Navigation Trouble

[bcaldoc]

Anyone having navigation trouble in Florida? We are cruising west on the Okeechobee Waterway and our brand new Garmin GPS is 200 ft off! Is this a fault in the GPS, or is this something to do with the solar storm? Any ideas?

[Wotname]

Originally Posted by bcaldoc

Anyone having navigation trouble in Florida? We are cruising west on the Okeechobee Waterway and our brand new Garmin GPS is 200 ft off! Is this a fault in the GPS, or is this something to do with the solar storm? Any ideas?

Very unlikely for solar fares to cause errors, perhaps very occasionally they might cause loss of lock but not errors see IPS - Space Weather - GPS Interference By Solar Radio Bursts

More likely a datum issue. Is you GPS set to the same datum as your reference source.

What was your baseline reference? For how long the suspected error occur or is it permanent? Did you observe any suspected error in other locations?

[bcaldoc]

Thanks for your help Wotname. Not sure what you mean by "datum" or "reference source". The chartplotter is brand new so we have only observed it in this location. The problem is intermittent but mostly present, we are from 50 to 200 ft off, always south, i.e the GPS says we are 200ft further north than we are.

[IslandHopper]

Where exactly are you and what is the Datum for that area, you can easily find it on a "real chart" for instance, then make sure the Datum of the area you are in matches the Datum settings of your GPS.....

But with all due respect, if you don't know what Datums are......get a nav course under your belt before going to far...........

[btrayfors]

Is it the GPS or the chartplotter? You might get the Lat/Lon as indicated by the GPS, and compare it to a paper or raster chart.

Problem with all chartplotters which use vector charts -- and most of them do -- is that vector charts may be more subject to errors of commission or omission which do not show up on raster charts (which are essentially photo copies of the paper charts which underlie all charting in the U.S.).

Does your chartplotter use NOAA vector charts or other? NOAA does a pretty good job of making sure that their vector charts agree with the raster charts; other private charts may not be so closely linked.

I believe that many Garmin chartplotters use Blue Charts, licensed from Navionics. There have been many reported discrepancies in these charts in several areas of the U.S., some of which resulted in groundings.

BTW, last year while delivering a boat with a large new Garmin chartplotter I noted an E-W error which was about 200'...showed the boat about 200' further west than it actually was.

Bill

[goboatingnow]

Originally Posted by btrayfors

Is it the GPS or the chartplotter? You might get the Lat/Lon as indicated by the GPS, and compare it to a paper or raster chart.

Problem with all chartplotters which use vector charts -- and most of them do -- is that vector charts may be more subject to errors of commission or omission which do not show up on raster charts (which are essentially photo copies of the paper charts which underlie all charting in the U.S.).

Does your chartplotter use NOAA vector charts or other? NOAA does a pretty good job of making sure that their vector charts agree with the raster charts; other private charts may not be so closely linked.

I believe that many Garmin chartplotters use Blue Charts, licensed from Navionics. There have been many reported discrepancies in these charts in several areas of the U.S., some of which resulted in groundings.

BTW, last year while delivering a boat with a large new Garmin chartplotter I noted an E-W error which was about 200'...showed the boat about 200' further west than it actually was.

Bill

Don't think Garmin is relicensed, I beleive they are modified transas charts. Also Garmin have their own cartography group, which can create charts as well.

Dave

[lolarose]

Originally Posted by bcaldoc

Not sure what you mean by "datum" or "reference source".

for heavens sake, i do not wont to believe that people like this "cruising" somewhere out there

[btrayfors]

Originally Posted by goboatingnow

Don't think Garmin is relicensed, I beleive they are modified transas charts. Also Garmin have their own cartography group, which can create charts as well.

Dave

Well, maybe, but lots of references to Blue Chart being licensed from Navionics.

Also, re: their own "cartography group", IMHO that's all the more reason to be wary of the vector charts in their chartplotters :-)

Bill

[Wotname]

Originally Posted by bcaldoc

Thanks for your help Wotname. Not sure what you mean by "datum" or "reference source". The chartplotter is brand new so we have only observed it in this location. The problem is intermittent but mostly present, we are from 50 to 200 ft off, always south, i.e the GPS says we are 200ft further north than we are.

Originally Posted by IslandHopper

Where exactly are you and what is the Datum for that area, you can easily find it on a "real chart" for instance, then make sure the Datum of the area you are in matches the Datum settings of your GPS.....

But with all due respect, if you don't know what Datums are......get a nav course under your belt before going to far...........

Unless you operating some sort of differential GPS or a WAAS GPS, then the error of 50 to 200 ft can be considered normal especially given the various chart errors Bill mentioned.

As to datums, lets try to keep this simple. The GPS doesn't REALLY know where it is, all it can do is work out a lat. and long. with reference to some system. Over the years different folk have have invented many different systems in many different countries essentially using slightly different starting points for each system.

Your GPS could have a 100 different systems pre-loaded into it. Each system (lets call them by their proper name - Datums) is identifiable by a code or name. Probably by far the most common one is WGS 84, standing for World Geodetic System established in 1984.

Now the datum of your chart has to match the datum of your GPS otherwise the starting point for one will be different from the starting point of the other. That is, the same lat. and long. from each datum will actually be two spots on the earths surface. Remember the GPS doesn't know where it actually is, it only knows what lat. and long. referenced to a particular datum.

OK, surveyors and navigators can back off, this is simple explanation and not entirely technically correct but I think Bcaldoc only needs a simple primer here. He can google for a deeper understanding.

[bcaldoc]

Thanks again Wotname. I appreciate your helpful attitude. Too bad it is not universal.
If I understand you correctly, the question is whether the chartplotter GPS is using a different datum from the chart that came preloaded into the chartplotter. Seems unlikely if not negligent. The charts are up-to-date NOAA charts using WGS 1984. Now that I know what a datum is, my initial research on the internet tells me that this is also the standard for all commerical and recreational GPS units, and has been since 1997. So it seems that conflicts between charts and GPS data are a thing of the past.

I will check the GPS co-ordinates with the paper chart next time I experience a discrepancy.

[MarinerJo]

Originally Posted by lolarose

for heavens sake, i do not wont to believe that people like this "cruising" somewhere out there

I would think we would help someone that doesn't know rather than take cheap shots at them for not knowing.

[IslandHopper]

Originally Posted by Wotname

OK, surveyors and navigators can back off, this is simple explanation and not entirely technically correct but I think Bcaldoc only needs a simple primer here. He can google for a deeper understanding.

I 100% disagre with statement's like this, google?? GMAFB! ....

Go and get yourself a basic navigation course, and in doing so it will (should) explain how Datum’s fit in and there importance in navigating safely....and it will be "technically correct"....

[colemj]

It could be that the chart itself is wrong. I don't know about the particular area you are in, but charts in general are only as good as the accuracy and precision of the original survey that created the data for them. A constant offset sounds like a chart data issue.

In a lot of the world, the charts are off significantly, and 200' would be considered very accurate.

Mark

[donradcliffe]

Why don't you let us know the model number of your chartplotter, and maybe we can help you find out whether you can improve things. The chart datum selections are buried deep in the menus, and some chartplotters have a very dangerous way of letting the user correct the datum.

[Wotname]

Originally Posted by IslandHopper

I 100% disagre with statement's like this, google?? GMAFB! ....

Go and get yourself a basic navigation course, and in doing so it will (should) explain how Datum’s fit in and there importance in navigating safely....and it will be "technically correct"....

Which bit do you disagre (sic) with? The bit about the explanation being simple and technically incorrect or the bit about the OP only needing a simple primer or the bit about google. If the latter, I suggest a google search will return as much factual information as any basic navigation text book and probably much more. For instance try a google search on "the difference between there and their"

I agree that a basic navigation course is a good idea but it isn't the only way to learn.