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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance > Electronics
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Old 08-10-2010
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GPS and Compass show different headings

Hi all,

I know this is a beginner question, but here goes. On my new (to me) boat, the compass and GPS show fairly different headings. I have a Raymarine C-60 and a Richie compass. I would assume that the GPS is the correct heading and that the compass needs calibration, but how do I know for sure?

Thanks, Bill
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Old 08-10-2010
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In many areas, there are ranges setup for things like navigating channels. You can use one of these to check the magnetic compass heading, since the range is a known bearing line. Granted, you may have to correct any deviation in the compass by swinging it, but checking it against the range will tell you whether you're in the ballpark or not.

If there aren't any ranges convenient to you, you can always setup your own ranges using things like radio, water towers, etc., to create a known bearing.
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Old 08-10-2010
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Quote:
Originally Posted by montenido View Post
Hi all,

I know this is a beginner question, but here goes. On my new (to me) boat, the compass and GPS show fairly different headings. I have a Raymarine C-60 and a Richie compass. I would assume that the GPS is the correct heading and that the compass needs calibration, but how do I know for sure?

Thanks, Bill
Ok 1st thing, your GPS is not showing a heading. It is showing a traffic across the ground usually in magnetic but that will depend on how you have yours configured. Of course your track will vary with current, winds and your leeway. Also your compass needs to be calibrated for your installation area (deviation) and it is also subject to magnetic variation. So it is no small wonder there is a difference between the 2.

Jerry
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Newbie question to your newbie question, montenido:

are you familiar with the difference between actual and magnetic headings?
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I'd point out that some GPS units, especially the multifunction display type units, may be showing a magnetic heading based on a FLUXGATE compass, rather than on GPS-calculated movement over ground, as Jerry describes.

It would help if the OP said exactly what electronics he has aboard. The C60 is a relatively vague description, since the C60 can have both a fluxgate compass and a GPS antenna attached to it.
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There are three headings... True, Magnetic and Compass. The Compass heading is what is read on the compass card, but does not take deviation and variation into account. Adding the deviation to the compass heading will give you a magnetic heading. Adding the variation to the magnetic heading will give you the true heading.

There is no "actual" heading, since it depends on what you're looking at, unless by "actual" you mean "true".

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Originally Posted by CapTim View Post
Newbie question to your newbie question, montenido:

are you familiar with the difference between actual and magnetic headings?
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This may help...

True Course (TC): This is the course measured from your navigation plotter when you plot your trip on your chart.

True Heading (TH): Now that you have a true course, we need to correct for winds/current which will give us a true heading.

Magnetic Heading (MH): The difference between true north and magnetic north is known as variation. Lines of variation are shown on a sectional chart as dashed magenta lines and called isogonic lines. By adding or subtracting variation from your true heading you will get your magnetic heading. Remember east is least (subtract) west is best (add)

Compass Heading (CH): Items from inside the airplane (boat?) can actually affect the performance of the compass. This is your deviation. This is normally on a small card mounted close to the compass. When you sum everything up this is your best guess on what you will read on your compass to maintain a course. This will obviously have to be updated as you make progress on your journey. You can also do this entire operation in reverse to plot your winds/current.

Have fun.

Jerry
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Last edited by jerryrlitton; 08-10-2010 at 11:15 PM.
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lol.. there's just no such thing as keeping it simple with you folks, is there? that's why I luff joo guys
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Old 08-11-2010
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Quote:
Originally Posted by jerryrlitton View Post
Ok 1st thing, your GPS is not showing a heading. It is showing a traffic across the ground usually in magnetic but that will depend on how you have yours configured. Of course your track will vary with current, winds and your leeway. Also your compass needs to be calibrated for your installation area (deviation) and it is also subject to magnetic variation. So it is no small wonder there is a difference between the 2.

Jerry
Double that.

GPS does not show headings, it shows a course over ground and in any place but a widless pool your heading is always very different from your course...

Some more complete GPS units feature a fluxgate though...And those can show more accurate true and/or magnetic headings...
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Don't confuse a multifunction display with a plain GPS Chartplotter. A multifunction display may be showing the heading based on a FLUXGATE compass, which should be at least as accurate as a magnetic compass once it has been calibrated properly. Without knowing what equipment the OP's boat has, it isn't possible to determine whether his MULTIFUNCTION DISPLAY is outputting a GPS-generated heading or a fluxgate based heading.

Again, using known terrestrial ranges is the only way to learn if the magnetic compass is off or not. Unless the compass was installed without regard to interference, chances are pretty likely that the magnetic compass is going to be more accurate, out of the box, than a GPS-generated heading, at least in regards to where the boat is POINTING.

Quote:
Originally Posted by pedcab View Post
Double that.

GPS does not show headings, it shows a course over ground and in any place but a widless pool your heading is always very different from your course...

Some more complete GPS units feature a fluxgate though...And those can show more accurate true and/or magnetic headings...
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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