPS can be seriously wrong at times. If you cant take sights then you shouldnt be there or should be dead stopped until visibility gets better. A prudent navigator relies on what he sees, not something a black box tells him.
For those who say charts are wrong, well, even with gps they would be wrong and you are still screwed.
OK, fishermen need gps to get coords of good spots and I use mine to give me good speed measurements and a position measurement when out of sight of land but I tend to rely more on my dead reckoning first and if they don't agree, somethings wrong.
In 27 yrs of extensive sailing, I've never needed "waypoints" and still dont get why people do. Instead, for example exiting Sea of Abaco near Little Harbor, I get to where the "white house" lines up at X degrees and follow the reciprocal course until my GPS Lon. reads zz.zzz to indicate that I am well clear of all the hazards. Then I can head on a course of 170 to Spanish Wells. Waypoints? Why?
Even going from Marsh Harbor down to Little Harbor where there are numerous turns, reefs and other hazards, I better be using my compass to sight on known points cuz that also alerts you to stuff that isn't on the chart. Why would waypoints help?
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Re: GPS Accuracy - how much is actually needed

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Originally Posted by Frogwatch View Post

If you cant take sights then you shouldnt be there or should be dead stopped until visibility gets better. A prudent navigator relies on what he sees, not something a black box tells him.
You rarely sail in places where fog can occur, it would appear...

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In 27 yrs of extensive sailing, I've never needed "waypoints" and still dont get why people do. Instead, for example exiting Sea of Abaco near Little Harbor, I get to where the "white house" lines up at X degrees and follow the reciprocal course until my GPS Lon. reads zz.zzz to indicate that I am well clear of all the hazards. Then I can head on a course of 170 to Spanish Wells. Waypoints? Why?
Well, waypoints can be be pretty damn handy, at times... Not everywhere has conveniently placed white houses, after all... :-)

To each his own...
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