GPS and compass read differently

[davidmhull]

Good morning. I went sailing yesterday and one of the things I wanted to do is check out my new depth gauge/GPS. On the recommendations of others on the forum I bought the Garmin 44dv. It seems to be really nice and the shallow alarm is very handy. I was just putzing around in the James river so I didn't really need navigation but I turned the GPS screen on just to take a look. My compass is mounted above the GPS and I notice they do not read the same. I'm not talking about a little difference, It's like 180 degrees. I have never had a GPS on my boat so I'm not sure what's going on. I will admit I did not spend a lot of time even thinking about it since I had other things to do but I'm curious. The navigational chart put me right where I knew I was and the little boat symbol had me going in the right direction. It's just that the heading was wonky. Any ideas? Thanks.

 

[Zed]

Good morning. I went sailing yesterday and one of the things I wanted to do is check out my new depth gauge/GPS. On the recommendations of others on the forum I bought the Garmin 44dv. It seems to be really nice and the shallow alarm is very handy. I was just putzing around in the James river so I didn't really need navigation but I turned the GPS screen on just to take a look. My compass is mounted above the GPS and I notice they do not read the same. I'm not talking about a little difference, It's like 180 degrees. I have never had a GPS on my boat so I'm not sure what's going on. I will admit I did not spend a lot of time even thinking about it since I had other things to do but I'm curious. The navigational chart put me right where I knew I was and the little boat symbol had me going in the right direction. It's just that the heading was wonky. Any ideas? Thanks.

Compasses have to be calibrated for the boat. Was yours ever calibrated?

 

[davidmhull]

No it has not been calibrated but it is generally correct checking it against other compasses that I have. I really don't think this is a calibration issue. If my compass is off then it is only slightly off. I'm talking about 180 degrees or so difference between what my compass reads and what the GPS reads. Thanks.

 

[uncledom]

No it has not been calibrated but it is generally correct checking it against other compasses that I have. I really don't think this is a calibration issue. If my compass is off then it is only slightly off. I'm talking about 180 degrees or so difference between what my compass reads and what the GPS reads. Thanks.

You just mounted an electrical device below your compass. This might affect the magnetic field. Turn the GPS on and off and see if the compass changes heading. You might want to recheck the compass reading with a hand held that you know was close originally and see if mounting the GPS has had an effect. Good luck.

 

[davidmhull]

Thanks uncledom. I did not notice any difference in the compass reading with the GPS on or off. I checked the compass with my old box compass I had on my 40' sailboat and they read very close if not the same. As I said I haven't calibrated the compass yet but it is very close. Of course just setting a box compass on the boat doesn't mean it is calibrated either but again they read very close to the same. I wonder if there is a feature on the GPS that tells you where you have been instead of where you are going!!! Just kidding.

 

[DougM]

Did you leave anything metal near the compass?
It wouldn't necessarily affect the GPS, but it would confuse the compass.

 

[Don S/V ILLusion]

Since we're playing a guessing game, mine is your compass is calibrated properly for the Southern Hemisphere.

Sail it to Oz or NZ and it will be fine.

 

[Brian D]

I read, and reread your original comments but do not understand which device is 180 degrees off. Are you saying that your compass if off or the reading on the GPS is off? The way I read it was the compass was pointing in the wrong direction, but then you said you compared it to another compass and the readings were basically the same.

If you GPS is off, then ensure your settings in the device are correct and you are using the correct map datum. Your GPS may somehow be set for South and West instead of North and West when dealing with degrees. i.e. 19.123N 33.123W vs 19.123S 33.123W

Does this make sense?

 

[Warren Milberg]

I'm not familiar with your specific GPS device, but on my hand held Garmin, you must first select how you want the readout to show north. Sounds like yours may be showing south, for example.

 

[davidmhull]

Brian and Warren....I think you probably hit the nail on the head. I must admit I have not gone into the GPS much. I flipped through the book to figure out how to program in a keel depth offset but that was about it. This darn thing will do your laundry. It has a lot of features that I will probably never use but who knows. It may be very possible that there is a way to "flip" the bearings. It would not surprise me with as many features as this thing has. To be fair to you guys I need to get out the book and do some reading. I thought someone may have gone through this and have a direction for dummies answer. I will get back with you guys when I figure it out. I must say for under $300.00 this is a very nice unit. It does everything I need and much more. The down vue has so much resolution that I can see my anchor clearly at 35 feet. Amazing. I did not buy it as a fish finder (I'm sure they know where they are and probably don't want to be found) but the detail of the bottom is incredible. Thanks again.

 

[Brian D]

It is very possible that the unit is designed to operate inverted (upsidedown) while keeping the chart orientation correct. In that case, the reading would also be inverted, South being North. Just a thought.

 

[Stu Jackson]

Try the SETUP menu.

RTFM F stands for Funny.

Some time ago some dumb manufacturers were using, get this, MAGNETS TO CLOSE THE DOORS ON THE BACK OF THEIR GPSs to keep them closed, where the SD cards were inserted.

Guess they never figured their marine GPSs would be used on boats!!!!

 

[25yearslater]

Is it possible your compass is correct and the GPS is set for 'Ship's heading up,' instead of North up? That's all I can suggest.

 

[JamesG161]

RTFM F stands for Funny.

I thought F=Frustrating, oh well...

I thought that this thread would solve my similar FRUSTRATING GPS "OFF", situation.

My real steering compass and boy scout compass are dead on each other.

I set the options on both the "electronic" and GPS to be Magnetic versus True.

Set a calm day steady course and calibrated my "Electronic" to match steering compass.
YESSSSS!!
Finally they read and tracked!!!:dance:

But...

The GPS (Magnetic=M) doesn't match the real course. For a while it did.

This may not be your answer but it was how I kinda got GPS (M) to get close to the other 2.
Since GPS must take position to position points to calculate a course, I figured (even though it uses 15 satellites) the GPS precision is not perfect as the real world compasses.

Jim...

PS: My Autopilot now steers a good course since it uses the calibrated Electronic compass.

 

[Ward H]

Hi David,
If you can't figure it out, try Garmin tech support.1-800-800-1020 promts 4 then 2.
I had great luck with them answering my questions.

I think 25yl has it. The Garmin software is pretty good but you do have to play with it a bit to get it where you want it but with default settings it probably is something as simple as North up or Heading up.

Try going to Navigation Info/Compass. That should display the GPS compass. That is really what you want to see to compare to boats compass.

On the GPS screen, showing your the satellites, you should also see the accuracy.
You can turn on /off different GPS settings to see what gives you the most accuracy. Mine is 9 -10 feet. Close enough.

 

[davidmhull]

Going to take her out to Lewisetta this week to leave it moored on the Chesapeake bay for the next three months or so. At least until it gets too cold to sail. My first trip will be out to Tangier island and I will definitely have a good opportunity to play with the GPS settings. I will let you guys know what the results are. Thanks so much for all the feedback.

 

[ifmdalvey]

I am still unsure which is incorrect, if you had a secondary handheld compass reading similar to your binnacle compass then it the GPS at fault.
Remember, the GPS is providing COG which is not the same as heading, and the COG readout maybe in True rather than magnetic, providing a bit of difference.
If you were puttering around and not going in a straight course then the GPS is always playing catchup because of the way it derives COG. Go as fast as you can in a straight line for a few minutes and then see what the difference maybe, have an additional heading source available, (iPhone) but keep it away from your binnacle compass.

 

[JamesG161]

If you were puttering around and not going in a straight course then the GPS is always playing catchup because of the way it derives COG.

That is what I guessed. Thanks for setting me straight!! (pun intended)

For most short distance trips, Magnetic versus True is best, since the steering compass is magnetic and no power needed.

However COG is what shows on my chart plotter routing.
Jim...

 

[davidmhull]

Interesting. Thanks.

 

[philwsailz]

A compass will always point to the north. Unless you have induced a deviation my magnet or other ferrous object. A GPS.only tells you where you are, really... Especially if you are not moving...


Consider this scenario. You have your boat pointed north. Your bow and the compass needle are aligned. Your knot meter indicates 4 kts boat speed. You are in a current 90 degrees to your heading. The current has an SOG of 4 kts. Your GPS will indicate your heading as NW with a speed of 5.66 kts. What gives?

Compasses tell you where you are pointed. A GPS tells you where you are going. 5.66 is the vector sum of the two velocities.


If you still don't get it, point north doing 2 kts indicated by your knot meter in a 5kt current straight on your bow. Your compass will still tell you that you are going north burg the GPS will correctly tell you that you are going SOUTH at 3 kts...