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Old 14-03-2011, 09:01   #16
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Re: Deviation table from gps

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Originally Posted by estarzinger View Post
I think we all mostly agree about the cruising practicalities of autopilots and fluxgates and bouncy conditions and that magnetic compasses are not used very often or very accurately any more by most of the cruising fleet.

BUT the OP specifically asked about building a deviation table, in particular using the COG as a reference to do so. You are basically suggesting that whole exercise is a waste of time because no-one uses the binnacle compass anymore. Perhaps . . . But if the OP does not agree with that and still wants to do it, I do think its worth pointing out for the OP that #1 the typical inaccuracy of COG makes it a poor reference for a deviation table and that #2 there is a different but still easy way to use the gps to create a much more accurate reference.

A COG based deviation analysis will tell you if you have huge gross errors in your binnacle compass, but it will not (accurately) correct the smaller sort/size deviation errors normally corrected for. Its simply a matter of what the OP is trying to accomplish. Looking at his location and boat, he may not have an autopilopt or fluxgate and may actually be using his compass to navigate!
I believe the gist of the OP was why PAY someone to swing your compass when one could do it themselves with a GPS... In a sense, I was agreeing that it was a waste of money..... AND TIME!

That said, IF one really wanted to make up a card, I would use the static plotted positions of distant visual targets from a KNOWN (GPS derived) position, as opposed to underway bearings.
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Old 14-03-2011, 09:50   #17
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Re: Deviation table from gps

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I would use the static plotted positions of distant visual targets from a KNOWN (GPS derived) position, as opposed to underway bearings.
It's handy to be able to point the boat at the target because most binnacle compasses only have lubber lines for dead ahead and 45 degrees either side. It's usually hard to get a decent compass reading on a bearing off the lubber line.
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Old 14-03-2011, 10:07   #18
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Re: Deviation table from gps

True-that, but as the distance to the target increases the less the sighting error will be (percentage wise)... If I remember high school geometry, I think it is one of those inverse square thingums
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Old 14-03-2011, 10:23   #19
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Re: Deviation Table from GPS

While sailing upwind on a compass heading, one should compensate for leeway. The boat's centerline points slightly higher than the CMG, on most boats in the vicinity of 3 degrees, depending on conditions. Of course, with GPS, this isn't a problem.
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Old 14-03-2011, 11:00   #20
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Re: Deviation Table from GPS

Using GPS to establish deviation works well under the right conditions. Remember to set the GPS to read magnetic, not true.
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Old 14-03-2011, 16:30   #21
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Re: Deviation Table from GPS

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- - Your autopilot, the real helmsman on a cruising boat uses a "fluxgate" compass which you cannot access to build deviation card. And if you had one what would you do with it? Autopilot compass headings are normally all over the place and when compared to your old fashioned binnacle magnetic compass, rarely even close to each other.
Most fluxgate compasses can be electronically corrected for deviation. The reason the readings differ is that most of the time the fluxgate is more accurate that the yachts compass.

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Old 14-03-2011, 16:46   #22
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Re: Deviation Table from GPS

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fluxgate is more accurate that the yachts compass.
Not really but you can set a course with one based on a GPS and be very close. A fluxgate requires magnetics. Deviations of the boat still apply just like a compass. The ugly part is they are not the same as the compass given they are not located at the some point.

A gyro does not. A ring laser gyro is pretty nice, and I've used them but not on a boat. They tend to cost more than the boat. The mathematical integration of changes in velocity is pretty exacting. The devices are amazing in that they take a while so they can cancel out the rotation of the earth.
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Old 14-03-2011, 17:21   #23
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Re: Deviation table from gps

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If there’s a difference between experience & theory, one of them is wrong. Often, the error is a misinterpretation of experience, by one who doesn’t understand the theory.
Not really....having taught the theory and walked the walk as a cruiser, pro captain as well as a USCG helo pilot for quite a few years before LORAN/GPS...I clearly see the two application possibilities in navigation...just like in all professions....

Also as a hands on boating instructor for over 10 years now...I see plenty of "experienced" boaters who clearly know the theory that I would never follow through the fog....

To the OP...I say go for it...there's plenty of info on the internet and compass instruction booklets that tell you how to do it...you can make it complicated or keep it simple...your choice
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Old 14-03-2011, 17:24   #24
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Re: Deviation Table from GPS

Why not do it the proper way?

And if you navigate by GPS, why not let it be? (I mean - is your boat made of steel, etc.?).

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Old 14-03-2011, 17:26   #25
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Pblais , the reason I said they are more accurate is that ( a) they are decoded to a higher resolution but more importantly they are capable of easily having their deviation Offsets calulated and such offsets are often computed for 0. 5 degree changes sometimes even more.

This on practice makes the on board fluxgate much more accurate then the traditional , poorly maintained yachts steering compass.

I wasn't commenting on the basic technology. Ps I played with solid state optical laser ring gyros in my time too.
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Old 14-03-2011, 17:59   #26
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Re: Deviation Table from GPS

As I said, buried in the middle of my post about the uselessness of actually having a deviation table - is the fact that there are a few marine surveyors who will cite you for not having a current deviation table if you have a magnetic compass. So Yes, you do need to make one.
- - And for practical purposes you can make one up out of thin air or simply put a current date on an older one. Like the silly "bell" that you must carry on larger boats - who is really going to stand out on the deck ringing it in the fog? But we must have one - and it does make a nice call to dinner bell.
- - But to build a real deviation table you need to physically point the boat in the exact cardinal directions and then compare the magnetic compass reading to a variation corrected true heading. Not an easy task given you have a floating object that does not like to stay rigidly put very long.
- - However, in a still river you can plot a cardinal direction on the GPS or via a nautical map and then motor down that course for a period of time taking magnetic compass readings. With a little by gosh and by guess you can average them out to get a deviation value. Then repeat the process for the rest of the cardinal directions.
- - Again, not an easy task, but heck, it is a good way to kill a Saturday and you will probably learn a lot about how your boat reacts and holds a course either on autopilot or hand steered. If nothing else it will impress upon you the significant differences between what is on your nautical maps and what the magnetic compass displays. The more you learn about your boat and how it works the better is my personal philosophy.
- - So I would recommend doing it once, - the right way or as close as you can get to it - and then thereafter use a little literary license to make a new "current" deviation table.
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Old 14-03-2011, 18:27   #27
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Re: Deviation Table from GPS

So why is a GPS's COG not an accurate way of measuring a true coarse?
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Old 14-03-2011, 20:02   #28
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Re: Deviation Table from GPS

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So why is a GPS's COG not an accurate way of measuring a true coarse?
A GPS receiver determines COG by differencing two positions taken a short time apart (usually a few minutes). Let's say the averaging interval is 6 minutes. On a sailboat moving at 5 knots the position changes 0.5 nautical miles (926 meters) every 6 minutes. Given that GPS WAAS position accuracy is about 10 meters, the COG error based on two position measurements taken 6 minutes apart may be as much as 1.2 degrees (arcsin(20/926)). Typically, the error will be less, however.
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Old 14-03-2011, 20:15   #29
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Re: Deviation Table from GPS

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So why is a GPS's COG not an accurate way of measuring a true coarse?
You mean for compass adjusting?as in Post 19 ......the compass is
to steer straight but it doesn't indicate where you've been or where you are going to end up. It's just the first step.Imagine setting a course aiming for something crosswind or cross tide....If you keep the compass course,you will end up somewhere downwind,downtide.What will your gps cog have to do with the compass? Nothing.but you steered straight and that's the first requirement of old fashioned navigation!Which was a lot of DeadReckoning.But compasses are still important!

That's some reasons magnetic compasses are hard to adjust underway so pick the right day,the right place for, at best,a near-enough small craft check.You can't actually steer as straight with a gps as with a compass.2 different tools....Oh,I'm nit picking but there's also heeling error in a steel keeled sailboat,a heavily engined powervessel etc. ...
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Old 14-03-2011, 20:19   #30
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Re: Deviation Table from GPS

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So why is a GPS's COG not an accurate way of measuring a true coarse?
For one thing GPS's are not as accurate as you think they are. In the marine GPS world we have "2d" or "ordinary" GPS which has significant Fix location errors if you intend to use it for 1-2- or 3 degrees course tracking purposes. If you have a "Differential/WAAS" GPS you get a significant improvement in fix location errors but still the fixes are averaged internally and still not very good for "precise" course purposes.
- - You can see this in real life by using a GPS utility program such as "VisualGPS" and others. These programs have a plot survey function that shows on a screen graph how the individual "fixes" jump all over the place with time. You can also see this situation if you use the GPS as an anchor watch in the plot track mode on the map screen. You will over time see a crescent develop as your boat swings at anchor. But you will also see some "wild" track fix dots show up considerably away from your actual crescent plot. These "spikes" are errors generated by multiple causes inside the GPS.
- - So trying to rely on a GPS for "precision" tracking of a COG will most probably involve some "out of the box" track heading and course plots. Trying to resolve 1, 2 or 3 degree differences for a deviation card will indeed by a statistical challenge especially if you can only travel a very short distance. If you can hold your course for long enough over a significant distance then the errors should average out to a reasonably reliable plot. But here you are getting into some serious math.
-- And although it might be fun to play with trying to build a deviation card using a GPS, the result most probably will not be very accurate. And how would you know if it is accurate or not? Deviation changes with adding/subtracting or operating/not operating different systems on the boat and even a guest with a steel plate in their head or other part of their body (or underwire bra) might throw off the whole exercise.
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