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Old 09-01-2013, 23:22   #16
noelex 77's Avatar

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Re: Mounting GPS on a wheeled boat

Originally Posted by Straightrazor41 View Post
I use a cetrek chartplotter, it has a compass page that gives heading so I thought they all did. .
I think you will probably find your chart plotter is showing COG not heading. It's important to understand the difference.

You can do this simple test to what is displayed. Look when you are at anchor, or tied to a dock. If its COG, as I expect, you will find the "compass" page on the chart plotter is all over the place and has nothing to do with the way the bow is pointing.
If it is heading data it will be steady and will reflect the direction the boat is pointing. (Just like an ordinary compass).
If you have an autopilot leave this on as sometimes this will feed heading data into the chart plotter.

COG is very useful figure to have. It's one of the wonders of GPS that we get accurate information on something that used to take considerable calculation and guesswork to estimate.
The difference between COG and heading tells is a lot about what currents and leeway is effecting the boat. If we loose the compass we loose heading data and can no longer see the difference. Not understanding the difference between COG and heading data does the same thing

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Old 10-01-2013, 10:57   #17
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Re: Mounting GPS on a wheeled boat

noelex77, thank you for that detailed and informative post. I never considered that it wasn't a true compass bearing. It's a cetrek 3400. I went to the manual after reading your post and the page shows a compass stip that gives heading when not traveling a route and when your following a route this page displays course and cross track error. I just assumed it was a compass bearing but unless it has a built in flux gate compass there's nothing connected externally.

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Old 10-01-2013, 11:29   #18
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Re: Mounting GPS on a wheeled boat

If it's just a GPS, and not a chartplotter, I'd mount it down below at the nav station.

Rick I
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