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Go Back   SailNet Community > On Board > Gear & Maintenance > Electronics > GPS and Compass show different headings
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Thread: GPS and Compass show different headings Reply to Thread
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Topic Review (Newest First)
08-12-2010 07:55 AM
jerryrlitton
Quote:
Originally Posted by montenido View Post
Thanks everybody for the great replies. My GPS is actually a multifunction display - Raymarine C80, not 60 (sorry, my mistake). The compass is made by Richie. All of the equipment is relatively modern and probably works as intended.

Good points made about the difference in a bearing on a compass and the COG on the GPS. I just thought that the bearing and heading should be close. I don't remember exactly, but I think they were 20+ degrees different.

The reason it came up is that I set a waypoint for my marina 25 miles away and made towards it using my chartplotter. Even though I tried to follow the most direct route shown, I ended up doing more of an arc than a straight line. This was with no wind or current, just motor sailing. Not a big deal, but I probably covered a few more miles than necessary.

Next week I am going to Catalina Island, a trip of about 60 NM. I thought I would plot a heading using my charts as well as using the chartplotter with waypoints. On a 60 mile trip I would rather not add any additional miles if I can help it.

Thanks again for the great input.

Bill
If your track described an arc then I would guess you put the bearing needle (or equivalent ) and kept it pointing at the bow of bow of the boat and a current indeed was present and you arced to the destination. Very common for pilots too. If your GPS also has a function to read lateral drift and you adjust course to zero (lateral drift) then your track will be a straight line.
08-12-2010 07:45 AM
Maine Sail Find yourself a local compass adjuster and have them come out with you and professionally swing the compass. I do mine every two years or whenever I change anything near my helm. It runs about $125.00 here in Maine. You will steer cardinal positions then the inter-cardinal positions and they will adjust your compass as close as can be then make you a deviation card.

If you do not own a deviation card, set up specifically for your compass, there is no way you are getting accurate readings...
08-12-2010 12:59 AM
montenido Thanks everybody for the great replies. My GPS is actually a multifunction display - Raymarine C80, not 60 (sorry, my mistake). The compass is made by Richie. All of the equipment is relatively modern and probably works as intended.

Good points made about the difference in a bearing on a compass and the COG on the GPS. I just thought that the bearing and heading should be close. I don't remember exactly, but I think they were 20+ degrees different.

The reason it came up is that I set a waypoint for my marina 25 miles away and made towards it using my chartplotter. Even though I tried to follow the most direct route shown, I ended up doing more of an arc than a straight line. This was with no wind or current, just motor sailing. Not a big deal, but I probably covered a few more miles than necessary.

Next week I am going to Catalina Island, a trip of about 60 NM. I thought I would plot a heading using my charts as well as using the chartplotter with waypoints. On a 60 mile trip I would rather not add any additional miles if I can help it.

Thanks again for the great input.

Bill
08-11-2010 02:28 PM
BarryL Hello,

A real simple way to determine if your compass is close or not is to get another compass! I have a hand bearing compass on my boat, many people have a cheap hiking compass too. Spend $5 for a cheap one and if it is closer to your GPS than to your compass, the compass is wrong. If your new hand compass agrees with your mounted compass you have bigger problems.

Personally, I almost never use my mounted nav compass. Most of my navigation is line of site (sail towards that smoke stack) or by chart plotter heading.

Barry
08-11-2010 08:21 AM
davidpm The OP didn't mention how much the difference was and if the difference was the same in all directions.

Most of us on small boats find that a few degrees off is the best we are ever going to get.
I'm pretty happy personally with about 5 degrees.
08-11-2010 07:21 AM
sailingdog 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...
08-11-2010 03:24 AM
pedcab
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...
08-11-2010 12:19 AM
CapTim lol.. there's just no such thing as keeping it simple with you folks, is there? that's why I luff joo guys
08-10-2010 11:51 PM
jerryrlitton 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
08-10-2010 11:46 PM
sailingdog 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".

Quote:
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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