Compass vs GPS

Oct 2, 2008
3,807
Pearson/ 530 Strafford, NH
Hi all,

We have done a check of heading comparison between our compass and the GPS heading from our autopilot and chart plotter. They have never been in agreement, usually a difference of 10 degrees or more. No problem because we always would make our destination, but thought it would be nice to have the entire boat heading the same way. The marina tech asked me to check for any offset in the autopilot settings which I hadn’t done since the upgrade seven years ago. It was minus 8 degrees. I noted that and slowly returned it to zero.

Yesterday we went out and followed the eight points of sail by the autopilot and logged it within five degrees + or - with the compass. We did reduce the dry weight of the boat when we replaced the old genset with a new one that was 600 pounds lighter. It is probable that this could have influenced the compass which may have to be swung a some point. I thought I should wait until I could check again against other electronics having GPS headings (like the iPad). Or I could wait until the poles are done shifting.
 

DougM

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Jul 24, 2005
2,242
Beneteau 323 Manistee, MI
Is the chart plotter reading magnetic north to coincide with the magnetic compass?

That said, even with everything set to read magnetic north, I can’t say mine ever agreed.
Like you, I am seeing a 10 degree or so difference. I could never quite determine whether the manual compensators in the binnacle compass were even functional. Theres no doubt that the electronics in the instrument pod located less than a foot from the binnacle compass are having an effect.
 
Sep 25, 2008
7,068
Alden 50 Sarasota, Florida
Magnetic compass deviation around New England is close to 15 degrees different than what your GPS will indicate
 
Feb 14, 2014
7,377
Hunter 430 Waveland, MS
I calibrate my Raymarine Flux compass to match the Steering compass each year to ±1°.

This allows my Chart plotter to have a Correct Radar overlay on the charts.

I am not sure why I have to calibrate each season. Guessing the SeaTalk network loses power and does a Reset.

Magnetic setting works fine for 300 nm or so.

GPS course is a calculation that has a lot of variables.
Jim...
 
May 17, 2004
5,009
Beneteau Oceanis 37 Havre de Grace
Are you comparing it to a GPS “heading” (actually Course over Ground) or a flux gate compasss (heading)? The course over ground will be off because of any current but also leeway when sailing. A flux gate compass might be able to be calibrated by doing several 360 degree turns while in a calibration mode, in addition to the fixed offset.

The other way to check which one is correct is to point at an object on a known bearing and see which one reads the right heading.
 
Nov 13, 2013
723
Catalina 34 Tacoma
Then there is the fact that the magnetic pole is moving towards Russia.


Who did this?
Our universe is not a static place. Change is constant. We are on space ship earth with a molton core and thin crust of terra firma we cling to. We are revolving around the sun with another large object (moon) rotating around us and influencing our own orbit, climate, weather, and tides. When I hear the climate is changing and the poles are moving I say really?
 
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Nov 13, 2013
723
Catalina 34 Tacoma
Consider neither of them are probably on point. I just use them to get me close enough to where I want to go.
I think the satellites make necessary corrections. GPS puts me right on the buoy. Compass gets me close enough.
 

capta

.
Jun 4, 2009
4,765
Pearson 530 Admiralty Bay, Bequia SVG
Hi all,

We have done a check of heading comparison between our compass and the GPS heading from our autopilot and chart plotter. They have never been in agreement, usually a difference of 10 degrees or more. No problem because we always would make our destination, but thought it would be nice to have the entire boat heading the same way. The marina tech asked me to check for any offset in the autopilot settings which I hadn’t done since the upgrade seven years ago. It was minus 8 degrees. I noted that and slowly returned it to zero.

Yesterday we went out and followed the eight points of sail by the autopilot and logged it within five degrees + or - with the compass. We did reduce the dry weight of the boat when we replaced the old genset with a new one that was 600 pounds lighter. It is probable that this could have influenced the compass which may have to be swung a some point. I thought I should wait until I could check again against other electronics having GPS headings (like the iPad). Or I could wait until the poles are done shifting.
LOL
I doubt any of will be alive then.
Our autopilot and GPS tend to agree most of the time, but our deviation card works pretty well to keep all 3 synced up.
 
Jan 1, 2006
7,025
Slickcraft 26 Sailfish
Deviation card! A blast from the past of my USPS courses. I guess the electronic version is the 360's mentioned in Jim's post. I'm so analog.
 

Tom J

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Sep 30, 2008
2,296
Catalina 310 Quincy, MA
If I calibrate the autopilot compass, it will be close to the GPS. If I don't shut off the electronics, it stays calibrated, but when I shut off the electronics while in my slip, the autopilot compass will be way off the next time I turn it on. I'm not going to do a slow circle and wait for the compass to calibrate every time I leave the slip, so I just point the boat to follow the chartplotter, and note the heading on the magnetic compass and ignore the readout on the autopilot.
It is amazing how sensitive the flux capacitor (I mean compass) is. I have learned the hard way that when I sail over the granite ledges off Cohasset, MA, the autopilot will go crazy, with it's compass reading + and - 30 or 40 degrees every few seconds. I now hand steer over that part of our route.
 
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Apr 8, 2010
1,937
Ericson Yachts Olson 34 28400 Portland OR
Better to just have your compass swung and corrected. The compass guy/gal will compose a new and accurate deviation card for you.
I love all of my digital and electronic toys, but they can lie without warning or remorse. Your ship's compass is your basic foundation-level instrument. It should be true and reliable, all the time.