GPS question

[wallm]

I have a somewhat lengthy and technical GPS question. I have a Garmin handheld 76S but I suspect this question may be generic. I charter frequently and use the unit in the cockpit. It is usually found propped up near the binnacle. One night I was using the unit to follow my heading only to eventually figure out it was about 30 degrees off compared to the binnacle and my handheld.
As I read through the manual it noted that the unit is set up from the factory to use an electronic compass to establish heading at speeds <10 mph and to use the GPS receiver at speeds > 10 mph. Clearly I am always going < 10 mph. I can change the default setting so it will use the GPS receiver for navigation instead of the electronic compass at speeds down to 0 mph.
So here are my questions:
1. I assume the GPS was way off that night because I had it propped up (the instructions say the electronic compass likes to be flat) and it was near the binnacle compass and a radio. Does that sound right? Is an electronic compass subject to the same interference problems as a magnetic one? I really do not know what is meant by an electronic compass.
2. Is there any reason not to change the settings so that the unit is always using the GPS receiver to navigate, i.e., I can change the default down to 0 mph? I know that a GPS only navigates "correctly" when one is moving but of course in a sailboat when not moving I am generally anchored and using my handheld regular compass anyway.

[mikehoyt]

Not to sound too obvious but is it possible that one of the instruments is using TRUE and the other MAGNETIC headings?

[wallm]

No. The variance was only 6 degrees plus I was real careful about that. GThe GPS reads true and I converted the binnacle reading to true.

[mdougan]

You''ve pretty much answered your own question. I probably have the same unit (mine is the GPS Map 76S)

Anyway, the way I understand it is this, the reason they set the default speed for using the gps for compass readings at 10 miles an hour is that this is about what it takes to get an accurate reading between gps sample times (maybe you can reduce the frequency of how often the unit tries to update it''s position information) and you will have moved sufficiently between readings that it can deduce your course.

However, you shouldn''t have any problem using the compass at lower speeds, but in the directions, it says pretty plainly that you must hold the unit horizontal or the reading will not be accurate. I image the same thing would apply to a hand-held magnetic compass.

Don''t really know if the internal compass can be thrown off by electronics or metal, but I doubt it, as the unit itself is electronic, and that would be pretty hard to engineer.

Make sure that you''ve calibrated your compass. Go into one of the setup menus and select calibrate, then it will ask you to rotate your unit through two horizontal revolutions. I think that there is also a place where you can enter deviation, though it''s not clear to me why that should be necessary (they should know where you are, and have that info available to the unit)

Oh well, good luck.

[hamiam]

not sure if the gps compass is a fluxgate compass like that on an autopilot but I found that I had to move my fluxgate compass away from my boom box as it caused the magnitude of error of which you speak. The speakers in the boom box have magnets which throw off the fluxgate. Same is true for electric motors and any metals, like a knife, that have been magnatized. Just a thought.

[Yodagwb]

Doesn''t the GPS compass use the change in location to determine the course, therefore any slippage or cross current would factor into your electronic course. Could you of had that kind of deviation from compass course?

[wallm]

The night my GPS heading way way off I had the unit upright and it was near a radio. I think both conditions could have thrown it off. However, I like to have the GPS upright so I can see it when I am at the wheel and since I charter different boats who knows about differet magnetic forces in various cockpits. As I mentioned previously, the GPS comes from the factory set so that at speeds <10 mph it uses the electronic compass for heading and at speeds >10 mph it uses the GPS fix from satellites. However, one can change this default so that the unit uses the GPS and not the electronic compass all the way down to 0 mph. I just do not understand GPS units well enough to figure out why they even build in the option, if one would lose accuracy using the GPS as the determinant of heading at slow speeds like 1 or 2 mph, etc.

[hamiam]

Call Garmin tech support and ask one of their tech geeks.

[mdougan]

Well, obviously, they do it for people who want to use the compass while the the unit is vertical, but they know that it''s not going to be as accurate as your boat''s compass, and at least they say that plainly in their documentation.

I think that the compass itself is just a feature added to make it look better on paper. They even recommend turning it off to help conserve battery life.

Next time you see such a variance, just lay the unit down and you should see it assume a more correct reading.

[39512]

With the boat at the dock, take a magnetic compass reading. Then turn on the GPS and place it near the binnacle. I would guess that the magnetic compass card will deflect.

Turn on the GPS. Stand near something ferrous. See if the heading changes.