Compass, AP, GPS conflict

dlochner · Jul 18, 2018

KennyBene said:
1999 B352 with C80 MFD, ST 50 plus autopilot and compass

None of these instruments show the same heading. For instance, the C80 shows COG of 240 degrees but the AP says 233 while the compass reads 234...
I presume the GPS heading is correct but why is there a difference between the 3?

The AP compass is fixed to a bulkhead in the cabin with no metal nearby.

Any advice on what to do?
Thanks!

Those readings are remarkably close.

The gps based COG is affected by current and leeway. It is the direction that the boat is actually traveling over the bottom.

The compass readings are the direction the boat is pointed towards. +/- 1 degree is remarkably accurate. In real life it won't make a big difference.

jviss · Jul 18, 2018

Yes, what Dave said: AP and compass show heading, the direction the boat is pointed, while COG, course over the ground is just that. A little current from any direction other than directly forward or aft will cause a difference.

Will Gilmore · Jul 18, 2018

I understand that the magnetic pole is moving south relatively quickly of late. Perhaps the GPS doesn't rely on magnetic north, but rather, a gyroscopic orientation.
https://www.scientificamerican.com/article/are-the-earths-magnetic-p/

Absolutely COG is different than heading.

- Will (Dragonfly)

JamesG161 · Jul 18, 2018

Also one minor difference... AP and steering compass is guided by Magnetic North.
COG is calculated by GPS satellites.

From your numbers I suspect your AP is set to Magnetic and NOT True direction.
Jim...

dlochner · Jul 18, 2018

Will Gilmore said:
I understand that the magnetic pole is moving south relatively quickly of late. Perhaps the GPS doesn't rely on magnetic north, but rather, a gyroscopic orientation.
https://www.scientificamerican.com/article/are-the-earths-magnetic-p/

Absolutely COG is different than heading.

- Will (Dragonfly)

The direction to N or Magnetic N on a gps is calculated. The GPS receiving unit, takes the information it gets from the satellites, determines your position on the ground (or air or water) and then calculates the direction to either of the norths based on your known position and the known position of n or mag n. I believe Declination tables are built into the software.

DougM · Jul 18, 2018

That looks about the same as what I am experiencing except that I see more of a variance between the compass and the autopilot heading as well as compass and GPS. I always attributed it to the compass having never been calibrated. I never worried too much about it because if the GPS fails, as long as I head generally west or generally east, I will hit land eventually.

dlochner · Jul 18, 2018

DougM said:
That looks about the same as what I am experiencing except that I see more of a variance between the compass and the autopilot heading as well as compass and GPS. I always attributed it to the compass having never been calibrated. I never worried too much about it because if the GPS fails, as long as I head generally west or generally east, I will hit land eventually.

Compass deviation due to local magnetic fields (boat characteristics) will vary based on the heading. If the compass is swung, the deviations can be noted and accounted for when plotting a course. Most quality compasses can be compensated so that the deviation is minimized.

capta · Jul 18, 2018

When is the last time you've had your compass swung? Do you even have a deviation table aboard and if so, did you apply the errors?
Is your GPS reading true or magnetic, a setting you must choose?
It really is of little consequence what the numbers on your autopilot read as most are set by heading and maintain that heading while operating. Again, if you want that compass to be correct, you must ensure that no ferrous metals near it ever move, and have it swung by a compass adjuster and apply the deviation card data to the readings.
Probably the single most important piece of navigation gear aboard our boats is the compass and yet, without a current deviation card, it is just about useless. All the classes, video watching and book reading in the world won't help if your most basic and easiest to use emergency navigational instrument aboard isn't usable.

FastOlson · Jul 18, 2018

First things first ...
Get your compass corrected and then regularly read your deviation table produced from that effort.
Then, make damned sure that all of your electronic devices are installed further from the compass than the minimum distance specified in the manufacturer's install instructions. (BTW, this part of the install is sometimes ignored or even denied by unscrupulous vendors in their haste to get your $$.)

And finally, do not temporarily park any steel devices like a screwdriver, or magnetic devices like a radio remote, near the ship's compass.
We do have a corrected compass, and have dialed in the AP readout within a few degrees of it. As others have noted, all of your devices may vary a little bit. Try to remember that all this digital wonderfulness can fool the end user into believing that precision to several decimal points can be assumed 24/7. Close maybe.

Upon further thought, just print out copies of reply #9, and post them all over your boat! (with non magnetic tape)

Ron20324 · Jul 18, 2018

MY b323 HAS THE SAME ELECTRONICS AS YOU. There is a way to program the autopilot so it uses your mfd/gps as it's degree source. It's in the maual, which buttons to set to GPS. Last weekend my auto, helm, mfd read 252/262/272 degrees.

capta · Jul 18, 2018

Just a note. No compass will be exactly correct on all points. A compass adjuster may take some of the error out of the compass, but, like a chronometer, it is the known error (from the deviation card of the compass) applied to the visual reading that gives one an accurate reading.

dlochner · Jul 18, 2018

Been thinking about this thread off and on through the day.

One of the lures of electronic devices is that they yield very precise numbers. The chart plotter says the next waypoint is on a bearing of 113 degrees. So staring at our fluxgate compass readout we try desperately to steer a course of 113 degrees. But that's a fool's errand. Wind, waves, currents, measurement error, and our own inattention cause us to steer a course that is hopefully somewhere near 113. What we need to do is steer a course that averages 113 and constantly adjust that bearing based on leeway, currents, and so forth.

Then there is the precision of a traditional magnetic compass. At least on my compass, the lubber line is about 4 degrees wide. That means I can only read a compass direction of about +/- 5 degrees if I'm lucky. So does it matter if I have my compass swung and a deviation card created? At what point does deviation even matter?

Unless we are racing on a very short course, course swings of 5 or 10 degrees don't really matter if we are reasonable navigators, because we will periodically determine our position and adjust the course to hit our waypoint.

I guess the upshot of this, is to know if on some point of sail your magnetic compass is way off due to local magnetic fields, however, if the deviation is less than a few degrees, don't sweat it. And if your GPS, Mag Compass, and chartplotter all show different headings, but they are within 10 degrees of each other, don't sweat it, until you get real close to the rocks. In which case, you need a lot more navigation skills than simply blindly following a compass course.

capta · Jul 18, 2018

A standard compass may have deviation errors of 20 degrees or more, especially considering that our electronics are often installed on the pedestal near the compass. But you won't know the error if you don't swing it.
When your compass comes in most handy is when you have lost your electronic navigation and visibility is poor. You may only need it once in your boating lifetime, but if you don't know it's errors, your compass is a worthless piece of junk when you do.
As for steering a compass course, it isn't the minute to minute steering that is important, but the average over time. If you need to steer a specific course, say in the fog, and your GPS has failed, do you really think inattention will be a factor?
Most of us sail in areas we know, so setting a course taking into account your expected set, drift and leeway should be automatic. All should be known factors in your home waters, that's why they give away tide tables. So you shouldn't need to constantly be making course corrections.
These forums are full of safety advice; wear your PFD, always carry charts just in case, wear a harness, never leave the cockpit (for many even on the calmest, most beautiful days) without snapping on, don't operate your dink w/o the kill cord, a checklist a mile long before you leave the dock, etc. etc. etc. but having your compass swung and an up to date deviation card at hand is not worthwhile?

jviss · Jul 18, 2018

capta said:
but having your compass swung and an up to date deviation card at hand is not worthwhile?

I generally agree with you, but I must confess I've never been on a yacht, including two I've owned, where I knew for certain the condition of compass adjustment, or had or saw a deviation card. I wouldn't even know where to go or who to ask to have the compass professionally swung, and I don't doubt that it would be considered unusual for a moderately sized yacht, and expensive as well.

What I have done over the years (since I am a bit of a nerd) is keep track of my steering compass, when "things line up." It's very tough where I sail, as there are strong and variable currents constantly. But you can observe the sun's azimuth at sunrise and sunset, sighting over the compass card; and when it's clear the current is slack, compare the heading to GPS COG. After a while you develop a sense of how well adjusted it is. Mine have never been off more than a few degrees, and as David points out, for coastal cruising, that's enough.

For offshore sailing, yes, I would find out how to have it done!

dlochner · Jul 18, 2018

capta said:
A standard compass may have deviation errors of 20 degrees or more, especially considering that our electronics are often installed on the pedestal near the compass.

capta said:
but having your compass swung and an up to date deviation card at hand is not worthwhile?

If the compass is off by 20 degrees, that is a problem and you need to know it. Perhaps I'm responding to the torturous lessons of True Virgins Make Dull Company Add Whiskey days of the Power Squadron nav course a half century ago. Back then we were adding and subtracting 1 or 2 degrees of deviation to plot the course without rounding the magnetic variation after calculating it from the chart and the known drift..... But I digress.

There are fairly easy and straight forward ways of determining the effect, if any, of electronics near the compass. If you suspect the electronics on the pedestal may affect readings, simply turn off the electronics and note any changes in the compass. Do this 8 times on each of the 8 compass points and you have down and dirty sense of your compass's accuracy.

But wait, there's an app for that. Download an app that measures magnetic fields. Measure the magnetic field near the compass, turn on the electronics and measure the field again. If there is a significant increase in the magnetic field, there's an issue to address. Now you know whether to swing the compass or not.

Drifting back to this thread and the OPs post, he was concerned about the different readings on the 2 compasses and the GPS. Precise looking numbers, like 233 degrees give the illusion of a level of measurement precision that is either not there or irrelevant. Another way of asking the question is how much difference is a difference worth noting?

@capta your point is well taken, it is important to have some idea about the accuracy of your navigation instruments, however, it is easy to get sucked into worrying about differences that don't make a difference. A 2 or 3 or 5 degree deviation error is not all that big considering that most compasses on our boats can only be read to +/- 3 or 4 degrees in the best of conditions. Understanding measurement error, precision, and accuracy are equally, if not more important, especially when we have instruments that give the illusion of great accuracy to high levels of precision.

Davidasailor26 · Jul 18, 2018

dlochner said:
There are fairly easy and straight forward ways of determining the effect, if any, of electronics near the compass. If you suspect the electronics on the pedestal may affect readings, simply turn off the electronics and note any changes in the compass. Do this 8 times on each of the 8 compass points and you have down and dirty sense of your compass's accuracy.

But wait, there's an app for that. Download an app that measures magnetic fields. Measure the magnetic field near the compass, turn on the electronics and measure the field again. If there is a significant increase in the magnetic field, there's an issue to address. Now you know whether to swing the compass or not.

Unfortunately the errors from electronics aren't always from having the electronics on, but just from their presence. Magnetic latches on memory card doors for chartplotters are particularly problematic. To really see the effect of that you'd need to remove and reinstall the whole chartplotter a few times as you go around a circle and take measurements.

dlochner · Jul 18, 2018

Davidasailor26 said:
Unfortunately the errors from electronics aren't always from having the electronics on, but just from their presence. Magnetic latches on memory card doors for chartplotters are particularly problematic. To really see the effect of that you'd need to remove and reinstall the whole chartplotter a few times as you go around a circle and take measurements.

Hmm. Hadn't thought of the magnetic latches. I am most familiar with the B&G Zeus2 which does not have them.

Using one of the magnetic field apps will show how much, if any, magnetic interference is caused by a MFD. Test with no MFD, with MFD installed, with MFD installed an powered up. If the magnetic field is disrupted by the MFD, then it would be necessary to determine how big the effect is, as well as determining how much of a real difference it might make.

Rich Stidger · Jul 18, 2018

WOW! I am amazed at the complexity of the answers. I would not expect any of these instruments to agree, and if they did it would be a unique situation.

The GPS heading is the COG. It is calculated from the sats by knowing position and time intervals.

The compass shows the heading that your bow is pointed. If current or wind is pushing you sideways, the boat will be crabbing and the COG will not be the same as the heading of the bow.

The AP heading is the heading that it must steer in order to stay on your desired track. It will be greater or less than your desired track depending upon the effect of current and wind.

The only time all three will agree is if the compass is corrected perfectly, and there is no wind or current pushing you to port or starboard. And of course all three must be reading in magnetic for your particular location.

jssailem · Jul 18, 2018

@jviss your not alone in the issue. Try your local chandler. They may know a person. It is a learnable task you can do but local knowledge of a site with a known heading helps. We have a site in our bay near the Navy base.

In order to swing the compass your going to need a calm day no current, when you can align the boat on a specific course, ideally to a fixed site on land, and make the compass adjustments as the helm is guided on course. You need to go in both directions making adjustments.
We have a compass shop. That may also be a resource.