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Bycrick · 23-09-2021, 11:09

"Ground truth" has nothing to do with this question. With a GPS compass, you have a piece of equipment with a theoretical base line of at maximum 18” although mine is less than 12”. That base line isn’t marked anywhere on the equipment. On the boat, you have a theoretical line from the center of the stem to the center of the transom. What you’d like to do is stretch a chalk line on the boat, extend the baseline on the compass, and make sure the two lines are parallel. Good luck doing that within +/- 1/2 degree.

Stu Jackson · 23-09-2021, 11:40

Quote:

Originally Posted by Elleroo

I do compass adjusting occasionally.
....................

These suggestions were the first I thought of when I read you OP, before I read any of the comments: essentially, what do "The Big Boys Do?"

The boat-centerline-chalk-line was fun, too.

When confronted with these types of questions, I usually go back to my early navigation lessons, one of which included the proverbial "have more than one method" but which also included: "Not so good to have two watches, because it creates doubt as to which one is correct."

How good is "close?" seems to be what you're asking, is it not?

Good luck, please keep us posted.

Fishzine · 23-09-2021, 11:49

Check out.

https://www.youtube.com/watch?v=iIRLTyp8vhg

While they are talking about a typical magnetic compass, they cover how to do very accurate bearing readings using a sundial and a maneuvering board to take very accurate solar bearing readings easily.

Cadence · 23-09-2021, 12:41

What about a chart and some range lights?

stinkpots · 23-09-2021, 12:48

I have just installed a new Simrad system with GPS compass and radar etc. I now have to calibrate the system.
Taking a hypothetical case that if the radar dome is not physically lined up fore and aft on the centerline of the boat, let's say 5 degrees to stbd. Now let's assume the gps antenna is also physically not on fore and aft centerline and is pointing 5 degrees to Port.
Clearly lining either one of these to the other would be a mistake as they would now both be misaligned to one side.
Both the radar and the gps have adjustments for correcting this problem BUT a proper point of reference is needed.
My method will be to align the compass first using a geographical reference preferably pointing the bow along a Transit Line (two objects in line on your chart which give a precise bearing). Now adjust the compass so it matches the transit. This should be done with no wind or tide so the boat is not crabbing along the line. Alternatively do the same thing at anchor which gets around the cog and heading being different due to wind and tide.
Now the gps compass is correctly aligned the chart plotter will display the chart orientation correctly, so now you can adjust the radar overlay to match the chart.
The main thing here is getting that initial transit, getting the boat heading exactly on a geographic bearing with no drift, otherwise the boat will be crabbing along the transit line and the heading will not be the same as the transit line and the error could be huge.

If you did this the other way around and aligned the radar overly to the chart first you would be aligning to the gps compass which would be out by 5 degrees in the above example.

**Dockhead** · 23-09-2021, 13:02

Quote:

Originally Posted by stinkpots

I have just installed a new Simrad system with GPS compass and radar etc. I now have to calibrate the system.
Taking a hypothetical case that if the radar dome is not physically lined up fore and aft on the centerline of the boat, let's say 5 degrees to stbd. Now let's assume the gps antenna is also physically not on fore and aft centerline and is pointing 5 degrees to Port.
Clearly lining either one of these to the other would be a mistake as they would now both be misaligned to one side.
Both the radar and the gps have adjustments for correcting this problem BUT a proper point of reference is needed.
My method will be to align the compass first using a geographical reference preferably pointing the bow along a Transit Line (two objects in line on your chart which give a precise bearing). Now adjust the compass so it matches the transit. This should be done with no wind or tide so the boat is not crabbing along the line. Alternatively do the same thing at anchor which gets around the cog and heading being different due to wind and tide.
Now the gps compass is correctly aligned the chart plotter will display the chart orientation correctly, so now you can adjust the radar overlay to match the chart.
The main thing here is getting that initial transit, getting the boat heading exactly on a geographic bearing with no drift, otherwise the boat will be crabbing along the transit line and the heading will not be the same as the transit line and the error could be huge.

If you did this the other way around and aligned the radar overly to the chart first you would be aligning to the gps compass which would be out by 5 degrees in the above example.

The problem with this is there is no such thing as no wind and tide, and no way to accurately (within a few degrees) observe visually where the ship's head is even if you're not crabbing a bit due to current.

So this simply doesn't work.

stinkpots · 23-09-2021, 14:22

Hi Dockhead.
If you can't find a transit without wind and tide then you can anchor on the transit line, there are loads of places you can find in our area where this can be done but I accept some areas this can be a challenge.
I don't think I will worry about a degree or two, if you navigated pre gps then an error of 1 or 2 degrees was considered fantastic. We had to contend with deviation, variation and vague tidal and current data, now with twin gps receivers we don't have to mess with compasses including the old fluxgate, the twin receiver gps eliminates the need for compass input.

We should not allow highly accurate gps to lead us away from good seamanship and common sense. If you are navigating using all available means then any small errors will be corrected as you go.

The transit method has stood the test of time and it will work for me just fine as it has for my last 50 years of boating.

You said it does not work, what would you do instead?

stinkpots · 23-09-2021, 14:33

HI again Dockhead
Here is a video which shows it well. If you do the manouver quickly wind and tide will not affect your result.

https://m.youtube.com/watch?v=1NpdA-rz6hI

Clivevon · 23-09-2021, 14:55

Use a transit??

Clivevon · 23-09-2021, 14:57

and a paper chart ?

stinkpots · 23-09-2021, 15:01

Hi Clivevon

The transit is explained in the video I attached, you can use your chart plotter rather than a paper chart to do the dame thing.

Fishzine · 23-09-2021, 15:02

Quote:

Originally Posted by Dockhead

The problem with this is there is no such thing as no wind and tide, and no way to accurately (within a few degrees) observe visually where the ship's head is even if you're not crabbing a bit due to current.

So this simply doesn't work.

Which is why the video I linked is such a fantastic solution to the problem you are trying to solve. You make a relative angle measurement to the centerline of the boat compared to the expected shadow angle cast by the sun for your current location/time. This method is as accurate as you want to put the effort into it being. You just have to get the compass rose flat and the sundial vertical.

Take it to the extreme. If you wanted to be extremely accurate you do do this while the boat was on the hard to eliminate boat motion, with a pumpbob as your shadow source to create a really long shadow increasing the accuracy of your angle measurement. I have no doubt you could exceed the accurate of your satellite compass.

Personally I think tied to a dock on a calm day I could pretty easily get a centerline angle within a single degree,