Visit our Popular Forums

IslandHopper · 27-01-2012, 08:56

Quote:

Originally Posted by 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

I agree, and doing a Nav course will also teach you how to read a chart,
including understanding it's accuracy.....

HappySeagull · 27-01-2012, 09:04

...a 200 feet error seems a lot, if your charts are large scale harbour plans. But larger chart errors are not uncommon with smaller scale charts... if the charts are a more general nature, I mean.
Still, I would add to the above advice to consider where the gps is...not inside a steel or aluminum wheelhouse,I hope. Gps Satellite reception works ok through certain thin materials but a clear "view" of the sky is always best. I'm not familiar with where your gps's antenna is but, say, is there a pair of binoculars on top of the antenna? and so on.

**Wotname** · 27-01-2012, 09:19

Quote:

Originally Posted by 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.

So if I understand your setup, you are using a chartplotter loaded with current NOAA vector charts rather than say Garmin Blue Charts and new Garmin GPS unit interfaced to the chartplotter.

Yes you are correct in that you should expect both are referenced to WGS 84 but since you have a discrepancy, you can't assume anything. I.E check everything.

I suspect that everything is actually fine, don't forget that Garmin GPS receivers only have an average accuracy of 15 metres (about 49') so occasional errors of 200' could be expected and 50' would be common. Again I assume you don't have a WAAS unit.

Mexdon · 27-01-2012, 09:25

I tend to agree with Islandhopper, when in confined waters we should all use pilotage skills and paper charts rather than being glued to a screen. Watch keeping means what is says and being aware of what is around us means we are abiding by the rules of the road to avoid collisions.

GPS has virtually replaced the sextant for knowing where we are in the open ocean, but nothing can replace a good bearing compass, binoculars and awareness when we are in a busy harbour.

**Wotname** · 27-01-2012, 09:35

Quote:

Originally Posted by Mexdon

I tend to agree with Islandhopper, when in confined waters we should all use pilotage skills and paper charts rather than being glued to a screen. Watch keeping means what is says and being aware of what is around us means we are abiding by the rules of the road to avoid collisions.

GPS has virtually replaced the sextant for knowing where we are in the open ocean, but nothing can replace a good bearing compass, binoculars and awareness when we are in a busy harbour.

Yes, I think most of us would agree but let's remember the OP is only asking about his suspected discrepancy in his GPS accuracy. There has been no mention of not using good pilotage skills or of poor watch keeping.

It could be inferred that he has been keeping a very good watch and maintaining a high state of situation awareness from the fact he has picked up small (50') suspected errors.

lolarose · 27-01-2012, 09:41

Quote:

Originally Posted by MarinerJo

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

no i do not wont to help people like this one. i would like to help them in "classroom" but not when they do not try at least to GOOGLE (to find something, actualy, very important for navigation) before they set sails out., what would you do if driver ask you what is meaning of "stop" sign or railway crossing sign, or pilot ask you what is PAPI or VASI lightning and you are already up there.hey, someone is out there with the boat not even knowing what is bloody CHART DATUM.

Mexdon · 27-01-2012, 09:45

Quote:

Originally Posted by Wotname

Yes, I think most of us would agree but let's remember the OP is only asking about his suspected discrepancy in his GPS accuracy. There has been no mention of not using good pilotage skills or of poor watch keeping.

It could be inferred that he has been keeping a very good watch and maintaining a high state of situation awareness from the fact he has picked up small (50') suspected errors.

Maybe I did not explain myself fully. We all know GPS has errors especially when we are wanting accuracy within a foot or two. So why use GPS in the harbour at all. I can understand in a fogbound situation and combining that with radar but on a sunny clear day it should not even be turned on.

Why do we know what GPS says when we know where we are?

**Wotname** · 27-01-2012, 10:02

Quote:

Originally Posted by lolarose

no i do not wont to help people like this one. i would like to help them in "classroom" but not when they do not try at least to GOOGLE (to find something, actualy, very important for navigation) before they set sails out., what would you do if driver ask you what is meaning of "stop" sign or railway crossing sign, or pilot ask you what is PAPI or VASI lightning and you are already up there.hey, someone is out there with the boat not even knowing what is bloody CHART DATUM.

Who knows why the OP didn't understand "Chart Datum" but not helping him (and berating him for not knowing) doesn't actually help the situation IMHO.
He may be a very fine sailor or a complete noob but that is not the point.

Would you help David Pescud?

Well you couldn't help over the internet as he can't read or write but he did win his division in the 1998 Sydney to Hobart Yacht Race. The race where 6 men died, 55 were airlifted from distressed vessels and 71 yachts failed to finish (retired or sunk).

The point being if you knew ONLY the fact that he couldn't read or write, would you want to share the same waterway as him.

See David Pescud | Crew and Recruits | Disable Bodied Sailors on SBS

Or perhaps I am the silly old fool for providing information that I know something about to someone who doesn't yet know.

**Wotname** · 27-01-2012, 10:08

Quote:

Originally Posted by Mexdon

Maybe I did not explain myself fully. We all know GPS has errors especially when we are wanting accuracy within a foot or two. So why use GPS in the harbour at all. I can understand in a fogbound situation and combining that with radar but on a sunny clear day it should not even be turned on.

Why do we know what GPS says when we know where we are?

And perhaps I also haven't explained myself well (or perhaps I am just grumpy tonight

)

I am supposing that the OP's stated NEW GPS is exciting and what better way to get to know all its functions (and accuracy) than playing with it on a fine sunny clear day when good pilotage is easy and its safe to discover the benefits and shortcomings of the new toy.

four winds · 27-01-2012, 10:24

My Garmin 441s is consistently 50 feet off, a north/south offset.

I can only really notice this when I'm in a channel. On the ICW traveling east/west in a narrow section with my boat dead center of the channel the plotter shows me 50 feet offset to the north or south (can't remember which) but it's no real problem.

With the route marked dead center of the channel, an "off course" reading of 50' is readily visible on screen and corresponds with eyeball on the channel seeing dead center.

I practice this as an aid to my piloting in case I get caught out after dark with no moon and need to navigate in narrow waters. (no radar)

So some of you may get nervous if you see a blue boat named four winds around. Just a heads up, so you know to put your "idiot ahead" plan into action.

SteveT · 27-01-2012, 10:27

The OP was asking for help and abusing him for not being sure as to why he may have errors is a bit uncouth. He is in an inland waterway where he would really struggle to get lost and is using equipment which he is unfamiliar with and experimenting with it - sounds sensible to me. As for being 200 feet different to charts or whatever datum he is using: if he was offshore 200 feet means absolutely jack, it is meaningless. if he was approaching land again he'd hopefully also be using other means of determining his position as well in which case small errors (and 200 feet is small most times) are not that important. I agree the most likely cause of the apparent error is different Datums. Remember that some inland waterways are charted using whatever datum the land is mapped from, in the UK this is the Ordnance Survey and sometimes there can be noticeable differences in areas where there is an overlap.

IslandHopper · 27-01-2012, 10:37

I fail to see how suggesting doing a course can be termed abuse.....

But then i admit to not been a new age sensitive guy....

SteveT · 27-01-2012, 10:44

IslandHopper - It was not your post i was referring to. I absolutely agree that before you venture very far in any kind of boat some kind of formal navigational training is a big help and particularly if you go offshore. FWIW I am definitely not a new age type - not even sure what one is. I am advised that I am a Grumpy Old Man, although I prefer to think I am Master under God (and my Wife) and not necessarily in that order.