GPS ACCURACY

[Trevor_swfyc]

Nearly every month an article appears blaming GPS for, hitting a rock, running aground and many more mishaps. GPS was quoted as being accurate to within 100 metres 95% of the time. I believe it is much more accurate than that now. So why are we getting printed comments such as its only any good for getting LAT & LONG but not for anything else.
Am I being cynical but could it be a case of rubbish in = rubbish out, whats going on?
Whats your experience / view.

Its all a worry.
Trevor.

 

[rogerroger]

GPS is accurate now to about a boat length. Trouble is, many charts are not as accurate so there lies the problem.

My chart of Chi harbour got my boat on it's mooring absolutely spot on. Up the River Dart at Dittisham it put me well on the land ! There are many instances when I'd gladly dump the GPS for a good echo sounder.

Roger Holden
www.first-magnitude.co.uk

 

[bedouin]

Re: A bad workman always blames his tools

It is difficult to blame GPS for anything - it is a tool like anything else; and anyone who tries to blame it for hitting something is probably just making an excuse for their own incompetence!

The problem with GPS is that it is just too accurate. With the switching off of SA errors seem to be down to a few feet - this means we tend to rely on it too much rather than using it as one of many tools for navigation/pilotage. I have never known my GPS to give a wrong position but there is plenty of evidence that they do occasionally, and there is plenty of scope for user error in inputting the waypoints, and errors in the charts, many of which are not as accurate as GPS.

So if you are navigating by GPS alone you are trusting
(i) The chart is accurate
(ii) You entered the waypoint correctly (including Datum &c)
(iii) The GPS is working correctly and is accurate.
That's an awful lot of trust!

 

[trev]

GPS is an AID to navigation NOT a means of navigation.
All aids should be treated with a degree of scepticism and to rely on a system to such a degree that it could put you in danger is not good seamanship.
The mark-1 eyeball is still the most reliable.

Trev

 

[billmacfarlane]

I'm not sure about GPS only being good for LAT/LONG. It surely depends on what type of GPS is being used and the functionality of the set. If a boat hits a rock , I'm afraid it's got nothing to do with the GPS but a lot to do with the navigator. GPS is a great tool , but it's not a substitute for traditional navigation but to be used in addition to it.

 

[Guest]

I know my three GPS's are accurate, an old Garmin, an old Apelco, and an old Magellan because they agree fairly well with my old Loran! Yes, I still rely more on my Loran. The primary reason why I rely on the Loran is because my charts are not new and they are Loran charts, not GPS charts.
The difference between the two is enough to cause the exercise of significant caution in my home waters. Reliance on any electronic device in these waters is foolhardy at best. For instance, one of our charts that is recent states that the survey datum is very, very old and has not been updated....so the possibility of shoaling and changes due to storms is highly probable.

There have been significant changes in land masses and bottom features here in Florida due to hurricanes in the past. So, it behooves one to neither rely solely on charts or on their GPS's or Loran's for that matter. So, in the long run it is the "prudent mariner" who utilizes all available
faculties and instruments while boating, not relying solely on electronics.

 

[rex_seadog]

Roger
My boat is moored up the Dart off Dittisham and my 5 year handheld Garmin appears to locate our mooring correctly (as near as damn it) on our Imray chart.

The only significantl mis-positioning I've noticed was on a chartered boat off the east coast, south of Harwich, when the boat's GPS claimed we were 1/2 mile west, on the land! (Trouble with a chartered boat is that you don't know how the instrument was set up. When chartering now I always take my own GPS as a back-up).
I have to agree with other replies that as accurate as GPS usually is, it should not be the only means of navigation. I get a little alarmed when I read of some peoples reliance on electronic charts etc. I get enough glitches from my land based PC without putting too much faith in a computer on board a small boat!

 

[rogerroger]

My point was more about the charts - I reckon the GPS DID give the correct location in Dittisham (we managed to find the pub without it!!) - so perhaps the Admiralty chart I was using was out.

Guess there's no way of knowing for sure really!

Anyway, anyone who needs to use GPS in a river really shouldn't be sailing I reckon !!

Nice place to have your boat - we really liked in up there and spent several days there listening to forecasts of F7 out so sea!!

Roger Holden
www.first-magnitude.co.uk

 

[rogerroger]

Indeed. In fact it's probably too acurate now.

When it gave you a position to about 300 metres you simply had to confirm it another way. Now it's too easy for a novice to rely on it completely and get into trouble.

The more sophisticated electronics get the more untrained people take to the water... the GPS tells you where you are, the DSC radio brings a rescue at the touch of a button...

Roger Holden
www.first-magnitude.co.uk

 

[claymore]

I agree with the bravery factor. I think years ago, people tended not to stray too far offshore because they didn't trust their navigation, certainly I know a few at my yacht club who wouldn't do the 50 miles to the Isle of Man without having someone else to sail in company with.
I'm amazed by the accuracy of GPS - I have it linked to a software package on my laptop and last summer I was showing someone how to do 3 point fixes whilst we were in Tobermory. The fix and the GPS icon's position were practically identical. The problem is then that you get brave and allow yourself to completely trust the electronic chart, then you sail a little closer.......

 

[brianrunyard]

Mine is accurate to within about a boats length.
But then I'm using the correct Datum, remember charts have a note giving the correction to GPS positions.

 

[HaraldS]

Haven't really read any GPS blaming stories lately. I thought by now most people know that this has nothing to do with accuracy, rather than stupidity. The GPS position is much better than anything we ever had before. If the charts are of latest survey the accuracy is ovewhelning, like in Norway where I noticed that markers showed abeam, when they were actually abeam of my stern, where the antenna sits.
But seriously I wouldn't go into a difficult passage, solely relying on a few meters of accuracy anywhere, without additional means of verification.
I think these stories are as absurd as the BMW story I read a year ago, where a guy with a car navigation system drove straight into the river, after coming around a bend, following the street on the display onto a ferry path. There was no ferry and the car sank, and he tried to blame BMW! How silly can you get?

 

[colin_jones]

Open sea and wide, recently surveyed estuaries are fine as long as you are on the correct chart datum. Now, very impressive.

Ours goes a bit adrift in smaller harbours - where the chartlet has not always been written to the same datum as the big chart which it overlays. The errors are consistent on a big chart plotter ie we always seem to clip the corner of Brixham Marina and are shown on the wrong side of the river in Audierne.

Here in Lyme Regis it shows me out side the harbour master's office - where I am keeping my head down in case he notices me and asks for some money.

 

[LadyInBed]

No one can doubt the accuracy of GPS, its normally finger trouble that causes the problems.
In DECCA days it was said that the position given might not be accurate, but it was repeatable.
i.e. if you were over a wreck, or on mooring or whatever, even if you were not at the charted position, if you entered your location as a waypoint you would get back to the same position next time.
So I always put in an ‘approximate’ waypoint until I actually get to the position I was aiming for, then enter that position as the waypoint. This is a practice I've continued with GPS.

 

[Piers]

The main problem is complacency of the user, putting far too much trust in the GPS without the knowledge to back it up, such as Chart Datum, correct reading of the lat and long, faith that the GPS has not gone wrong, not knowing how to check it to see if it has gone wrong, and so on.

GPS is really excellent, provided you know what you are doing with it, and not just following it in blind faith.

Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk

 

[Trevor_swfyc]

Summing Up

Thank you all who have spared the time to have an input into this interesting subject. It looks as though most agree GPS is currently far more accurate than the charts and waypoints we are using. Errors most common extracted from postings.

(1) Input error that goes undetected.
(2) Using non WGS 84 charts with the GPS set to WGS 84 (GEODETIC DATUM).
(3) Setting up a route and not checking that it doesn't run over a hazard.
(4) Accepting any waypoint as being correct without knowing its source accuracy.

I think the point made by NigelB "A WAYPOINT SHOULD ALWAYS BE CONSIDERED APPROXIMATE UNTIL CONFIRMED BY GOING THERE AND THEN ENTERING THE WAYPOINT AGAIN IF NECESSARY". sounds like good advice to me.

Many thanks to all.
Trevor

<P ID="edit"><FONT SIZE=-1>Edited by Trevor_swfyc on Tue Jan 29 13:01:07 2002 (server time).</FONT></P>

 

[jfkal]

Fully agree. Always keep in mind that GPS is more accurate than many charts who date back to the 19th century. So you get a precise position on an imperfect chart.
And- the BMW story is hilarious. I would sue them as well for not having their GPS linked up with the ferry operators GPS )). They should at least update their US version of the manual: FERRIES MIGHT BE FURTHER AWAY THAN THEY APPEAR! Only proceed if you can see the smoke from the ferry captains pipe