GPS and Compass show different headings - Page 2 - SailNet Community
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post #11 of 15 Old 08-11-2010
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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.
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post #12 of 15 Old 08-11-2010
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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

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post #13 of 15 Old 08-11-2010 Thread Starter
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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

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Channel Islands, CA


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post #14 of 15 Old 08-12-2010
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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...

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post #15 of 15 Old 08-12-2010
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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.

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Last edited by jerryrlitton; 08-12-2010 at 06:58 AM.
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