GPS tracking

[Mr B]

Originally Posted by Yellowtulip

It sounds to me like the datum of the section of charts in use when you were at anchor is different to the datum on the section of charts in the Channel.
Incidentally, if you've got piles or poles marking the Channel why are you telling your helmsman to stop using them and peer at a GPS? In my opinion it's better to say "keep to starboard (or whatever) of those poles" and have the occasional look at the GPS yourself. This keeps your helmsman's situational awareness up too.

When I told my Helmsman to follow the GPS to the points where the previous tracking was and our new course was intersecting,

That point was in the dead centre of the two channel markers,


Which were too far away to be seen, 3 miles,
Then the previous track moved to the left, Unnoticed by me,

Thats when we realised the GPS was up the Putt with the previous track moving,

My Helsman also happens to be a pilot that flew planes into Vietnam to bring our soldiers home,
She does know how to operate a GPS,

Thats the reason Im asking on here to find out whats wrong with it,
And a Possible solution,

Cheers, Brian,

[Yellowtulip]

Originally Posted by Mr B

When I told my Helmsman to follow the GPS to the points where the previous tracking was and our new course was intersecting,



That point was in the dead centre of the two channel markers,





Which were too far away to be seen, 3 miles,

Then the previous track moved to the left, Unnoticed by me,



Thats when we realised the GPS was up the Putt with the previous track moving,



My Helsman also happens to be a pilot that flew planes into Vietnam to bring our soldiers home,

She does know how to operate a GPS,



Thats the reason Im asking on here to find out whats wrong with it,

And a Possible solution,



Cheers, Brian,

I wasn't questioning the competence of your helmsperson. You raised the point of the possible unreliability of the GPS. I'm merely suggesting that GPS, like any other electronic aid, has limits on its reliability, so it's well not to rely on it. A compass course is a good way of navigating if you can't see the Channel markers with GPS as a check.

[Mr B]

Originally Posted by Yellowtulip

I wasn't questioning the competence of your helmsperson. You raised the point of the possible unreliability of the GPS. I'm merely suggesting that GPS, like any other electronic aid, has limits on its reliability, so it's well not to rely on it. A compass course is a good way of navigating if you can't see the Channel markers with GPS as a check.

A compass doesnt give depths or where you are in a channel,
Thats the job of the GPS,
To this date, The GPS has been dead accurate, Both of them,

I know precisely where I am with out looking at a compass, Especially in Westernport Bay,
There are plenty of land marks to look at,
Its hard to miss the Hastings shipping terminal over on our left side,

The depth sounder came into play here, She said we are in two feet of water,
Thats wrong, We should be in 30 feet of water, In the channel,
So I checked the GPS and found we were well off course,

Thats when we realised the GPS was off,

At low tide, Where we were, The land is actually above the water level,

[Wotname]

Originally Posted by Mr B

A compass doesnt give depths or where you are in a channel,
Thats the job of the GPS,
To this date, The GPS has been dead accurate, Both of them,

I know precisely where I am with out looking at a compass, Especially in Westernport Bay,
There are plenty of land marks to look at,
Its hard to miss the Hastings shipping terminal over on our left side,

The depth sounder came into play here, She said we are in two feet of water,
Thats wrong, We should be in 30 feet of water, In the channel,
So I checked the GPS and found we were well off course,

Thats when we realised the GPS was off,

At low tide, Where we were, The land is actually above the water level,

Mr B, you keep repeating the GPS was off but have you confirmed this as it seems unlikely both GPS units would be 'off' by the same amount?

I am thinking your chart plotter was off or at least the previous track has been displayed incorrectly - but why I don't know.

However in order to get to the bottom of this problem, you have to seperate (at least in your mind) the GPS functions and the chart functions as shown on the chart plotter. Although they might appear on the same instrument, they are completely seperate functions. Once one of the functions are proved correct, you can concentrate the fault analysis on the other function.

[Mr B]

I will post a piccy of my GPS when I am down the boat next,
Then you can see what I am talking about,
In 4000 Nmiles, This is the first time it has happened,
I will reroute the 12 volt wires from the stove,

Thats the only thing that is different other than putting the Garmin GPS above the ONWA GPS instead of below it,

[Mr B]

Here are the pictures of my Garmin and ONWA with the tracks moved to the left,
I came up the channel in the centre, As per the Track in the channel,
The east west tracks havent moved,
But the previous tracks have all moved to the left,
Where the tracks are north south is all above the tide level,

I have disconnected my new stove from the wiring,
I will take the boat out tomorrow to see if the wiring removal has effected the tracks again,
The only other change was putting the Garmin above the ONWA GPS as per Picture, ,
If the tracks move tomorrow, I will take the Garmin down and put it back where it was,

After that, I dont have a clue,

Cheers, Brian,

[Hesti]

If you find the solution, let me know. I also have an Onwa (rebranded version, but an Onwa) and have the same problem on exported tracks. They are considerably off. When underway, the thing is as accurate as I could wish for, within a handfull of metres, but the exported tracks are not worth anything in tight spaces.


Something about chart systems possibly, corrections for wgs84 not exported or watever, but no way to change settings for this.

[tanglewood]

Oh, I see what's happening. Your track recording interval is too high. It needs to record points more frequently. You have recorded points in the upper channel, then the next recorded point is in the lower channel. When that's later displayed as a track, it just draws a line between the two points.

[Mr B]

Originally Posted by tanglewood

Oh, I see what's happening. Your track recording interval is too high. It needs to record points more frequently. You have recorded points in the upper channel, then the next recorded point is in the lower channel. When that's later displayed as a track, it just draws a line between the two points.

AAAAHHHHH, You might have it there, I didnt mark the way points at all,
But previously, It did automatically just mark where I had been,
With out drawing a straight line,

I will try putting in more way points, Especially when I change direction,