GPS vs. Ship's Compass

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Oct 11, 2007
Vessel Name
Circuit Breaker
Vessel Make
2021..22' Duffy Cuddy cabin
You have just completed charting a course for your cruise, using the proper charts and charting tools, and your chart plotter is in good working order and your ship's compass has been swung within the last week. To hold the magnetic heading that you determine from your paper chart, which do you trust most.....The GPS mag heading or the ship's compass?
Well, it's easier to steer by the compass, since it's a little more damped than the GPS readout. OTOH, the you don't have to calculate deviation for the GPS, saving you a step.
Scew it... just point yer boat the direction you wanna go just once, and don't go any faster than you're willing to hit something

j/k... couldn't resist
YOUR EYEBALLS!!!!!! Go where your eyeballs say to go!!!!!!
I prefer going with the chart and keeping a reasonable amount of thick water under the keel.

In Fl and on the Loop in Canada , the use of GPS to stay clear of the bottom would be folly.

Too many twists and turns , sometimes between buoys that are 35 ft apart , or running a range that is primitave at best.

When the AP is in charge , it is only in zero danger areas where poor steering , slightly off course , or system failure would be a minor hassle.

A run between Norfolk and NYC or Block Island , so what if your 5deg off ? its only 5 miles in 60 and sure to be found easily with an hourly plot.

Baker wrote:

YOUR EYEBALLS!!!!!! Go where your eyeballs say to go!!!!!!

Soooooooooo, you don't go offshore much, do ya?
Thanks Keith, for answering the question.

I didn't ask whether you should use paper or electronic charts.....or just point the boat and go...
I ask "Which do you trust the most...the ship's compass or the mag compass on the GPS chart plotter?" I simply want to know what the group's opinion is!!!!!! Now, maybe this group doesn't actually cruise much and that's OK! But for the ones that do, what's your answer to my question?
My magnetic compass has not failed in 10 years but one of my two GPS's did fail.* And I do have a backup handheld compass.* Watching helmsfolks snake wake the boat by chasing the GPS compass vs a much better wake w/ a magnetic compass puts my vote w/ the mag compass.* Of course you should be able to dampen the GPS compass which would help dampen out some of the heading changes.* Ever watch your GPS compass at the dock - interesting.* Perhaps the newer GPS compasses are better but big $$$$.*
Interesting....only the Krogen guys have answered the I know why I like those boats so much.
Feed the magnetic variation into your plotter and go with that , but also keep referring to your magnetic compass.
Be aware that even though you have recently had you compass swung ,the deviation can be put out by something stored too close or some other recent or local magnetic annomoly.
If the GPS fails at least you know it is kaput.
The compass can vary but you may not be aware of this change.
The beauty of this modern era is we have many choices. I also carry a Coleman hand held battery powerd digital compass which is also another check.

I spend a lot of my time offshore in and out of the reefs of the Great Barrier Reef once in the reef it is daylight visual travel only between the hours of about 8 am to 3 pm.
Most folks do not have a compass that has been swung and has the required deviation table, with 15deg breakdown

Most do not know how to shoot a sun line to check the accuracy of the dev table on the desired course.

Many do not know "Can dead men vote twice" Compass, deviation, magnetic, variation, true


True virgins make dull companions,, true, variation, magnetic, dev, compass.

So with an unrated compass and a frequent lack of ability to plot a course on a chart and convert it into a compass heading , the GPS is first choice for most "boat drivers ".

I guess when GPS goes south, Sea Tow will help, as for some it is an emergency.

Keith wrote:
Baker wrote:
<<Soooooooooo, you don't go offshore much, do ya?
That is funny because the last time we went to Port Aransas, we went offshore and this boat I have has no autopilot.* I was definitely doing the "snake wake" deal.* I then used my wake as part of my "crosscheck".

To answer the question, I use the GPS more probably.* I am not saying that is right.* But I am just being honest.* The compass does have undesirable traits in that the "display" is difficult to read.* If I am truly in need of something to steer by I will put the GPS in the "vertical compass card" mode and steer by it.* Ideally, we would all have vertical card gyroscopic direction indicators that eliminate all of the undesirable traits of the compass.

In reality, the compass is just a backup or standby instrument.
** I agree Mr. Baker, a compass, in this day and age, IS a backup instrument but I would not be without one.* I find that reference to the chartplotter, which is just a glance away from the helm is handier than constantly refering to paper charts (the CP automatically*"turns the page").* I do not find the compass display that difficult to read because the card is quite large and it is at*such a*distance that I do not need my glasses to read it and I have used the compass to steer a course.
** One good thing about the chart plotter is that the view can be zoomed in and out to give some indication as to what is either next, or around the next corner.
The GPS gives you the "course made good" after deviation, current, and wind have all been factored out.* Since what you're interested in is getting from here to there, it's the optimum navigation aid to follow.

But you're generally best off steering off of the compass - so determine the compass course based on the GPS.* You'll have to keep an eye on it, as changing current, etc, will change the compass heading that you have to steer in order to maintain a given course-made-good.

"When the clock goes BING I change my course."

Aren't you glad Lincoln didn't "free" the Clock Springs or Electrons?

In my opinion the only instrument you can truly trust in an airplane or a boat is the magnetic compass. We have two large GPS chart plotters (C-Map) on our boat, the Echotec 310MP we installed when we got the boat ten years ago and the Furuno NavNet VX2 radar/plotter we installed last year.

We use both plotters when running the boat but we hold our course with the compasses, which up here takes a lot of "work" without an autopilot because of the boat's progress through constantly shifting currents among the islands.

The reliability of modern electronics is such that they can be depended on pretty much 100 percent of the time (I'm talking about dedicated plotters, not laptop-computer based plotters which have an almost infinite number of failure modes when you add in the glitches the Microsoft kids have generously included in the operating system).

But despite the reliability we have experienced so far with our two plotting systems (three, actually, counting the Magellen C-Map handheld plotter we have as well). the magnetic compasses are the only instruments on the boat that I know will get us within shouting distance of our destination if the electricicals suddenly stop running around through the wires.

For the same reason we have the relevant paper charts at the helm at all times, too. Granted we're lucky up here in that most of the areas we boat in have large but manageable chart books covering them as opposed to having to deal with a series of huge, rolled-up NOAA and Canadian charts although we have them as well.

Now if the question is which is more accurate I would say the plotter compass or bearing indicator probably is.* But I think*there is a subtle difference between the information coming from a magnetic compass and the information coming from a plotter compass or heading display.* The electronics tell you what's happened.* The magnetic compass tells you what's happening.* I find that I do a much better job of holding a course with the compass (using the plotter to confirm or correct the compass heading) than I do holding a course with the plotter alone.

-- Edited by Marin at 20:06, 2008-09-23
"In my opinion the only instrument you can truly trust in an airplane or a boat is the magnetic compass."

Even more so in a voyaging boat as the chances of total electric failure , or an errant wave are much higher than for the canal cruiser.
Wouldn't that errant wave wash your paper chart in your open cockpit overboard too? How about those who run their boats from the bridge and the wind blows the chart away? How likely is it to have a complete and utter electrical failure of your electronic chart plotter versus having someone accidentally put something too close to the magnetic compass?

I think I see some pretty rigid thinking. I can't trust the electronics because they're "magic" and I can't see them work. I can trust a compass and chart because that's what I first learned on.

If you truly distrust the electronic charting systems then you don't fly in airplanes, you don't drive farther from home than you could walk back, and you don't have any credit cards. Those items are all dependant on the electronics working properly. If they failed catastrophically then major problems would arise and there is no paper backup for them. Your entire life savings is kept track of electronically. Do you trust that system? It might even be run on a Microsoft operating system. Oh the horror of terrible crashes!!!
What if you were making a large deposit when the system crashed? (parallel this with the chart plotter going out just when you were entering a tricky channel) Do you demand a written paper ledger entry when you put your paycheck into the bank? Does anyone still have a passbook account where deposits and withdrawals are hand written into the book?

Interestingly I boat in the same waters as Marin. I'd bet I can get back from anywhere without charts or a compass. Might I take a wrong turn or have to retrace my steps once or twice? Sure. But we are always in sight of land. (excepting fog of course and I think most would rely on radar to take them home, don't you? Isn't that an electronic device which could be catastrophic if it failed?)

Now if I was offshore and wanting to be sure I hit that little dot of land called Hawaii, which of us would set off with only a compass and chart and which would take a chart plotter and GPS as our primary navigation tool?

Just my thoughts,

2bucks wrote:

Interestingly I boat in the same waters as Marin. I'd bet I can get back from anywhere without charts or a compass.
I would agree with this if one was very familiar with the area.* But there are lots of places--- the entrance to Comox Harbor from Cape Lazo on Vancouver Island*being one we just experienced the other week for the first time--- where if you didn't have a chart or didn't have local knowledge you could end up in a very frustrating and time-consuming situation.* In the case of Comox, not knowing*in advance where the very narrow channel across the wide and long*bar is, you could easily put the boat aground.* Monitoring the depth sounder would hopefully prevent this, but without knowing the one safe*way across the bar the only*alternative is to go many, many miles out of the way around two large islands*to the southern entrance.

So while one could eventually*find their way into Comox without charts or a compass, the operative word here is "eventually."* Most of us don't have the time to boat on an "eventually" basis.

Also, given the almost infinite number of rocks and reefs all over this area (PNW),*many of which lurk just*below the surface most of the time,*I would not want to venture out without charts--- paper or electronic---into an area that I was not 100-percent familiar with.* Some of these rocks and reefs regularly collect boats including large, fancy, fully-equipped yachts.* They earn the local name "Money Maker Rock" for good reason.* If you don't know exactly where they are, you could easily end up contributing to the local economy.

Ken's point about our depending on electronics in almost every aspect of our lives today is totally valid.* But one difference between banks, credit card systems, commercial aircraft, etc. and the typical recreational trawler like most of us have is redundancy.* Commercial aircraft have multiple power sources and several layers of system redundancy.* So the likelihood of a total electronics failure is very, very slim.* Even if the flight crew manages to run the plane out of fuel there are batteries to keep the critical attitude and guidance electronics operating long enough to get them*to the scene of the crash.* And ground-based systems like banks, credit card companies, etc. have all sorts of layers of redundancy in their power systems and their computer systems.

By comparison our little trawlers have very little redundancy.* Some have no redundancy.* So things like paper charts and magnetic compasses have, I think, a more important potential role.

-- Edited by Marin at 12:33, 2008-09-24
Automation in airplanes was viewed with the same scrutiny as y'all are discussing. We are now flying into Quito, Ecuador with VNAV/LNAV approaches that use only GPS and onboard computer systems for guidance through some extremely challenging terrain.

We did have a 757-300 that lost all power between SEA and ANC and I think they landed in Ketchican. The airplane "only" has 3 generators on board to begin with(2 engine driven, 1 APU...auxillary power unit). They began the trip legally with one generator inop. This required them to run the APU for the duration of the flight. The APU shutdown/failed so that left them with only one generator. That generator did not properly load shed so it overloaded and went offline. So now they were down to only standby battery power over nowhereville Canada. FAA certification requires standby power last for only 30 minutes and then it is lights out. They touched down 28 minutes after the emergency began. Our ETOPS(Engines Turning Or People Swim....really Extended Twin Engine Ops)) 757s also have a hydraulically powered generator....our -300s are not ETOPS certified.

The FAA comes up with a "one-in-a-billion" chance scenario for critical systems. I have no clue as to how they calculate it.

The way we do it in airplanes is "levels of automation". Maybe you could set something up like that in your boat or at least think in those terms. Fully electronic would be the top level of automation. Then maybe the next level would be hand steering guided by electronics. Then the next level might be paper charts while hand steering,etc......always being able to go in each direction of automation. If it gets too complicated in full auto, then click the autopilot off and take control. If things still aren't looking right, crosscheck on radar and maybe paper get the idea. Keep going down in automation until you are sure you are totally situationally aware. The problem airplane pilots have is that you can't stop and hover......time is compressing and you gotta do something. I do think there is still time compression doing 7kts. It is amazing how fast things can happen at 7kts....especially in low visibility/low light conditions.* At some point, there will be consequences(good or bad) to your decisions.

-- Edited by Baker at 14:57, 2008-09-24
But you see Marin, you've said you can only totally rely on your magnetic compass. Your example of entering Comox is a perfect example of where your magnetic compass will tend to lead you into trouble.

To chart your way into Comox with paper chart and magnetic compass what do you have to do? Draw a straight line from Point Here I Am to Point This is the Gap, parralell rule to the compass rose and get your heading to steer? No, not if you want to successfully navigate the channel. You instead check the state of the tide, you consult the Current tables, you do a little math to determine where you are in relation to what time you'll be at the gap. Then you calculate mag dev for the area, and hope all your numbers come together at the right time.

Or, you take off from Here I Am and steer the heading line on the chart plotter to the Gap. Then you sit back and watch the scenery. The GPS tells you where you are and where you're headed, automatically notifying you about set and drift when the bearing line is off.

Now, the knotheads who are running into things are not doing it because of any chart plotter. They are doing it in spite of what the chart plotter, radar and depth sounder are telling them. If those knotheads were calculating set and drift how much more attention would they be paying to looking out the window?

I'll bet that Marin used his chart plotter to know exactly where the gap was, going in to Comox. While he was following along with a paper chart, depth sounder and visual confirmation, he also looked at the technology to confirm what everything else told him. Most prudent mariners do.

My piloting is done with two chart plotters below and one above on the flybridge. If something doesn't look right, needs confirmation, or a failure occurs, I have paper charts, tide tables, current tables and plotting hardware a few steps away. It only takes a few minutes to calculate where I am and begin the more tedious process of paper charting my course. As I've mentioned before, I spend one day each vacation doing the paper chart. This keeps my skills at a level that I am comfortable with. Your level of comfort may vary and your history of paper charting may vary. In 1979 when I started in the San Juans and Gulf Islands Loran was out of my price range. Every trip was charted on paper and notes were made to make it easier next time.

2bucks wrote:

But you see Marin, you've said you can only totally rely on your magnetic compass. Your example of entering Comox is a perfect example of where your magnetic compass will tend to lead you into trouble.

I'll bet that Marin used his chart plotter to know exactly where the gap was, going in to Comox. While he was following along with a paper chart, depth sounder and visual confirmation, he also looked at the technology to confirm what everything else told him. Most prudent mariners do.
The original question was assuming a properly swung compass, properly plotted course, etc., which would one trust the most to follow the magnetic course, the magnetic compass or the GPS magnetic heading indicator?* Assuming "trust" means which one is*least likely to crap out, the magnetic compass is the more reliable in my opinion.* So in that respect, it's the only instrument I feel I can totally rely on (not to fail).

However the magnetic compass won't help you compensate for changes in current that you might encounter as Ken has pointed out.* The plotter will, assuming you've entered a pair of waypoints to track between.* But if you're just using the GPS to hold a specific heading that you've plotted rather than track a course then it's not any more accurate or helpful*than the magnetic compass.

And yes, we most certainly did use every aid we had at hand negotiating the gap into Comox harbor for the first time.* We used both chart plotters zoomed in tight and the paper charts for the bigger picture.* And the depth sounder.**

But our primary instrument for finding and negotiating the gap*was neither electronic nor magnetic--- it was the*pair of binoculars we used*to monitor the ridiculously small range markers on*shore.* Had either or both of the chart plotters shown we were not accurately*tracking the gap, we would have ceased paying attention to them and continued concentrating on keeping the range markers lined up.

In our "order of dependability" list the electronics are at the bottom.* At the top are physical things like paper charts, binoculars, the lead line,*and navaids like buoys, range markers, etc.* Then come the magnetic compasses.* Then come the radar, GPS plotters, sounder, Loran, etc.

Of course in practice we use all these things (except the lead line
).* But we operate the boat as though the electronics are supplementary, and we are ready to dismiss their information if it's not confirmed by what we physically see around us and what we see on the chart, assuming daylight and decent visibility.

If it's foggy we boat in the fog and so place much more dependence on the electronics.* But even then we have the charts out and basic courses plotted so if the electricicals stop holding hands we can still head for a safe haven with an acceptable chance of getting there.* Eventually
Whooooooo........ You had me worried when you tossed in the lead line. Thanks for clearing it up that you don't really use it.

Oh, and welcome back. I hope you saw some great things. Stretching boundaries and going new places is both fun and nerveracking.

The only time I've had reason to use the lead line (complete with the star-shaped tallow bowl in the bottom of the weight for bringing up "samples") is when we're at a dock in an extreme minus tide. There have been a few occasions when we wanted to know if we were sitting in the mud, or to confirm that we had enough clearance under the boat to start the engines without sucking stuff into the through-hulls. Other than that the line and its weight stay in the storage bag. But it's there if we need it.......

And yes, we saw lots of great things. This first trip was more of a runaround than we'd like, but we wanted to see a fair number of places to get an idea of the ones we'd like to spend more time in on later trips. Since you (Ken) know the area, we visited Ganges, Silva Bay, Pender Harbor, Grace Harbor, Prideaux Haven, Refuge Cove, Squirrel Cove, Von Donop, Herriot Bay, Rebecca Spit, Comox, Nanaimo, Telegraph Harbor, Port Browning and Prevost Harbor (Stuart Island). We'd been to some of the southern locations before, but everything above Nanaimo (including Silva Bay) was new to us.* We cleared customs by phone in both directions so did not have to go anywhere out of the way to clear in.

-- Edited by Marin at 14:59, 2008-09-25
Well thats the difference between Boeing and Douglas.

Chevvy VS Caddy.

The old DC 10s had ADG , air drag generators (if memory serves that was the designation) , that simply fell out when actuated that provided unlimited time on the emergency buss.

Probably cost too much for Boeing , on their cheapos.

That's a great loop that Marin took. I think we're headed to the Broughtons next year which will be a bit of a stretch for us. Although I don't think I'll ever stop going to some of the places, Victoria, Ganges, Nanaimo, Vancouver, and Pender Harbor, on our way up, we're starting to feel the need to see some of the more northwestern spots away from Desolation. According to some of the guidebooks a few of the places are starting to disappear as the owners get older and can't find someone to take over. Some of the places are getting bought up to become private and I imagine some will become Yacht Club outstations with no reciprocal.

It's never too early to start dreaming about next year.
2bucks wrote: I think we're headed to the Broughtons next year which will be a bit of a stretch for us....According to some of the guidebooks a few of the places are starting to disappear as the owners get older and can't find someone to take over.
That whole area up there is really neat.* We've not taken the GB up there but we've trailered our 17' Arima up to Telegraph Cove and then gone halibut fishing and exploring out*in Blackfish Sound, Knight Inlet, etc.* And we've flown through the area a lot in the floatplane and stopped at Sullivan Bay and Minstrel Island (when it was still in operation).* And you're correct, some of the resorts and little marinas are starting to close down or are for sale.* Not that this makes the area any less desireable to visit, but it will mean that you may not be able to count on getting supplies as often, and you may have to anchor out more where you can now tie to a dock if you wish.* Fortunately, the distances between places are not that great once you're in the area, so places like Port McNeil, Echo Bay, and Sullivan Bay are always within easy reach.
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