GPS Accuracy?

  • Thread starter Thread starter bob_tyler
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Re: GPS Accuracy? + EGNOS + Admiralty Charts

I completely agree with you that GPS is only an aid to navigation and also about the Mk 1 eyeball. However, the Mk 1 eyeball will fail in thick fog, at night or when an obstruction (rock?) is submerged.

From the correspondence above my doubts are increasing over the accuracy of the paper charts we all use and from which the digital information is derived.

It has always been my practice in cases of doubt to give a wide berth to obstructions such as rocks and wrecks if I can't see them. I think I may well increase the allowance for error in the future where there is searoom to do so even though travelling a little further..

In the English Channel (La Manche for our French friends) I think that I will increase the margin to about 100yards or metres from my present practice of 50 yds/mtrs in plotting a GPS track. I have been sailing since the mid sixties and have managed to avoid hitting anything hard so far and want to keep it that way.
 
Re: GPS Accuracy? + EGNOS + Admiralty Charts

Bob,
Next time you are on the boat read off the position of something in Fowey harbour and I will check it against the NT+ chart cartridge and see if I show the same long lat.
 
[ QUOTE ]
At the end of the day does it matter, who navigates by chart plotter in a relatively small area such as a harbour?

[/ QUOTE ]

About a month ago we made the passage round the Kyles of Bute. For interest sake, and to see if it worked, we set up a route on the GPS which should have taken us round the Kyles, using the northern channel round Burnt Island. We used waypoints taken off the largest scale Admiralty chart. Then we linked the GPS to the autopilot and sat back to see what happened.

The boat held to the track perfectly. Going round the dogleg in the Burnt Island channel there were four hands hovering over the tiller, all ready to take over should anything go wrong, but I don't think the boat could have been more than 5 yards away from the centre of the channel at any time. The only thing that really worried us was finding the Waverley coming through in the opposite direction, timed so that we should just about meet in the narrows! We slowed right down to let her pass before we entered the narrows; not a spot in which to argue about Rule 9.

We had good visibility, and could have taken over in a few seconds had anything gone wrong. It did, though, give us a greater degree of confidence in the accuracy of GPS and the boat's instruments. I hope I'll never find myself in the position where I'm forced to rely on them, but if I am ........
 
[ QUOTE ]
At the end of the day does it matter, who navigates by chart plotter in a relatively small area such as a harbour?

[/ QUOTE ]

About a month ago we made the passage round the Kyles of Bute. For interest sake, and to see if it worked, we set up a route on the GPS which should have taken us round the Kyles, using the northern channel round Burnt Island. We used waypoints taken off the largest scale Admiralty chart. Then we linked the GPS to the autopilot and sat back to see what happened.

The boat held to the track perfectly. Going round the dogleg in the Burnt Island channel there were four hands hovering over the tiller, all ready to take over should anything go wrong, but I don't think the boat could have been more than 5 yards away from the centre of the channel at any time. The only thing that really worried us was finding the Waverley coming through in the opposite direction, timed so that we should just about meet in the narrows! We slowed right down to let her pass before we entered the narrows; not a spot in which to argue about Rule 9.

We had good visibility, and could have taken over in a few seconds had anything gone wrong. It did, though, give us a greater degree of confidence in the accuracy of GPS and the boat's instruments. I hope I'll never find myself in the position where I'm forced to rely on them, but if I do ........
 
I have used Navionics for 5 years now and Generally it has always been correct.
However I did find inaccuracies in the Biscay chip. No lights shown on the trevenec [raz du Seine] and buoys out of place further South. Yeoman was however correct.
Told Navionics and now response!!!
 
I did say harbour. However I agree with what you say, it is very comforting to have a route plotted in a tricky area, especially if it is new to you. I do stick with what I said about harbours as the electronic mapping can be quite poor in many places, well that is to say out by quite a few metres!
 
Meeting the Waverley at the Burnt Islands - been there, done that! There's a lot of her and not much channel...

I've got a series of waypoints set up for going through the Cuan Sound with which I have tried to do the same "hands off" exercise as you. Each time, however, I have chickened out at the critical point and taken over manually since I was convinced that, although following the GPS course exactly, we were actually heading for the normally covered rock NE of the Cleit. Certainly there is a significant discrepency between the charted depths along the plotted line and those measured. Of course the fact that the tide is under you at the time and things are happening fast does nothing for meticulous observations! I intend to sometime go through at slack water so that I can poke about slowly to properly resolve this, but as of now I have the impression that there's a slight chart offset.
 
Not generally known ....

Having run GPS Nav group for some significant time now and watched / seen various different chart formats ....

There are anomalies in the digitising of charts and it is not uncommon for some charts to diplace a position when changing from another.

Let me put that better ..... (and before some smart arse jumps on me - I KNOW the original post is plotter not PC ....) .... A Maptech chart of a position will put you on spot A. Don't move but change chart to a Fugawi / SHOM chart and position often jumps a number of metres .... in case of my home berth about 150m NE.

Now why would I illustrate with a PC based plotter ..... the ability to see that different chart systems / formats can actually give different results. Now lets extrapolate this out and look at Plotters .. with their single proprietary chips ....

Who can say that all charts and datums used in the digitising process are correct. And who says they use the same origin of chart for the whole set. For example .... English Channel folio chip may be a collection of UK Admiralty, French SHOM, US NOAA etc. Each having potential to displace position ....

As we are all finding out .... GPS is an AID and not the be all and end all of Navigation. It is a very fine piece of gear that used well can save lives and improve our use of boats .... but it is an aid.

In Fog .... the Echo-Sounder and GPS together make a formidable team ... along with gently, gently speed etc.
 
Re: GPS Accuracy? + EGNOS + Admiralty Charts

[ QUOTE ]
my doubts are increasing over the accuracy of the paper charts we all use and from which the digital information is derived.

[/ QUOTE ]

This is a little displaced from your initial question about what could be causing the original issue you saw, but I'm suprised at the comment above.

People have been saying on these forums for years that much surveying is based on original surveys from over a hundred years or more ago, and that gps is far more accurate than the chart data. Many of the boat mags have also had article which have made the same point.

Are people really not reading the comments here, or the articles, or just based on commensense, until a real life situation occurs to make them suddenly question the validity of chart data, compared to gps accuracy?
 
Re: GPS Accuracy? + EGNOS + Admiralty Charts

Brendan,

Of course I have read all this several times before. This is the first time I have been in the situation where my position on the Admiralty Chart differed so much from the position on my plotter. I always allow for some error on both and steer well clear of rocks etc.

I was trying to get others' opinion on whether the inaccuracy was in the boat's position on the plotter as derived from GPS compared to the actual position taken both by bearings plotted on the chart and by Mk 1 eyeball.

It was as though the plotter chart had slid North East a bit under the boat position. I completely agree with your comments.

I wish I had permanently recorded the positon taken from bearings and figures and chart on the plotter. The position on the chart, from the bearings and from eyeball, was much closer to actuality.
 
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