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Old 09-05-2011, 16:01   #1
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What Heading Am I Really On ?

I have wondered this in the past; but the other day according to my different instruments I was on 3 different headings. My autopilot was set to 135 my GPS said I was COG of 145 and my boat compass was around 160-170!

I'm more inclined to believe the GPS is giving me the real course as it is taking into account drift etc. But I've never really understood why the boats compass (on 2 different boats now) always is so much different that the autopilot.
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Old 09-05-2011, 16:16   #2
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Re: What heading am I really on?

Are all of your instruments set for magnetic or true north, and if not, did you figure that in with comparing the different readings.

Generally, though useful, your magnetic compass will be a bit less precise than the others depending on how well dampened it is for boat motion and how well it has been adjusted to compensate for metal structure.
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Old 09-05-2011, 16:16   #3
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Re: What heading am I really on?

You have heard of Variation and Deviation haven't you???
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Old 09-05-2011, 16:16   #4
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pirate Re: What heading am I really on?

Are your GPS and Autopilot set on True...
Has your compass been swung to check/correct it since you bought the boat...
PO may have fitted something that affects it..
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Old 09-05-2011, 16:32   #5
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Re: What heading am I really on?

Heading called “Ship’s Head” and Course over Ground (COG) are two different things

Compasses indicates Ships Head which is a physical installation complete with fixed and variable errors that you need to adjust for to get your "True Heading"


Your GPS measures COG and is simply an electronic aid that includes drift and leeway


Do not try to calibrate your compass to your GPS

Best you read up on basic navigation to appreciate how to survive without GPS
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Old 09-05-2011, 16:37   #6
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Re: What heading am I really on?

Like Boatman 61 says, swing your compass and no I'm not talking about square dancing.
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Old 09-05-2011, 16:44   #7
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Re: What heading am I really on?

just as a follow-up ... regardless of the question I'm not an idiot!

all the electronic units are set to magnetic

wonder if the mounting of the autopilot and GPS a foot away from the comprass could effect it this much (was a little closer when the seas settled down some) both these should both be measuring the same magnetic heading
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Old 09-05-2011, 16:46   #8
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Re: What heading am I really on?

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Originally Posted by cchesley View Post
You have heard of Variation and Deviation haven't you???

Yes, have you? I'll let you read up on which term has nothing to do with reading the various instruments and which has to do with a chart!
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Old 09-05-2011, 16:59   #9
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Re: What heading am I really on?

You might also want to take into account the GPS has some lag time in it. That might not cause you to see things differently, but it may. The GPS could be a second or two behind.
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Old 09-05-2011, 17:08   #10
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Re: What heading am I really on?

Compare your handheld to your compass. There may be something installed causing a drastic difference.........i2f
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Old 09-05-2011, 17:14   #11
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Re: What heading am I really on?

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You might also want to take into account the GPS has some lag time in it. That might not cause you to see things differently, but it may. The GPS could be a second or two behind.

I probably shouldn't have included the GPS COG as this is the sum of all factors of heading drift, current etc etc etc.

But all the autopilot is using is a magnetic compass and it is a lot different than ships compass. Much more than expected. Wonder if it is worth having the compass checked and getting a deviation table (like I can read those swinging numbers that close to start with).
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Old 09-05-2011, 17:19   #12
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Re: What heading am I really on?

Does your AP have a flixgate compass amd if sp where is it located? Hopefully not near any electronics or metal.
Turn your AP off and see if the compass reading changes.
l
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Old 09-05-2011, 17:40   #13
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Re: What heading am I really on?

If your autopilot has a flux gate compass, it was set to some arbitrary reference heading when installed. Usually that is to the boats compass heading though it always seems to vary, at least in my experience with the one on my Raymarine X5 pilot. Might try re-synch'ing the autopilot compass to the ships compass if you think it's reasonably correct.

The GPS heading always seems to be 10 or more degrees out from all my other compasses even when I have reason to believe that there is little leeway or current.
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Old 09-05-2011, 17:48   #14
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Re: What heading am I really on?

Quote:
Originally Posted by Don Lucas View Post
I probably shouldn't have included the GPS COG as this is the sum of all factors of heading drift, current etc etc etc.

But all the autopilot is using is a magnetic compass and it is a lot different than ships compass. Much more than expected. Wonder if it is worth having the compass checked and getting a deviation table (like I can read those swinging numbers that close to start with).
The AP compass may have been calibrated but not corrected. When I installed my AP I had to go do circles to calibrate it but then you still need to set a known course to be sure it is correct, and you can change that during the sea trial.

It could also be that pesky interference.
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Old 09-05-2011, 18:09   #15
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Re: What heading am I really on?

You would have to compare readings on all points to see what is going on but GPS without other inputs is COG. Applying "set" would then give heading. A calibrated hand bearing compass will resolve the gross error. Recheck the GPS options. Sometimes the preselect hdg selections from menu may have changed. (Would look like the autopilot calibration is the problem)
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