How do I synchronize my compass reading to the GPS ?

[TheOtherLine]

I notice that the heading reading on my GPS is different than the heading on my compass, by as much as 5 degrees. I know compasses can be adjusted to compensate for magnetic or electrical interference. Anyone done this? Is it easy to do?

 

[Fishinfireman]

Below is a link to compensation instructions from the Ritchie site. You must use a non magnetic scewdriver. A little brass screwdriver should be in the Parker pouch that came with your boat, there was one in mine.

http://www.ritchienavigation.com/servic ... ation.html

If you have your boat on an aluminum trailer you can do the adjustments on the trailer. Have someone tow you down a straight road that runs N-S(+or- 10*) and then one that runs E-W(+or- 10*). Get the COG from your GPS and use it to adjust your compass. I used this method on my compass and it worked real well. You could do this in a large parking lot also. You just need to travel far enough to get a good COG from the GPS, then you can stop and make an adjustment.

 

[DaleH]

First off ... understand this:

Even if DEAD ON, while heading to the North heading, AND you make a compensation adjustment, you still could be off in any or EVERY other heading :shock: ! [Art least those ~30-degrees away ...]

Pay heed to the comment about creating a deviation card. Example: If you wanted to head East, you might need to add or subtract 3 degrees [more or less] from your compass reading to indeed get it to match the GPS and 'real' heading.

FWIW a truly FASCINATING read on this topic is the book Compass: A Story of Exploration and Innovation by Alan Gurney. It talked about how when ships were made, whether wood or iron, how the ship was oriented in the ways (east to west, north to south, etc.) had an effect on the ship's inherent magnetism and it could affect the compass readings. Also, many ship Captain's were know to keep pistols under the compass for self defense from a mutiny :shock: ! Compasses from either case could be off up to 20-degrees, which on voyages of hundreds to thousands of miles - could be HUGE!

 

[TheOtherLine]

:shock: Wonderful....

 

[Outta Here]

First off ... understand this:

Even if DEAD ON, while heading to the North heading, AND you make a compensation adjustment, you still could be off in any or EVERY other heading :shock: ! [Art least those ~30-degrees away ...]

...not to mention the effects of current and wind. Most small boats have the compass surrounded w/electronics. More problems. GPS costantly recalculates correct heading, so what if we're 5 degreres off. That's closer than I can steer in most seas anyway.

No one asked, but my advise would be "leave the compass alone" :wink:

 

[Stank Bait]

If you can steer your boat so that 5 degrees makes any difference you are better than any others out there. Usually within 15 degrees either side of the heading is about all you can expect if there are any wind and waves. Would not want to concentrate on a compass even in calm water enough to try to keep it within 5 degrees. Too much stuff floating around that could cause a nasty collision. It's not worth fooling with on a small boat because the course will not be very straight no matter how hard you try. Way too many corrections in steering.

 

[TomS]

Is your GPS set to display your heading relative to true north or magnetic north?

I looked up the magnetic declination for St. Petersburg, and its just about 4.5* - which may explain your delta if your GPS is displaying True North.

-- Tom

 

[TheOtherLine]

Is your GPS set to display your heading relative to true north or magnetic north?-- Tom

I think my GPS is set to True North. It's not that I am worried about running a true linear route, what I guess I was thinking was that, when approaching a waypoint, like a fishing spot, my gps will say, head XXX degrees. So I begin steering in that direction. Well, by the time the GPS "repositions" itself and gives me the current bearing, I'm off target. So then I try to use the compass to dial into the heading I was supposed to go to. But, the compass is off about 5 degrees.

When your trying to get on top of a small reef that might only be 20 feet long, accuracy counts.

I dunno. Maybe it's just me not used to my GPS yet.

 

[TomS]

That is my point, if your GPS is set to point to "True" north and your compass is pointing to "Magnetic" north they will be off by 5*.

Try setting your GPS to display magnetic north.

-- Tom

 

[DaleH]

That is my point, if your GPS is set to point to "True" north and your compass is pointing to "Magnetic" north they will be off by 5*.

Try setting your GPS to display magnetic north.

-- Tom

Exactly!

 

[TonyAmalfitano]

This is the most complicated subject in navigation IMHO.
But there is a handy formula. I'll try to reproduce it here.
T = true north
M = magnetic north
V = variation (found on the compass rose) ((in NJ it's 15*West)
D = deviation (caused by magnetic interference, ex. speaker magnet)
C = course ( the course you will travel)
+W = add west, -W subtract west, +E add east, -E subtract east.
v = down arrow, follow TVMDC down ^ = up arrow, follow TVMDC up.

+W v -E T +E ^ -W
V
M
D
C

EXAMPLE: T = 090
V = 015W
M = ?
we are now following the left side of the formula, working down (v)
T 090
V +15W
M = 105

EXAMPLE 2:
T = ?
V = 15W
M = 105
we are now working the right side of the formula, working up (^)
T= 090
V - 15W
M 105
Your compass reads Magnetic north, charts are true north, GPS can be what you set it at, most of us use Magnetic north. If you have the GPS set to Magnetic north, and GPS reads 090, but your compass is reading 095, you most likely have 5W degrees(*) of "DEVIATION." Your devation error will change with your course heading. In the formula you would plug in YOUR deviation for a givin course and add or subtract as needed.

EXAMPLE 3: T= 090
V= 15W
M= ?
D= 5W
C= ?
Working down (v)
T= 090
V+ 15W
M= 105
D + 5W
C= 110, Steer 110* to reach your destination.

In converting, or correcting from TRUE to MAGNETIC a west variation is always added to TRUE, and EAST variation is always subtracted from TRUE.

You would use this formula to create the "Course Card."

Everytime you add a piece of equipment with a magnet in it you will effect your DEVIATION, thus requiring a new course card.

Ain't that easy!!?? :roll:
OK, when I hit SUBMIT it moved things over so they don't line up TVMDC should be straight down between -E and +E where the T is. COMPUTERS :twisted:

 

[TheOtherLine]

Ain't that easy!!??

WTF? :? I'll just find a bigger reef.......

 

[TonyAmalfitano]

This is the crap one does in the winter when there is nothing else to do, and I mean nothing!
It's nice to have a paper chart back up with course lines plotted with the corrected headings should the electronics fail. Getting home is then no more a problem then breaking out the old chart and following the compass home, even works in the fog. There's always the
"Seaman's Eye" but that dosen't work in the fog.
I don't do the math, as in write out the formula and plug in the numbers, as in the examples, I work off a plotter and adjust the course from the compass rose on the plotter.
Those of you on the west coast of Florida, your variation should be 0, you lucky dogs.

 

[JC]

breaking out the old chart

Tony.....how old is the chart :shock: and where can I buy a "Seaman's Eye" :?: :lol: :lol:

 

[TonyAmalfitano]

The "OLD CHART" just turned 55 end of September. 8)
If you ain't got a "SEAMAN'S EYE" by now, you ain't gettin' one!

 

[JC]

:lol: :lol: :lol: :lol: Happy Birthday :!:

 

[Principal Reeler]

Tony A.

I was a Math teacher, and my brain hurts after reading your post :shock: :roll: :wink: :lol: !!

Mac

 

[TonyAmalfitano]

If you know how to use Parallel Rules you don't have to do any math. All you do is plot point A and point B, place your rules in line with A&B, draw a line, and walk the rules to the compass rose. There you will find True Heading on the outer scale, and Magnetic on the inner. Then adjust for deviation. (From your Deviation card)

My head spun a few times learning this ship, but once I caught on it was a snap. Kinda like algebra.

 

[hakr]

I'm sorry Parker wastes dashboard space for that honking big compass.
I have an installed GPS and a portable GPS, and a hand bearing compass, in case the GPS system craps out on me. I never use the installed compass.

 

[Outta Here]

I'm sorry Parker wastes dashboard space for that honking big compass.
.

I'm gonna take my compass out and convert the hole into something useful like a cupholder. What a foolish place for a compass...what were they thinking?