|Topic Review (Newest First)|
|08-12-2010 06:55 AM|
Originally Posted by montenido View Post
|08-12-2010 06:45 AM|
Find yourself a local compass adjuster and have them come out with you and professionally swing the compass. I do mine every two years or whenever I change anything near my helm. It runs about $125.00 here in Maine. You will steer cardinal positions then the inter-cardinal positions and they will adjust your compass as close as can be then make you a deviation card.
If you do not own a deviation card, set up specifically for your compass, there is no way you are getting accurate readings...
|08-11-2010 11:59 PM|
Thanks everybody for the great replies. My GPS is actually a multifunction display - Raymarine C80, not 60 (sorry, my mistake). The compass is made by Richie. All of the equipment is relatively modern and probably works as intended.
Good points made about the difference in a bearing on a compass and the COG on the GPS. I just thought that the bearing and heading should be close. I don't remember exactly, but I think they were 20+ degrees different.
The reason it came up is that I set a waypoint for my marina 25 miles away and made towards it using my chartplotter. Even though I tried to follow the most direct route shown, I ended up doing more of an arc than a straight line. This was with no wind or current, just motor sailing. Not a big deal, but I probably covered a few more miles than necessary.
Next week I am going to Catalina Island, a trip of about 60 NM. I thought I would plot a heading using my charts as well as using the chartplotter with waypoints. On a 60 mile trip I would rather not add any additional miles if I can help it.
Thanks again for the great input.
|08-11-2010 01:28 PM|
A real simple way to determine if your compass is close or not is to get another compass! I have a hand bearing compass on my boat, many people have a cheap hiking compass too. Spend $5 for a cheap one and if it is closer to your GPS than to your compass, the compass is wrong. If your new hand compass agrees with your mounted compass you have bigger problems.
Personally, I almost never use my mounted nav compass. Most of my navigation is line of site (sail towards that smoke stack) or by chart plotter heading.
|08-11-2010 07:21 AM|
The OP didn't mention how much the difference was and if the difference was the same in all directions.
Most of us on small boats find that a few degrees off is the best we are ever going to get.
I'm pretty happy personally with about 5 degrees.
|08-11-2010 06:21 AM|
Don't confuse a multifunction display with a plain GPS Chartplotter. A multifunction display may be showing the heading based on a FLUXGATE compass, which should be at least as accurate as a magnetic compass once it has been calibrated properly. Without knowing what equipment the OP's boat has, it isn't possible to determine whether his MULTIFUNCTION DISPLAY is outputting a GPS-generated heading or a fluxgate based heading.
Again, using known terrestrial ranges is the only way to learn if the magnetic compass is off or not. Unless the compass was installed without regard to interference, chances are pretty likely that the magnetic compass is going to be more accurate, out of the box, than a GPS-generated heading, at least in regards to where the boat is POINTING.
Originally Posted by pedcab View Post
|08-11-2010 02:24 AM|
Originally Posted by jerryrlitton View Post
GPS does not show headings, it shows a course over ground and in any place but a widless pool your heading is always very different from your course...
Some more complete GPS units feature a fluxgate though...And those can show more accurate true and/or magnetic headings...
|08-10-2010 11:19 PM|
|CapTim||lol.. there's just no such thing as keeping it simple with you folks, is there? that's why I luff joo guys|
|08-10-2010 10:51 PM|
This may help...
True Course (TC): This is the course measured from your navigation plotter when you plot your trip on your chart.
Magnetic Heading (MH): The difference between true north and magnetic north is known as variation. Lines of variation are shown on a sectional chart as dashed magenta lines and called isogonic lines. By adding or subtracting variation from your true heading you will get your magnetic heading. Remember east is least (subtract) west is best (add)
|08-10-2010 10:46 PM|
There are three headings... True, Magnetic and Compass. The Compass heading is what is read on the compass card, but does not take deviation and variation into account. Adding the deviation to the compass heading will give you a magnetic heading. Adding the variation to the magnetic heading will give you the true heading.
There is no "actual" heading, since it depends on what you're looking at, unless by "actual" you mean "true".
Originally Posted by CapTim View Post
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