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GPS Accuracy - how much is actually needed

5K views 42 replies 21 participants last post by  MarkofSeaLife 
#1 ·
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?
 
#2 ·
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.
 
#3 ·
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.
 
#19 ·
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.
 
#5 ·
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.

;-)
 
#8 · (Edited)
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

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... :)
 
#6 ·
"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.
 
#7 ·
"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.
 
#9 ·
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?
 
#10 ·
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...

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...
 
#13 ·
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.
 
#15 ·
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
 
#17 ·
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.
 
#20 ·
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?
 
#24 ·
OK, OK, I am almost convinced that chart plotters are nice. After considering the 4-5 times I have been really confused about exactly where i was I can see that a chart plotter would be nice.
Several years ago when we sailed from West End to Freeport, we wanted to go into a narrow channel east of the main Freeport channel and I just couldnt locate it with my binocs and didnt want to get in too close due to reefs. Suddenly, my daughters BF brought up his laptop to which he had a GPS connected and overlaid our track onto google earth. I was effin amazed.

So, do Ipads have built in GPS? I've never used a tablet computer.
That Lenovo Miix, does it run Win 8 or android? Could I get it with Win7 instead of Win8?

All this 21 century stuff is gonna kill me. Next they'll be putting flush toilets on boats.
 
#25 ·
I resisted chartplotters, too, I thought my Magellen 310, with nothing but Lat Lon's and a course track, and paper charts was all I ever needed. Then I bought a used center console that already had a Garmin chartplotter mounted on it (with an old black and white model screen). I have to admit, it made a believer out of me.

I now have two chartplotters on my sailboat. One just as a back up. :D
 
#26 ·
Here's an interesting article on WAAS and GPS accuracy :

The coastguard reported real world performance for Loran-C of 50m (return to position accuracy). WAAS-enhanced GPS guarantees 7.6m, although real-world performance is considerably better. I routinely see accuracy of about 1m.

Sometimes when I record my GPS track with my handheld and paste it into Google Earth, I can see where I walked down the dock, to the restroom, and back. The accuracy looks like about 1m to me.
 
#36 ·
Maine-
Let's see. With a chart scale of 1:25,000 and a common 0.5mm mechanical pencil, unsharpened, that pencil line would be 25,000 x 0.5mm wide in the real world. 12,500mm at 1000 mm to the meter, 12.5 meters wide.
But if you buy that mechanical pencil at a real supply store it can easily be 0.3mm lead, giving us 25,000 x 0.3mm, which will be 7.5 meters wide in the real world.
I'm not sure if you can sharpen a conventional pencil to a finer point.

Of course if the navigator is using a vintage 5/0 or 6/0 Rapid-O-Graph, they can do twice as well as that.

Personally I prefer the safety zone created by a nice dull Crayola Crayon. (G)

But back to the OP's question: Offhand, anytime you can increase accuracy or precision, that's a good thing. What they are seeing is a signal degradation caused by the hull and deck, and that probably will increase under heavy rain and cloud cover. Ten meters, twenty meters..."Good enough for government work."
 
#38 ·
I wish you guys were around when I took my ASA 105 test. I was trying to use similar arguments to explain why it was impossible for me to get the "exact" answer that the instructor was requiring of me. He seemed clueless about significant figures, loss of precision due to subtractive analysis, magnification of errors caused by long extrapolation, etc. I still scored 95%, but on the problems where he knocked off points, he seemed to expect me to shoot a golf ball with an arrow from a mile away.
 
#37 ·
Back in the summer of 2001 I was sailing solo up the east coast of the US, somewhere around NJ the fog set in as I was heading for a small inlet to pick up a mooring for the night. There was a marker at the mouth of the inlet with a bell on it that I could hear but could not see in the fog. Because of the fog, I could not determine what direction the sound of the bell was coming from. The night before, as was my custom, I put all the way-points for the next days sail in my handheld GPS. So I turned the GPS on and followed it directly toward the buoy. The buoy broke out of the fog at 12:30 off the bow at about 50 yards away. With map in hand I safely navigated past the rocky shoreline into the inlet where I was met by the harbor master's assistant who escorted me to my mooring for the night. Without GPS I would have turned seaward and spent another night on the open sea - something I really would not want to do in the fog. That little GPS was the best $150 I ever spent.
 
#39 ·
I remember getting caught out in the fog at dark in the 80's one time, and trying to find the pass through a spoil bank, using nothing but my depth finder and compass. One of the most miserable times I ever had in a boat. With today's gps/chartplotters, it would be so simple to get in when caught in that situation again.
 
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