GPS accuracy...

I

inkblue

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Jan 18, 2007
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24914
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I have a Garmin 540 GPS I got from Boatfix recently and upon testing it in Long Island, I see that their preloaded maps are kinda 'off'. Some buoys are way off on the GPS and some are spot on. I know GPS is not to be relied on solely, but am I the only GPS owner with this problem?

Just wonderin'...

[:-dunce]
 
when you say way off, how far is that? I find some off here in Florida on the St John's river. Also, in which direction are they off? A GPS can be off as much as 25'. That is why you sure wouldn't want to rely on the GPS at nightime thinbking you are in the middle of a channel. That is the quickest way to run into a maker pole.

Art
 
I boat in the mississippi river, have a furno gps with loaded charts, most everything is dead on with the exception of the bouys, I think they have a tendency to drift.
 
Bouys get moved around as needed.... Every year the channels move slightly... so ur charts a guide.... u always need to be mindful of whats going on and where u are...

Fixed objects is what u need to see how far off u are...

Rob
 
Hey engdockter-- It really doesn't matter what kind of gps a person has on the boat. It just depends on the gps accuracy at the time of passing markers. I have send my Garmin 3006C with accuracy of 4' and then I have seen it at 25'. I was coming down the river and checking the accuracy and my gps showed me in the middle of the channel, but I was within 10' of a marker on my starboard side. That is why a person can't just rely on the GPS of navigating.

art
 
inkblue:

You must learn to distinguish between the lat and long provided by the GPS and the position that your "little green boat" appears on the chart, either a PC or chartplotter.

I am forced to use a number of charts that are the current versions but are "vintage" charts, and unlike wine, they get no better with age. It can be disheartening to be traveling in a narrow passage and see your "little green boat" taking the overland route on the chart on one side or the other of the channel. A visual is a big help but can sometimes fool you, but then traveling with your radar on allows you to travel the middle of the channel by "painting" the shorelines and measuring distances on the radar screen to both shores.

I travel with two handheld GPS's that I take with me in the dinghy or kayak and frequently use them to check each other and the main GPS. Don't get uptight about .001 nm. That's only 6 ft. and if that's a problem, you shouldn't be there.[:-blindfold]

Old_Salt
 
There are many reasons the chart can be off, or you think it may be off. First of all, buoys (and any type of floating aid) are not to be relied upon as a means of position fixing, as their anchor chains allow for some swinging, meaning that the actual position of the buoy at any given time, may be off from its charted position. Strong storms or waves can cause the buoy to drag its snker, as can collisions with vessels. You will often see a note on a paper chart regarding this. Second, your GPS receiver must be selected on the correct datum for the chart, most likely WGS84. There should be an option to select this somewhere in the unit's menus. Third, GPS accuracy is based on the number of satellites it is receiving. An obstruction can reduce the antenna's vision of the sky considerably, resulting in lost satellites. Fourth, the U.S. Government reserves the right to scramble the GPS signal at any time, without warning. The offset is not constant and can jump around. While I doubt that this is the case here, every user should be aware of this. Finally, GPS even when operating at peak capability, is really only accurate to about 8.5 meters, depending largely on atmospheric (not necessarily meteorological) conditions. Differential GPS (DGPS) can get this down to about 3 meters. WAAS systems can reduce it even further to 1-2 meters. In addition, as the accuracy gets down to this small of a distance, you have to take antenna location on the boat into account.

The last thing to consider is the accuracy of the chart itself. Many charts are based on surveys done long ago and it's hard to keep every chart up to date with the latest surveys that are using the most modern and accurate techniques.
 
My Lowrance w/WAAS is dead on except for floating bouys. They drift or get moved from time to time. I use the trails feature a lot due to boating in skinney poorly charted waters much of the time. I've noticed when using trails that repeatability is just about perfect.

The part of the country in which you navigate can have a lot to do with the availability and quality of the WAAS signal. In the Mobile Bay area we get pretty good signals.
 
Thanks.

At some spots, the bouys shown in the GPS are more than 50ft off and sometimes they are dead on, and of course, some aren't even seen on the water... I'll play around with the GPS settings and follow the advice above with regards to landmarks.

Gotta practice more next weekend...

[:-graduate]
 
Inkblue- It is definitely worth going into your Garmin's menu and making sure WAAS correction has been enabled. Often it is not until operator configures it.
 
Ink,

Where were you running when the GPS was off target?. I run in both New York Harbor and the Great South Bay. In NY harbor I run 4 different vessels and all Garmin gps's are on target. In the GSB, I run on 3 different vessels with Garmins and they are all off a bit. When running Fire Island Inlet for example, The GPS shows me over land between the 6 and 8 bouys when I'm about 100 yards off the beach. What I would suggest is run your routes and set marks on your gps as you pass close to bouys or other concerns or save your tracks in the gps memory. At least if your running in the fog or at night, you can plot your route with the marks you've already set,

Adam
 
One common error is believing that the underlying base charts are accurate. They are not. Most marine charts were originally drawn based on older survey methods and the base chart can have rather large errors. In this case your GPS is dead bang on target, and the chart ( NOT the plotter, the chart ) is in error.

Look at the details on the charts you are using. You may find that the data represented on the chart can be quite old. In fact, in my area, the depth info between Chandeleur Island and the Louisiana marsh was collected via leadline just a few years after the Civil War. Due to hurricane activity, almost all of the shorelines differ from reality.
 
Ok Sandy I will check that.

Adam, I was cruising Freeport area...

[:-captain]
 
Lots of great information already written. Go on the boat and leave the GPS on for a number of hours while the boat is at the slip. You and see what the range that the gPS shows during this time. Generally I find my GPS's to be within 10 feet, but there will be occasional wanderings of over 75 feet. Floating marks are moored by large chains, and in fairly deep water, tide, current and wind will allow the bouy to drift out of position. Also bouys are to be moved to mark the channel and obstructions as they may change. Fixed makers are not moved.

Cartography is based on some surveys as much as 100 years old. 30 year old surveys are not uncommon. It depends on the cartography company as to the accuracy. Some have better corrections that others. Also as charts are digitalized there are errors, which may put bouys out of position. For the most part US waters are farily close. In some parts of the world, the charts are off as much as a mile.
 
See if they have a chart update at Garmin. You can upload the latest info.
 
Don't forget the Notice to Mariners and Local Notice to Mariners to tell you about changes to bouys and markers. Also there are many private aids to navigation that aren't necessarily maintained properly, These (usually smaller) bouys can drift a lot.

KPMid has a good point on comparing the chart you are using to the datum that the GPS is using. Also the scale of the chart matters. A 1:80,000 chart accurate to 1 millimeter would have 4 times the error of a 1:20,000 chart that is accurate to 1 mm. Make sure you are using the largest scale chart that is available. Put a waypoint on a marker on one chart and then switch charts. You'll see that the waypoint isn't nessecarily right on the marker on the second chart.
 
The 540 has an internal antenna - you should be able to see signal strength bars, similar to a cell phone, on the home page. Make sure you have WAAS enabled as well. If you are getting strong signals and have WAAS enabled, check it in relation to a fixed object, like a pier or set of range markers. You should have position accuracy with a variation of no more than a few feet.
 
Ok, went out again today and the GPS seems to be "settling"...

[:-magnify]
 
Nigel Calder in his book How to Read a Nautical Chart gives a very detailed description of the development of electronic charts from survey data and the limit of the accuracy (or lack of it) that is common with GPS based chart plotters. As the previous posts point out there are two sources of error - GPS accuracy and chart accuracy (not to mention positional error for floating objects like buoys!). In most cases GPS accuracy is much greater than chart accuracy. Basically, contemporary GPS systems can provide your position with much greater accuracy than the chart (read survey) can reflect. The generally held standard is that the limit of accuracy of a NOAA survey conducted prior to the mid 1990's is 1.5 mm at the scale on which the survey was done. If the survey was done at 1:40,000 (a pretty detailed survey) then the specified limit of chart accuracy is 60 meters, or about 195 feet. Add to this the limit of GPS accuracy and you can be off quite a bit. If the survey the chart is based upon was done at 1:80,000 you can double the limit of accuracy.

On GPS position Garmin maintains that their GPS receivers are accurate to 15 meters on average and that WAAS can improve accuracy to less than 3 meters.

The survey data and its conversion onto the chart seen on the plotter remains the greatest common source of error. The ability to "overzoom" chartplotters can lead to a sense of security about relative position that is certainly not justified. The general recommendation is not to use a chart plotter to navigate where you wouldn't with the corresponding physical chart.
 
GPS, as with most instruments, is highly repeatable but less absolutely accurate. E.g. a measurement with a yardstick may be repeatable but not absolutely accurate. So it is with GPS
 
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