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Old 20-11-2020, 21:13   #1
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GPS tracking

I have noticed my GPS tracks are moving to the west of where I have been,

The channel I am travelling up, Direct, North south direction,

The tracks previously Travelled are now 1/4 mile to the west of this channel,
I have an 8 inch ONWA and a 4 inch Garmin GPS,
And both are identical and doing the same thing,

I did connect my stove to the same power source from the batterys,
The remote antennas on both are very close to each other, ???????

Will the two GPS's being on top of each other cause interference as well,

Any info appreciated,

Thanks Brian,
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Old 21-11-2020, 16:26   #2
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Re: GPS tracking

what happens when you are in the channel? is the gps location correct in channel? or is it also 1/4 mile off. there is no way a track can move later.

I bet the channel maping is off, and you are also 1/4 mile off while going through it.
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Old 21-11-2020, 18:07   #3
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Re: GPS tracking

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Originally Posted by smac999 View Post
what happens when you are in the channel? is the gps location correct in channel? or is it also 1/4 mile off. there is no way a track can move later.

I bet the channel maping is off, and you are also 1/4 mile off while going through it.
The channel is very narrow, If your not in it, You are definately on the hard,
The main channel up, You keep to the left or your aground,
The Marina Channnel, you keep to the right or your aground,

Im a Cat and pull two feet, at low tide Even I scrape coming in or out, The channels are not very deep,
I have to be very carefull where I am in both channels,

It gives the track in the channel correctly, Where I am,
It gave the track correctly coming out,
But when I picked up the anchor and was going back, The track I had been on had moved to the west,
I had a friend driving the boat, I said to go there where the track was on the GPS,
I was watching the pilings, and we were going to the left of the piling and was in 2 feet of water,
I had a look at the GPS and the track had moved, Whoops, Thats not where we are going,
We have to go between the pilings or we run aground,
All the previous tracks had moved to the left.

Garmin and ONWA, Both had the same tracks on them and had moved to the left of where we had been,

My Garmin has never had the tracks move from where I had been,
Its very strange how the previous tracks have moved,
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Old 21-11-2020, 20:13   #4
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Re: GPS tracking

The GPS does nothing more than give you a position (ie lat and lon) referenced to a datum (usually WGS 84). If you take that lat and lon and plot it on a chart with the same datum, that is where you are.

If you look out the window and you are not where you think are in comparison to the chart, either the chart is wrong or the GPS is wrong or both.

One easy way to check is to see if both GPS units are reporting the same position and plot that on another chart (ie a paper one). It is unlikely an offical paper chart will be wrong in your area.
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Old 21-11-2020, 20:18   #5
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Re: GPS tracking

Quote:
Originally Posted by Mr B View Post
...........

I did connect my stove to the same power source from the batterys,
The remote antennas on both are very close to each other, ???????

Will the two GPS's being on top of each other cause interference as well,

Any info appreciated,

Thanks Brian,
You have an 12V DC electric stove?

Closely spaced GPS antennas usually do not give trouble but it is possible. I have installed dozens of GPS units (Garmins) in helicopters with the two antennas within a foot of each other without problems bit never say never.

How far are yours apart?
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Old 21-11-2020, 21:59   #6
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Re: GPS tracking

You nee to look at your DOP figure and see what location accuracy the system is claiming
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Old 21-11-2020, 22:34   #7
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Re: GPS tracking

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Originally Posted by goboatingnow View Post
You nee to look at your DOP figure and see what location accuracy the system is claiming
Good point .

For Mr B, DOP = Dilution of Precision; see more here https://en.wikipedia.org/wiki/Diluti...on_(navigation)
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Old 21-11-2020, 22:37   #8
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Re: GPS tracking

If I understand your situation correctly, the active GPS position is shown correctly on the display, and the track laid down previously was in the right position (in the channel on the chart) but at a later time, that recorded track was shown displaced some distance and was no longer in the channel.

If I have this right, it isn't a GPS problem, but something in the charting software that has changed. The most likely one that comes to mind would be the GPS datum that is used in the recorded track has changed. I've no idea how that could happen, but I can't think of another explanation. The file with the series of positions that form the track hasn't changed, but how the program is relating them to the chart has... and a datum change could explain this.

I think that you are using OCPN... perhaps one of the gurus can help with this interpretation.

Jim
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Old 21-11-2020, 23:06   #9
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Re: GPS tracking

Quote:
Originally Posted by Jim Cate;3279526:redface:[B
no longer in the channel.
If I have this right, it isn't a GPS problem, but something in the charting software that has changed. The most likely one that comes to mind would be the GPS datum that is used in the recorded track has changed. I've no idea how that could happen, but I can't think of another explanation. The file with the series of positions that form the track hasn't changed, but how the program is relating them to the chart has... and a datum change could explain this.[/B]

I think that you are using OCPN... perhaps one of the gurus can help with this interpretation.

Jim
Thats exactly it, Jim,

The GPS both give accurate position of where the boat is and its direction,
They both leave an identical track,

But all the previous tracks have moved to the west,

Brian,
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Old 21-11-2020, 23:22   #10
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Re: GPS tracking

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Originally Posted by Wotname View Post
You have an 12V DC electric stove?

Closely spaced GPS antennas usually do not give trouble but it is possible. I have installed dozens of GPS units (Garmins) in helicopters with the two antennas within a foot of each other without problems bit never say never.

How far are yours apart?
The antennas were next to each other, But I moved one about a foot away,
It was only temporary till I could fix the new one in permanently,

I have a gas stove with 12 volt igniter on it, ??????

But in wiring it up, After finding a way for the wire across the top of the roof internally,
It came out beside the inlet 12 volt power for the GPS and Radar,

Well it just so happens I plugged the 12 volt igniter cable into the GPS power cable as well,
They are all 12 volt,
Instead of pulling my dash out to get power independantly for it,

I thought, How often am I going to light the stove up, and for those few seconds it shouldnt matter,
I will change that power source for a start, Run its own wire totally,
Its the only new thing that I have added, The stove,

Then see what happens,


Thanks all,
Cheers, Brian,
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Old 22-11-2020, 00:33   #11
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Re: GPS tracking

Quote:
Originally Posted by Mr B View Post
The antennas were next to each other, But I moved one about a foot away,
It was only temporary till I could fix the new one in permanently,

I have a gas stove with 12 volt igniter on it, ??????

But in wiring it up, After finding a way for the wire across the top of the roof internally,
It came out beside the inlet 12 volt power for the GPS and Radar,

Well it just so happens I plugged the 12 volt igniter cable into the GPS power cable as well,
They are all 12 volt,
Instead of pulling my dash out to get power independantly for it,

I thought, How often am I going to light the stove up, and for those few seconds it shouldnt matter,
I will change that power source for a start, Run its own wire totally,
Its the only new thing that I have added, The stove,

Then see what happens,


Thanks all,
Cheers, Brian,
OK, I don't think the stove will be causing the problem then and the antennas won't be causing the problem either.

Sorry I can't help further.
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Old 22-11-2020, 01:14   #12
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Re: GPS tracking

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Originally Posted by Wotname View Post
OK, I don't think the stove will be causing the problem then and the antennas won't be causing the problem either.

Sorry I can't help further.
It was worth a try, Its out of my depth,

Thanks, Brian,
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Old 22-11-2020, 06:22   #13
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Re: GPS tracking

GPS antennas are receive only, so they can be very close together. Only if they fail to suppress the receiver's local oscillator would it be an issue, which is rare. That theory could be tested by running only one of the GPS receivers. This is probably not the issue as you say they give identical positions (direction is derived from successive position updates, so that doesn't matter).

"Will the two GPS's being on top of each other cause interference as well,"

If they are literally stacked on top of each other, the lower antenna will not be able to see some parts of the sky. This will cause that GPS receiver to use different satellites for its solutions, with possibly lower accuracy. It could even cause a loss of 3D position solutions. But you are really meaning within 1 foot of each other, I think.

You can check the DOP or HDOP (horizontal DOP), but if they both are giving the same answers, that wouldn't change the actual track vs mapped track consistently; it would cause more random appearing errors.

I would try a few test runs through the channel, or between two buoys so you have a know visual course. Then compare to the track.

- Try turning off all other equipment and re-running the test course.

- Borrow a USB GPS receive and plug it into your computer; see how it differs from your receivers.
- Check to position of a few "known" objects, like buoys, mapped vs. measured.
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Old 22-11-2020, 07:07   #14
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Re: GPS tracking

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.
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Old 22-11-2020, 14:25   #15
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Re: GPS tracking

Quote:
Originally Posted by Yellowtulip View Post
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.
You beat me to it - I was going to raise the question of chart datum. Unfortunately my knowledge there is stronger on topo maps than nautical charts, but a chart update in the GPS that caused a change could be the culprit.
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