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  #1  
Old 12-22-2007
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ok, this is just bs...

I noticed this again the other day, and it reminded to ask, do anyone else's instruments disagree with each other? I'm talking gps fixed heading. I have a computer chartplotter, pedestal GPS plotter, B&G auto helm, handheld GPS, and magnetic compass, and not one of them agrees with each other. sometimes the spread is as much as 10 degrees. I use the computer chartplotter as the final authority because I can visually see how accurate it is my comparing where it says i'm heading vs landmarks, but all the others? I don't even look at the magnetic compass anymore because it never agrees with any of the electronics.

this isn't an issue of stray magnetic or electromagnetic fields and such throwing stuff off
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Old 12-22-2007
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Different GPS receivers average over different periods of time. The basic signal is hopping all over the place, so an average is needed. Hence, if you put three together (like I have) they don't often agree down at the last decimal points. The direction is extrapolated from successive position points, so on a slow moving vessel, this can wiggle about quite a bit.
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Old 12-22-2007
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The magnetic compass is subject to variation and damping issues, the GPS is subject to update lag, and your speed will affect the accuracy of other methods. I find generally that my fluxgate is the most accurate compass in terms of immediate readings, but that the GPS and the Ritchie will eventually agree if I hold a steady bearing for a minute or so, one which I can confirm with a running fix and a paper chart.
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Old 12-22-2007
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Also, most GPS units don't have a fluxgate compass built in... so they generate the heading of your boat based on its actual movement, and this takes current, wind, and such into account... which may not be the same as what direction your boat is pointed in.
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Old 12-22-2007
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Don't GPS units indicate true north, whereas magnetic compasses are of course magnetic and pointing to that chunk of iron under Hudson Bay?
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This can be set on the GPS as one of the options... they can generally display either true or magnetic.
Quote:
Originally Posted by sailhog View Post
Don't GPS units indicate true north, whereas magnetic compasses are of course magnetic and pointing to that chunk of iron under Hudson Bay?
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Old 12-22-2007
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Quote:
Originally Posted by sailingdog View Post
Also, most GPS units don't have a fluxgate compass built in... so they generate the heading of your boat based on its actual movement, and this takes current, wind, and such into account... which may not be the same as what direction your boat is pointed in.
now if feel stupid, which takes a bit of doing good job sd. i've never stopped to think that gps gives you your true heading while the compass shows the direction you are pointing, and of course they could be very different.

the one time became acutely aware of this issue was the time i was heading home on the fraser river; i was till out in the straight of georgia and had her pointing towards the next waypoint, but since there was some wind i decided to sail as long as possible. but the landmarks didn't seem to be getting any closer (after an hour of sailing), so i double-checked the gps, and although we were sailing towards the river, we were actually going backwards at about 1 knot because of the current.

i just never extrapolated that to the bigger picture. another teaching moment brought to you by sailnet

and as for sh's question, you can switch the preference between true north and magnetic north on any gps i've ever seen.
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Just a bit of maybe useless information...GPS units more than 6-7 yrs old didn't have very fast processors and the time it took to average the calculations was significant...so much so that it was recommended that the antenna not be mounted high off the water, especially on a sailboat mast, to help reduce the effect of the exaggerated motion. I still like to mount the antenna somewhere at deck level if I can just because its usually easier to find an unobtrusive place for it.
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Since this thread has run its natural course, I'd like to hijack it with a quick question...

The screen on my Garmin unit has gone very dim. I turned up the brightness to its maximum setting, and it still is pretty damn dim -- visible, yes, but dim. I'm assuming this is a warranty issue, as it's about a year old? Anybody have the problem, quick fixes?
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Old 12-22-2007
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Why would you want to set the magnetic north or true north option on your GPS to true? Everything your doing needs magnetic readings, correct?
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