GPS errors

[kc]

Had a mate last week involved in a very seriuos accident in a big boat where the skipper "trusted" a top line GPS, travelling at speed, at night. The GPS positioned the boat 700 meters off course.

Interested if anyone else has has similar GPS errors.

At issue is that fisheries use GPS data to confirm a position in regards zoning and then book people wholely & solely on GPS data, which the manufacturers acknowledge is prone to error.

KC

[finga64]

biggest error with mine is the operator :-/
but it gets me very close to a tiny reef we fish all the time.
In the book they say that people should not depend soley on GPS's for navigation.

[theoldlegend]

Hi KC,

I'm surprised that your mate was allegedly 700 metres off course.

I thought that they got you much closer to the actual position than that. About 5 to 10 metres? How would you prove it though, if push came to shove, and an insurance matter was at issue?

Don't know.

TOL

[theoldlegend]

KC,

I forgot to mention that if the Fisheries mob book people for being inside a no go zone, how can they be confident that their bearings are correct?

Seems a bit sus to me. Could be a case that if they reckon you're in a no go zone, then you can ask to see their position on their instruments and compare them to your own maybe?

TOL

[Owen]

I was out on a charter a while back.
We were drifting in an area close to a green zone.
The skipper had just called "show hooks" as he thought we were getting too close, so we moved out again.
A few minutes later, off in the distance we see a boat, then after a while a smaller boat heading towards us.
It was the fisheries guys.
They boarded us and had a look around at everything and told the skipper that according to the RADAR he had been 100M inside the green zone!
He argued the point with them, showed them his position on his GPS and compared it with his charts.
In the end, they didn't book him (though apparently they do log the "breach"), but they wouldn't concede that he was correct with his bearings.
I don't know much about radar, but I wouldn't have thought they were accurate enough to have determined that he was 100M inside the zone when they were a good 3klm's away at the time.

To be 700M off course the GPS can't have been hooking on to hardly any sattelites.
Under my carport when I can only get a few my unit claims about 100M accuracy.
Out on the water it can get down to a claimed 5M.
Never tried to verify it though, so if I ever go near a green zone in my tinnie I'll be staying at least 500M away just to be sure :)

cheers,
Owen

[Jeremy]

where's Kerry when we need him?

Jeremy

[grex]

where's Kerry when we need him?

Spot on. I had trouble with my GPS 12 - 18 months ago & he was a wealth of information.

Generally speaking , most GPS errors occur due to differences in datum settings.
I recall playing around with mine at home , took a reading on WGS84 then changed the datum to another setting (can't recall which one) , into the car & drove to a spot which was showing the same coordinates.

I was , as the crow flies , approx 500mt from home

Geoff

[Ando74]

Incorrect datum is a major error in GPS, on most charts it will tell you how far to move your postion to make it correspond with WGS84. The number and elevation of satellites will also affect accuracy.
I work on a big 'boat' (185m oil tanker) and GPS is still used as an aid to navigation only.

[baldyhead]

Did the Radar also show the blokes were actually fishing in the 100m of green zone?
Thats a pretty hard one for them to prove I think

[baldyhead]

KC, I was in the middle of the Mulgrave river coming thru the heads and onwards up the river. My Lowrance GPS c/w the latest navionics chart zoomed right in said that the boat was actually 150m inland....in the mangroves...then in the cane. This was @ approx 3pm with 8 satellites showing. Usually it is much more accurate than this but I would NEVER rely on it at night in a dicky area that's for sure....cheers baldy

[theoldlegend]

Who's Kerry ??

Never hear of him.

TOL

[redspeckle]

With my GPS 72 every time you turn it on A warining

Warning All Data is presented for reference only.
You assume total responsibility and associated with using this device

Then you have press page Button to agree befrore can used rest of unit
So in the court of law Garmin have cover themselfs of any fault with the above instructions so if you run aground its you fault
I only use it as a guide as a 2nd navigation The main navigation use is the Beacons and Land marks, Charts/maps and Compass
KC So in a court of law someone in a green zone that is just inside it about 1km should be throwen out because GPS data is only a reference information only but unless the unit been officialy tested of its accuracy documented proof then the would stand up( maybe) [smiley=huh2.gif]
Some was in the green zone 5 km in yep should be fined
Mitch
Gee I miss kerry :'( would have given good adivce here on this subject
come back kerry we need you

[theoldlegend]

Hey Mitch,

Thanks for your words of wisdom.

Saw you at the servo the other morning when you were taking your niece out to get into the whiting.

I don't have a boat, so I'm open to any offers that may happen to be thrown my way.

Regards

TOL

[redspeckle]

Theoldlegend
I keep you in mind for next trip bay or offshore
I have sent you a pm
Mitch

[Lucky_Phill]

I am currently using 3 GPS units. All Garmin.

and old 120 in the tinnie and a 126 and a 172C in the big rig.

The 172C is ( IMO ) a far superior unit to the others for accuracy. BUT, having said that, the map shown on the 172C is NOT accurate. I do believe however, the actual GPS position, is spot on, due to me finding very small pieces of reef and shipwrecks where they are supposed to be.

The accuracy of each individual unit can vary, depending on exaclty where you " set-up " or " initialize " the unit. Best effort to initialize is ON WATER. Also setting up the correct Datumn is most important. WGS84 seems to be the rage, at the moment.

For a GPS system to be 700mtrs off KC seems a bit odd to me. Tells me a couple of things could have happened, like someone tinkering with the unit, or the unit not responding to the ' best ' available satelites. At any rate, the skipper more than likely had his Auto Pilot on, interphased with the GPS unit, thus the problem.

As for the Green Zone boundries, I believe the Fisheries must give the benefit of the doubt to the Skipper of said boat, using his navagation system to best tell his current position in relation to the " magic white line drawn on the ocean ". And how does one find this line, minus a GPS system ( which btw ) is NOT compulsory to have aboard ya rig while fishing near, in or around green zones.

I am still wondering what happened to the skippers " track log ", kc ?

Cheers Phill

[Wild_Thing]

What About The Old Days When Their Was No GPS.Think Its Pritty Easy To Blame A Nav Aid.
Was The Skipper Of Sound Mind And Body At The Time.Sound Like It May Have Been At Night!
These Things Do Happen!
I Heard Theirs A New System Being Released Soon Thats Even More Accurate Than The Currant One.
They Will Also Have GPS In Mobile Phones So That the Parents Of A Child Will Be Sent An SMS If The Kid Goes OutSide A Certain Area.But Then Again I guess the phone Needs To Be Turned On.
Cheers Payney!

[Volvo]

Firstly one should never rely on a GPS/Plotter for safe navigigation, they are an aid or so the manual sez??.
Yeah where is Kerry when ya need im:)...
Tell ya way i do it if you can make sence of it good if not, i tried:)..
The manual for your GPS/Plotter should tell you how you can adjust or correct your unit if its out a tad so you will have to read it on how to do so..
In front of our local VMR base they have a brass plate with the lat n long for where that plate sits so its quite easy on a not so busy day at the boat ramp to drive the boat over this plate and checkout if both the lats n longs match up with each other and if need adjusting.
Also sez whether its on 66 or 84.
Prior to that i used to nudge my boat up against one of the red channel markers to the creek entering the boat ramp and do the same thing as ide previousely logged in its co-ordinates and just see if there was any error and what side of the marker my boat sat against with the plotter..
Far as Gladstone goes with the charts i have they is on aus 66 so my GPS/Plotter is set on Aus 66 and today have had no problemo's with either the GPS or the plotter with either landing over the same bommie, wreck or whatever and always check to see how close or far i am from the leads/ nav aids in the harbour when under way..
Comming back in or going out of a night time i NEVER rely on either unit but stick the noggin out of the canopy and go by the leads , also occasionally watch which way the wake from the motor is heading just to let me know if the wind n tide may be affecting my boat if those leads are adjascent to rocks!!!..
Okay see what you can make of all that :-/ and if ned be ask away..If i can elp i will..
Cheers

[Angla]

I got pulled up about 5 odd years ago at Flinders reef and they also said they had me on radar. I told them they would have seen me move as they approached because a tour/dive boat had just left and I thought they would take its place so I moved in while I could and showed them the plotter marks on the gps. They said I was still in the wrong place and the argument went on for a while. In the end I talked my way out of 3 breaches Because I think they did not have enough evidence.

I do tend to take these type on when I'm unknowingly in the wrong. I don't always win but sometimes.............Hey it's a bit of a blonde defense but if it works.

I also said I would read the big boards at the boat ramp where I launched as apparently all the information is on them.

And dont forget that all these depth limitations are at lowest tide so if the depth minimum is 2 metres and you are at 2.5 metres of depth at a tide height above 0.5 metres.......you are illegal.

Angla

[kc]

The skipper involved in this accident was very experienced, stone cold sober and watching his top line GPS like a hawk and had 2 guys on the bridge with him and was getting a 700 meter variation.

There is a us navy example of a GPS fixed on top of a bulding which, during a 6 hour period, at one 4 minute stage the building "moved" 1/2 mile at 14 knots??

As to testing this in court it is clearly something we are considering.

When booking someone in a green zone, fisheries first establish the position on a GPS and then confirm that position on the GBRMPA maps.....both of which clearly identify themselves as "aids to navigation" and "not for navigation purposes".

Interesting situation given everyone is innocent till PROVEN guilty.

KC

[Wild_Thing]

Another factor that can make GPS have a variation is the position of the aerial or even heavy cloud cover.
When you concider how they work by sending and recieving a signal to a satelite and timed to the millisecond , or something like that.
I admit they do vary a bit even if perfect situations!
Trying to prove the GPS to blame in court won't stand up because of all the warnings on GPS at start up , then you need to press the OK button to confirm you understand the warning.
This is also worth a mention if you have been fined for apparently being where you shouldn't.
Best of luck to your mate anyway KC . But if no one was hurt boats can be repaired & i think we can all learn from the experence.
Cheers Payney ;)

[grex]

An interesting comparrison is to look at Google Earth & check the coordinates which appear in the bottom left hand corner of the screen against your GPS.

Not much good for checking spots out in the ocean but if you want to compare your GPS against another source this is an option.

Take a reading at home then check G.E.

On WGS84 the difference at my place was approx 20mt

Geoff