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post #11 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

Because they just painted the little white house.





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post #12 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

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Originally Posted by Frogwatch View Post
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?
Why waypoints? Well for starters, if you are crossing a body of water where you can't see the landmarks to guide you to your destination using the gps waypoints will allow you to maximize your VMG by compensating for current, leeway, etc.
Why accuracy greater than 1000'? I can think of two specific examples off the top of my head. Once we were navigating a stretch of water that was a natural channel with bands of reefs on both sides. We were looking for the gap in the reef that would allow us to get out of the channel and shave several hours off our trip. Our gps plotter showed us exactly where to look for the gap, which was very difficult to spot from a distance. The gap was exactly where the gps said it would be. Another time we were coming home, and as we approached the city the entire bay was engulfed in heavy fog. The gps told us exactly where we were in relation to the shipping lanes and the mud flats, so I didn't have to worry about that, and was able to devote 100% of my attention on watching for sport fishing boats and anchored freighters.

A gps is a valuable tool, but only if you can trust it's accuracy. I always double check with visual references when I can, and I have come to trust what the gps tells me when I can't see.

Of course there will always be those people who want to do it the old fashioned way with paper charts, but in my opinion the technology is reliable enough that the paper charts serve only as a back-up.
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post #13 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

With any notion that fog may happen, I keep a close eye on the proper compass course back to safety. I wouldn't consider entering a narrow rocky entrance in fog but would stay outside. In fact, I have done this several times while cruising. Seeing people underway in fog scares the crap outta me. Get almost run down once in the fog and you'll think better of being underway in it unless necessary.
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post #14 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

Now that NOAA no longer makes paper charts, I may have to modify my position on this. Still, do you really want to put your life on what a black box says? "Trust but verify".
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post #15 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

Hello,

To answer your exact question, is it worth it to spend money to get better than +/- 24 feet accuracy? The answer is no.

Really, even +/- 50' is more accuracy than anyone needs. Most charts are not that accurate, so knowing where you are on a chart that is not that accurate doesn't really help you. Do you really think that a rock or jetty or nav aid listed on a chart is accurate to within 25'? They aren't.

Personally, I try to stay at least 100' away from ANY hazards, and usually I can stay much further away from them than that.

Barry





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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?

Barry Lenoble
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Mt. Sinai, NY

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post #16 of 43 Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

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Now that NOAA no longer makes paper charts, I may have to modify my position on this.
You can still obtain NOAA paper charts, no problem, Print on Demand...

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Still, do you really want to put your life on what a black box says? "Trust but verify".
Guess you never fly in a commercial airliner either, huh? :-)
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post #17 of 43 Old 03-05-2014
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Re: GPS Accuracy - how much is actually needed

I think, as others have stated, that accuracy is entirely a non-issue. Another thought about having the sensor in the boat is reliability. Our VHF has an an internal GPS with an internal antenna and on occasions it will not be able to provide a position. I have never had this problem with any device with an external antenna.

Back home on Lake Ontario after something over 36,000 nm circumnavigator. Not surprisingly there is a lot of stuff I want to get done on Ainia both cosmetically and functionally. Getting an early start so it will be ready to go for next summer (Lake Superior?).
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Re: GPS Accuracy - how much is actually needed

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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.
I agree. I think the repeatbility of GPS today equals Loran at it's heydey.

The funny thing about selective availability was, at the time, GPS was still such an improvement, that none of us cared that it was off that much. It was still more accurate than any other navigation device available (except maybe Loran in the right area with TD's crossing at nearly right angles).

I can't imagine the chaos that would ensue today, if they ever turned SA back on. Towboat US, Seatow, and salvors would have a field day.

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post #19 of 43 Old 03-05-2014 Thread Starter
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Re: GPS Accuracy - how much is actually needed

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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.
I plan to mount it below decks because there is a radar/chart plotter already on deck, but this system gets the location coordinates from the chart plotter below decks. The chart plotter below decks also inputs the location to the VHF radio and sailing instruments.

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post #20 of 43 Old 03-05-2014
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Re: GPS Accuracy - how much is actually needed

I can understand that most people are not navigation fanatics and chart plotters do seem to make things so easy. However, you need to consider what has happed to some people on land who relied on GPS for nav and who have driven right into rivers or gotten lost. You really do need to be watching to verify that you are where it says you are or that hazards have not moved. Considering the necessity to be watching carefully, does extreme accuracy matter?
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