chartplotter off

[John R]

I have a Garmin 741 chartplotter. Recently I've noticed that it is off by maybe 20 feet. That's not a major problem out in the ocean, but if I were coming back into my dock in the dark, or trying to navigate into another marina or in a narrow channel in the dark or in a fog, it would be. In fact, if I were to follow it, it would have put me onto a channel marker or into the breakwater in my marina. Yesterday we came back from a short trip and the other thing I did was record some locations (latitudes and longitudes) at particular locations. Just now I compared what I recorded (e.g. at a particular point under a bridge) to what the charts said. The latitudes and longitudes I recorded were way, way off (as in on land in one case). Maybe I recorded them wrong, but something seems wrong.

Any suggestions or thoughts? (The chart plotter is about two years old, by the way. It has not seen any particular rough wear.)

 

[Johnb]

One place to start is to make sure you know exactly what the numbers you are recording mean. You might have
degrees, minutes and seconds or
degrees and decimal minutes, in which the numbers to the right of the decimal point are all decimal fractions of a minute or
decimal degrees in which the numbers to the right of the decimal are all decimal fractions of a degree or
an abortion in which the whole number is degrees, the first 2 decimals are whole minutes and the rest of the number is either decimal minutes or seconds.

If you are grossly off I would suspect that you might be getting seconds and decimal fractions of minutes mixed up.

That said, there are some strange things. In the pictures the one shows two Garmins being compared at the observatory in Greenwich England. They both showed the same position. The other picture shows the face of the GPS and in the background a line representing the prime meridian for visitors to enjoy. As you can see the GPS shows 0.073 minutes West, putting the meridian about 260 feet to the East of that location, which is where Google Earth puts it. The meridian just aint where they marked it.

I would be amazed (but that could happen) if your GPS was not accurate. Being a digital device it will either be correct within its limitations or not function at all.

With no disrespect intended if you are planning to use GPS navigation to come within 20 feet of things you could get some nasty surprises. Maps can be off (I had personal experience with the ones of the Erie Canal) or privately maintained marks can be moved seasonally as needed when channels move. The GPS is really just good for getting you to where your eyes and depth finder can take over.

 

[Bill Roosa]

Get a copy of Bowditch and look up GPS CPE
never rely on only one source of navigational information

GPS is only precise to about 30 feet. You can confirm your CPE by running the GPS while at the slip. The boat will be "all over the place"

 

[Rob2435]

From what I remember the precision of GPS for civilian use is not exact and the accuracy can be changed by the US Gov/Military without notice. I think when it was first released they kept the accuracy to within 300ft, then I Clinton made it 100ft. If you are using GPS to guide you into a "tight" spot at night, anchor off and make the move during daylight.

For the first few years I owned my boat, we never used GPS for navigation, we used paper charts. If we needed some clarity, I would turn on the GPS and check our position.

 

[RichH]

Its probably not your GPS that is that far off ... usually good for 30ft. EPE ... but probably the base charting that is used on your chartplotter.

That charting if not for waters that are heavily used by large commercial vessels, is probably derived from 1927, or earlier, charting that was established with lead lines, semaphores (flags) and theodolites. And ... if you use that charting at greater magnification than the original (NAD1927, etc. probably at 1:25,000 or greater) then any 'magnification' greater than those original base data can lead to some quite serious navigation mistakes.
What you dont want to do with a GPS chartplotter is, especially when in 'non-commercial' waters, use the greatest magnification possible - called 'resolution errors' and sure to cause some 'interesting' navigation issues. 30 ft. is a good value ... but IF and only IF you're near FIXED navigation aids that govern commercial waters used by BIG ships.

Simple speak: for waters not used by LARGE ships, its quite possible that the charting as found on 'chartplotters' is in error by sometimes LARGE distances ... AND if you *over-magnify* those charts the errors can also be significantly and additionally magnified.

 

[John R]

Thanks for all the explanations. JohnB may be right about confusing decimals with minutes and seconds.

But it's not only that. Where the chartplotter shows me on the screen just ain't so. I get it as far as how accurate we can expect the plotter to be, but my memory is that in the past it was more accurate. I remember, for instance, navigating up a narrow channel in Bodega Bay here in this area. It was a very foggy morning and I had somebody standing bow lookout, but even without this, the chart plotter put me exactly where I was. Could it be more accurate in some areas than others?

 

[shemandr]

You could also check the datum setting on the GPS. Mine was once inadvertently switched to something different, which had my anchored position on shore.
Some charts are in Minutes and Seconds, such as Fisher's Island Sound. Some years ago I had to anchor in terrible fog near Stonington and work out the GPS position in tenths of minutes (No chart plotter then), by extrapolating to minutes and seconds so I could put my position on the paper chart. I probably could have changed the GPS to read minutes and seconds but I never even thought of it. I was a lot stressed by a rough and foggy trip.

 

[Bill Roosa]

GPS is just like celestilal navigation if you choose 3 stars directly over head your cocked hat solution has large errors in it. if you choose stars that are seperated by 90 ish degrees your solution is "tighter".
I believe the GPS term is dilution of position, lower is better, numbers over 50 are pretty inacurate

 

[jimmcgee]

Standard GPS is accurate to 7.8 meters (http://www.gps.gov/systems/gps/performance/accuracy/) which is about 25 feet.

GPS with WAAS (Wide Area Augmentation System) is accurate to within 3 meters which is about 10 feet (http://www8.garmin.com/aboutGPS/waas.html).

If your 741 was more accurate before and is now off by about 30', WAAS may have been turned off on your plotter. You'll have to check your manual to access the setting, it's been a while since I configured mine.

 

[John R]

Thanks for the further suggestions, especially about waas. I will check on that.

I also spoke with technical support at Garmin. (Very easy to reach, by the way, and very helpful.) They said that I should update my unit, including doing a chart update (cost: $100). They also said: (1) To call them from my boat to let them know exactly what version of software I have; and (2) if my plotter shows me being off on my marina entrance to let them know and that's the sort of thing they update their charts with.

 

[Richard19068]

Have you checked the chart datum from your chart to the datum setting on the GPS?

WGS 84, NAD 27.. whatever it is on the chart you are using should be what the GPS is using. Its probably the same but worth checking.

 

[kloudie1]

Gps Telling Fibs

I think that Garmin took some shortcuts in translating some charts into their database.. There are some areas around the east end of the Lake Pontchartrain North Shore where the GPS does not work very accurately.. My Chartplotter 545 which has the charts and software all up to date and the correct datum set and WAAS on, showed me at center screen, quite a ways from the bayou.. The track was from a few days previous, showing that the problem is not consistent. I have also seen this same error about 5 miles east of this location where it showed me on the outside of a channel that I was clearly in!.. I contacted Garmin and got a reply that said that they'd look into the problem and get back to me.. That was a couple of years ago.. I haven't heard anything yet.. This problem appears nowhere else that I am aware of, and it happens in the same places whether sailing or motoring.

 

[Stu Jackson]

GPS & Repeatability

One of the things that GPS doesn't do as well as Loran did was to be able to repeat locations. Loran, given its land-based signal emitters, could repeat locations faithfully within pretty neat parameters. That's why fishermen liked it so much. GPS is, well, more global, and can work out of sight of land because of the satellites. It does not, and never has, nor was it ever intended, to be repeatable to the same extent than Loran. That's why it seems to me that "tracks" on GPS units are good for reference, but not exact on the next trip. Nor should GPS be used to get one into a slip, especially one's OWN slip. For the life of me, I can't figure out why why people would need to do that (how dark can it possibly be?) or would want to even try it. Maybe someone who tries to do this could explain it to me and help me understand.

The answer to Bodega Bay, compared to Emeryville, was answered earlier: commercial vessels. IIRC, there is a LARGE fishing fleet in Bodega Bay, which often gets down-on-the-deck that usually isn't seen (at all if not only rarely) in Emeryville.