GPS Accuracy?

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bob_tyler

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I popped up from Falmouth to Fowey, only about 20 miles, during Fowey Week to join in the fun and games.

Idly playing with my plotter (no rude comments please Jimi) on my mooring in Fowey it showed my position to be about 50 metres to the Southwest, almost out in the main channel, from where I really was (C Chart). It shows me right on the spot on my Falmouth mooring and everywhere else I have checked.

I checked it again frequently during the week but the position shown never changed, except to show slight movement when the boat had swung on the mooring.

I checked that it was still set to WGS 84, it was.

Have some charts been wrongly digitised? Or can one of the Forum boffins please explain?
 
Bob
I can't explain it but I can say that the GPS in my new motor has been absolutely spot on, that is, until this week when it appeared to be off by about 50 mtrs!!!! Depending on my "heading" it was either OK or Off and asked me to turn where there was no turning.

I'm not sure if my insurance company is going to pay for the new chip shop window I drove through when the polite voice of my GPS said "please turn right now" and I did /forums/images/graemlins/smile.gif
Peter.
 
My MapTech puts me right in position in Fowey (and Falmouth and Helford) to the extent that I have to allow for the fact that the antenna is at the stern and the boat is laying to the wind, to compensate. I have come out of L'Aber Wrac'h in poor viz on it and that's not an excercise I do willingly or lightly which means that I trust it (as an aid).

I don't think it's the GPS.

Local info - Helford Village Regatta this Sat - fireworks 2130.
 
Had it happened just once, then there would have been several possibilities. But to happen repeatedly, and to give accurate positions elsewhere is v difficult to explain.

The most obvious explanation is that you are wrong and you arent where you think you are! have you tried doing a 3 point fix to locate yourself on a chart for comparison?

The second possibility is that there is a local error in the digitising of the chart, but this process is automatic and I would doubt just a localised error.

The only other possibility is that someone has moved Fowey. /forums/images/graemlins/shocked.gif
 
[ QUOTE ]
Have some charts been wrongly digitised? Or can one of the Forum boffins please explain?

[/ QUOTE ]Here's a suggestion. use Google Earth (mentioned on other threads), 'Fly' to Fowey, England and then read off the coordinates (given to the nearest foot) of where you figure you were.

Haven't tried this myself, but intend picking up a GPS from boat today and seeing how it agrees with my positive ID of my house and boat from GE.

(well, GE has got to be good for something !! )

PS. Haven't yet figured if GE uses WGS84, ! assume it does, so may need shift for your charts, mine are European 1950
 
Did you check your long and lat against a paper chart? If you did you would probably find it was correct. C-Map does have errors for harbour detail, my boat was always in the "wrong" place for it's Dartmouth mooring. When I upgraded the cartridge to C-Map NT+ the boat then appeared on the correct finger pontoon.
 
Birdseye, yes, I did take a 3 point bearing which, when plotted on my Imray Chart, showed the correct position. The position shown on the plotter chart, when read off the plotter figures showed to be about 50 mtrs out on the Imray Chart.

Conditions were flat calm and no movement, ideal for taking bearings.

My C-chart is corrected to June 2004
 
[ QUOTE ]
My C-chart is corrected to June 2004

[/ QUOTE ]

Does not matter if it is corrected or not, the mapping is probably incorrect, I did not beleive it until I borrowed a C-Map NT+ and saw the difference. Next time you upgrade ask for a NT+ cartridge.
 
Although I did record it at the time it was with a very light pencil on the chart which I them deleted. I was not on a pontoon but fairly near the wall of the morrings maintenance area.
 
Just what is accuracy. Thought the yanks didn't allow 100% accuracy.Your reading from your gps can't be transferred without the loss of accurecy.100% accurate readings of the same spot will only be accurate for for that spot, once you transfer that reading you will lose accuracy, depending on the scale of your charts.A simple pencil line can be anything between 10 and 500 mtrs wide in real life depending on the scale of the chart you are using.
 
Bob, what might be an idea is to check the plotter against a different plotter to see if the difference is due to position or mapping?
 
Jimi

Although there were several other Club boats moored near me I was stupid enough not to think of this. Ah well, age is beginning to tell! /forums/images/graemlins/frown.gif

Bob
 
I was intrigued my your post as my Navman GPS does not show me at my berth in Chichester - it shows me on the next pontoon.

So I tried to check it's accuracy. I took took readings from 3 GPSs and they showed

Magellen H/H 050*48.294N 000*49.303W
Navman 050*48.293N 000*49.302W
USB linked to Laptop 050*48.292N 000*49.300W

The positions to 2 decimal places are, IMO, in agreement as makes no difference.

I have now plotted those fixes onto my C-Map NT + charts on the PC and they place me 0.1M 094 degrees from where my boat is in Chichester Marina.

When I travel down Chicester Harbour the C-Map charts on the Navman show my position accurately as I pass the various buoys in the Channel. Therefore it must be an error in the marina chart.

I also plotted the positions on the Magellan Bluenav charts and the position plotted is 0.1M in the other direction to my physical position.

I think the 'problem' is exaggerated because with digital charts we tend to zoom in, in effect turning them from charts into plans and we try to work at a far larger scale than we would have done in the past with a paper system.
 
I guess that is what I was trying to say. Interesting that moving to NT+ fixed it for me in Dartmouth, I guess that C-Map have only updated some of the the harbours. At the end of the day does it matter, who navigates by chart plotter in a relatively small area such as a harbour?
 
Re: GPS Accuracy? + EGNOS + Admiralty Charts

Malcolm, very interesting. If M means Nautical Miles that means about 200yards out. That is far more than most expect using GPS - could be quite worrying in confined waters in fog.

As my Navman 5500i is usually spot on, within reason, it was this anomally which intigued me.

I am wondering if the Admiralty Charts from which the digitised info is collected are out? After all, many, if not most of them, were surveyed before GPS was there to help.

Also, if that is the case, will EGNOS help when it starts? If the original charts are inaccurate to the extent shown in your posting, reducing the small inaccuracies shown at present won't help much without new surveys.
 
Jim, I have when thick fog rolled in unexpectedly and found my mooring safely. I probably wouldn't have found it if it had been in Fowey.
 
Re: GPS Accuracy? + EGNOS + Admiralty Charts

I should probably have clarified the distance. the distance was as measured on the C-Map chart software using an A-B line. I think that, that is its default minimum measurement. As I can determine in the marina it is definitely less than that, probably only about 20 - 30 metres.

Having said that, as the poster said, when you are in the marina, who uses GPS. When exitting the marina and going down Chichester harbour I use pilotage techniques with a paper chart, with GPS as guidance.

As far as I know, WAAS correction only affects USA accuracy, and EGNOS is not yet live in Europe. So I would certainly be relying on Mark1 Eyeball for pilotage and only using GPS as a guide as I would expect certain errors on GPS.

I think that we should all be aware that GPS Chartplotters are there as just one of our navigational aids.

I'm probably going off the thread a little bit and for that I apologise.
 
Re: GPS Accuracy? + EGNOS + Admiralty Charts

Down around the Sth Pacific way some charts are out, in places upto 2000mts. Ctp Cook did a great job but hardly to GSP accuracy. So there could be a smidgen on the chart.

Do you need to calibrate your GSP?. Some need it and like a 'hight from datum' measurement input when it is first kicked into life.

Some GPS also seem to pick up 'shadow' (for lack of the right word) signals when in a marina and busy places like that which can place you out a bit.

If your getting a postion inside 20-30 metres all the time that is pretty good. Some people belive these are accurate to the metre (which at times is correct but just 'when' is the tricky bit to work out) and unfortunatly one or two of those are now dead.

GSP is a navigational aid. Having done 30000 odd miles I belive the GSP to 100mts and then, as mentioned above, the Mk1 Eyeballs take over.

My theory is technology is a great aid in also sorts of areas but when it goes bad it is not always obvious. To find out the GSP is off 20-30mts by banging into a rock is not the ideal way.

Safe sailing all
 
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