GPS and compass read differently

[Jackdaw]

OK.

A magnetic compass of ANY sort will report your heading, ie the direction your boat is facing with respect to the magnetic pole. This can effected by local magnetic fields nearby. The boat does not have to be moving to have a heading.

A GPS unit uses the last several GPS fixes to calculate your boats COURSE, ie, the direction the boat is moving over the surface of the earth. Along with heading and speed it factors in drift, sideslip and current. It also does not care what way your boat is facing. It can be effected by poor GPS fixes, but has the biggest trouble with the movement is very slow (less than 1-2 knots). Many GPS units will not will not report a course value when the speeds are very low.

If you are off by less than 10 degrees that is probably drift etc. Unless motoring in flat current-less water the two almost never read the same.

If off by 10-170 you probably have a magnetic issue.

If off by close to 180 your compass is installed backwards or you are reading the wrong side of the card.

 

[PeterBld]

Magnetic or True

On our boat, all the charts are drawn up in true. (LOL, pretty obvious really) They have a rose to convert to magnetic if you really want (but you need to allow for variation since the chart was printed.)

Weather reports are always in true.

True is not subject to variation. Nor deviation.

The only item on board which must be magnetic is the compass.

So if true is available on any instrument why would you not use that?

I look at my chart, I want to go from where I am to a destination. I determine that COG required in True. Enter that in the auto-pilot which is also true, and go. The GPS is set to true so it checks that my COG agrees with the required COG. And maybe as back up, set that steering/heading marker on the binnacle compass so if all else fails, I still know what heading I need on the binnacle.

No need to worry about compass variation for my area. No need to worry about compass deviation. So simple.

Maybe I over simplify, but we sail in 1,000 mile lumps, so possibly have a different perspective.

But still, so why set any instruments to Magnetic??????????

 

[Brian D]

It is unfortunate that this discussion has migrated from a GPS pointing issue to a compass reading course. This issue is the GPS unit and we should really stay on course, unless your internal compass is pointing in the wrong direction.

 

[Stu Jackson]

Peter,

I believe this is a "philosophical" issue.

You're right, especially if you, as you said, are sailing in 1,000 mile chunks.

However, consider the "average Joe" sailor who is doing coastal or Bay sailing, for 4 - 6 hour stints.

The variation, in Joe's case, is NOT going to change during his "voyage." His compass is magnetic.

Charts are not drawn in True or Magnetic, they are exactly the same in any area WITHOUT a change in variation, which is what all daysailors encounter. The compass rose shows both T&M.

With no change in variation, the use of M makes sense so no conversions are required.

It makes sense to use M instead of T for the autopilot and the GPS, so all three instruments are using the same relationship.

That said, if one was traveling longer distances where variation changes, T is the correct way to navigate.

It's like the difference between navigation & piloting.

On our boat, all the charts are drawn up in true. (LOL, pretty obvious really) They have a rose to convert to magnetic if you really want (but you need to allow for variation since the chart was printed.)

Weather reports are always in true.

True is not subject to variation. Nor deviation.

The only item on board which must be magnetic is the compass.

So if true is available on any instrument why would you not use that?

I look at my chart, I want to go from where I am to a destination. I determine that COG required in True. Enter that in the auto-pilot which is also true, and go. The GPS is set to true so it checks that my COG agrees with the required COG. And maybe as back up, set that steering/heading marker on the binnacle compass so if all else fails, I still know what heading I need on the binnacle.

No need to worry about compass variation for my area. No need to worry about compass deviation. So simple.

Maybe I over simplify, but we sail in 1,000 mile lumps, so possibly have a different perspective.

But still, so why set any instruments to Magnetic??????????

 

[Brian D]

Found a section in the manual about heading line. It states it uses one of two available sources.

Page 7, Setting the Heading and Course Over Ground Lines

First being a heading sensor source. If you are configured for that source but do not have a heading sensor, it could be the cause of your issues. Or, as mentioned before, the heading sensor is wired backwards.

Second source is the internal GPS. If you are using that and have the anomaly, then I am at a loss. There was nothing else in the manual (poorly written) to indicate what could be causing a 180 degree variance.

Third option is set the source to AUTO.

Good luck on your testing.

 

[fastmover]

David,
I believe the answer to your question may surprise you. I was going to install my garmin gps on the cockpit bulkhead just below my existing compass and discovered that as soon as I placed the gps unit within 6 to 8 inches of the existing compass, the compass card would start to swing. This happened without the unit turned on. The gps has its own electrical field. This would be easy to test. Just unmount your gps unit and move it away from your compass and watch to see if the compass heading changes. Then with the gps unit in your hand slowly move it closer to the compass. You will be surprised but may be disappointed knowing that you have to find another location for the gps unit like I did.

 

[davidmhull]

Thanks again for all your replies. It would seem that the issue is what was discussed earlier. I pent the last 32 hours on the Potomac and Chesapeake bay and the GPS reading was exactly what my two magnetic compasses read. If you are going too slow, backwards etc. then this thing is just wonky. After you set a course and get to moving along at 4 or 5 knots then it reads right on the money. The only thing I need to figure out now is how to reverse the "compass tape" which scrolls above the charts. It is truly backwards although it does read correctly. I would prefer that it reads as a normal magnetic compass and I will play with that. I finally just turned the "compass tape" off because it was confusing and there is a perfectly good normal compass mounted just above the GPS so I don't really need it. Just as a side note I caught a lot of blues, spot, croaker, flounder and a happy little puffer which I returned to the wild. Since the bait fish came in about two weeks ago the fishing has been great. Thanks again for all your help.