Are pedestal compasses set to true north typically? I was trying to identify magnetic interference on my pedestal last weekend for a new autopilot control, and noticed my pedestal compass was slightly different reading than my hand held compass. It looked to be a couple of degrees off to the east.
Pedestal Compass, True North?
- Thread starter BAD ORCA
- Start date
Magnetic.
As a practical matter, a “couple of degrees” is insignificant.
you can find the precise deviation at your location on any internet search function
As a practical matter, a “couple of degrees” is insignificant.
you can find the precise deviation at your location on any internet search function
Correct.
That depends! It can be a big deal if you're transiting a narrow channel in fog, and using the compass.
That is incorrect. The two big compass errors we deal with are deviation and variation. Deviation are errors of the compass as a result of the boat upon which the compass is mounted, and the compass itself. These can be recorded on a compass deviation card and compensated for when steering. Local differences between true North and magnetic North are variation, and depending on where you are can be as great as 30º. They are printed on charts, and actually change year to year.
It is the nature of magnetic compasses that they work only by pointing to magnetic North. For very limited local sailing, they can be adjusted to point to true North, but as soon as you travel a few miles laterally, the variation between true North and magnetic North changes so that the compass is not pointing at true North. Thus, by keeping track of the local variation as outlined on the chart, there are no problems with the compass adjusted to magnetic North.
I don't know how sat/nav computers do it. They are certainly capable of using true North, but that could become confusing if combining with a magnetic compass
-Will
GPSs and Chartplotters have built in tables with variation information built in. They simply look at the device's location and apply the variation to the data.
The magnetic compasses we use on our boats are not very precise even if properly compensated because the disk is too small. Accuracy is at best +/- 5°. Of course if there are factors on your boat that cause deviation and knowing the amount of local variation is important.
Paper charts show true north. Magnetic campuses point to the northerly direction of the magnetic field that they are next to. It is (maybe) possible to attract the permanently mounted compass to the hand compass if you bring them close together. In a plastic boat a permanently mounted compass might more likely show a different reading for magnetic north with everything turned on than when every thing is turned off( or not ) because of nearby wiring, which is deviation. Variation on an old chart, the apparent direction of magnetic north, however may be quite different than the variation on a new chart because the magnetic North Pole is moving. For any given area I've always thought that magnetic north is accurate as it can be to a compass for any 50 mile location at sailboat speeds. Just take care when at the NE end of Puget Sound, GPS should be OK because it calculates true north.
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“Just take care when at the NE end of Puget Sound, GPS should be OK because it calculates true north.”
Huh? Please elaborate…Am I learning something new after 60 yrs?
Huh? Please elaborate…Am I learning something new after 60 yrs?
Quite often, questions asked on boating forums become useful to others because they raise "granular" issues within broader topics that may not have been discussed before. They are, therefore, helpful to many, and particularly to those interested in the nuts & bolts of equipment, material, and "how things work." We all know the valuable contributions by specialists in their field, like Maine Sail, and I applaud all of us who refer to his body of work and provide links to his informative articles and threads, as well as his Featured Contributor section on this website/forum.
Every once in a while, though, a basic question is posted that literally makes my jaw drop in fascination, and is exacerbated when it comes from a relatively frequent contributor and/or reader of a boating forum.
The question of whether a magnetic compass shows true north is simply mind boggling to me.
I have frequently been "accused" of suggesting that people do homework & research. To avoid that, I have often taken to provide links to previous discussions and quality information, sometimes for people who seem to be too lazy to do their own. I have even built two sections of our C34 Forum dedicated to this concept:
"Over the past decade or more, "regular" topics come up repeatedly. In developing responses to questions, many of us use the handy links to earlier discussions. I've developed these "101" topics, which are groups of links to popular subjects and those discussions. I'll leave this topic as a sticky, and add any new "101"s that may develop. Many of them include links to other websites and message boards, since boat systems are not always boat-specific."
This question about magnetic compasses is one that is answered in every single book about basic sailing.
I suggest that the OP crack a copy of Chapman's to avoid anyone having spend any time typing a reply to teach such basic navigation techniques. And if books don't work, there's this handy, dandy internet thing.
Every once in a while, though, a basic question is posted that literally makes my jaw drop in fascination, and is exacerbated when it comes from a relatively frequent contributor and/or reader of a boating forum.
The question of whether a magnetic compass shows true north is simply mind boggling to me.
I have frequently been "accused" of suggesting that people do homework & research. To avoid that, I have often taken to provide links to previous discussions and quality information, sometimes for people who seem to be too lazy to do their own. I have even built two sections of our C34 Forum dedicated to this concept:
"Over the past decade or more, "regular" topics come up repeatedly. In developing responses to questions, many of us use the handy links to earlier discussions. I've developed these "101" topics, which are groups of links to popular subjects and those discussions. I'll leave this topic as a sticky, and add any new "101"s that may develop. Many of them include links to other websites and message boards, since boat systems are not always boat-specific."
This question about magnetic compasses is one that is answered in every single book about basic sailing.
I suggest that the OP crack a copy of Chapman's to avoid anyone having spend any time typing a reply to teach such basic navigation techniques. And if books don't work, there's this handy, dandy internet thing.
The GPS will calculate and show either true or magnetic courses, it all depends on how you configure your gps.
Decaf might help some, but I find myself mysteriously drawn to his logic !
BTW, I admit that it's past time to probably have our compass checked out again, but in 2012 when it was totally rebuilt 'like new' and swung, the deviation card read "steer to compass" . Way cool!
Thanks for a chuckle... Since you have been sailing a long time, perhaps you recall the days of Loran, when we had to learn to reinitialize it off of Grays Harbor when it changed from one antenna chain to another and suddenly would show us about 15 miles off position. Exciting for a new crew, at 3 am, in the dark. Still way better for those coastal deliveries than the Olden Days of hand bearing compasses and the mysterious RDF...
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... and Will, if you travel a few miles vertically, it really will not matter to you what your compass said!
Love the old mnemonic for Compass to True adjustments "True Virgins Make Dull Company, Add Whiskey" for True + Variation = Magnetic + Deviation = Compass, adding Westerly. You can go in either direction.
Or invest in a few paper CHS charts to see what's going on. These are almost unheard of in our marina.
"Why bother when I got this handy, dandy pocket GPS in my back pocket"
And that's why CHS repeatedly brings up the subject of PRINT ON DEMAND ONLY where no vendor stocks charts anymore.
Anyone WITHOUT paper charts know what this is and what it means on a paper chart ?
I thought so.
There seems to be more going on there then just a simple compass rose. Why is there a rose inside a rose? It must be one of those double-roses.
-Will
A spinny wheelie turny gizmo? Where's the pointer?!?Anyone WITHOUT paper charts know what this is and what it means on a paper chart ?
Some would say that’s antiquated way of conveying the same information a GPS gives in the “variation” data item.