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Old 13-03-2011, 18:28   #1
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Deviation Table from GPS

Is there anything stopping me from making a deviation table just from the true heading on my GPS?

Here are my thoughts. Go out on a calm day make up a T,V,M,D,C table, swing the boat through 360 degrees making note of heading every 20 degrees. I would simply plug in the true coarse from the GPS, the Variation from a paper chart, and the compass coarse, from that I would calculate the deviation.

Is there a problem with my thinking? Why would anyone pay to have their compass swung?
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Old 13-03-2011, 19:07   #2
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Re: Deviation table from gps

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Originally Posted by unbusted67 View Post
Is there anything stopping me from making a deviation table just from the true heading on my GPS?

Here are my thoughts. Go out on a calm day make up a T,V,M,D,C table, swing the boat through 360 degrees making note of heading every 20 degrees. I would simply plug in the true coarse from the GPS, the Variation from a paper chart, and the compass coarse, from that I would calculate the deviation.

Is there a problem with my thinking? Why would anyone pay to have their compass swung?
I made a deviation table using my GPS, but in addition to a calm day I did it on a lake with no current. Current would be very bad. I also used my autopilot to get a much straighter course.

John
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Old 13-03-2011, 19:17   #3
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Re: Deviation table from gps

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Is there anything stopping me from making a deviation table just from the true heading on my GPS?
Obviously you have to do it with zero current and zero leeway.
GPS COG is typically accurate to ±3°, that's marginal for constructing a deviation table.

There are some high accuracy units, but they are unusual on a cruising boat and rather more expensive.
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Old 13-03-2011, 19:34   #4
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Re: Deviation table from gps

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Obviously you have to do it with zero current and zero leeway.
GPS COG is typically accurate to ±3°, that's marginal for constructing a deviation table.

There are some high accuracy units, but they are unusual on a cruising boat and rather more expensive.

not that important considering many compasses are only maked in 5 degree increments
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Old 13-03-2011, 20:01   #5
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Re: Deviation table from gps

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not that important considering many compasses are only maked in 5 degree increments

Perhaps. Although, most professionals aim for .5 degree accuracy in a deviation table, and most decent helmsmen can steer +- 2 degrees on a compass marked at 5 degree increments.

There is a relatively easy way to use GPS that eliminates this COG error.
(1) Plot your gps location on a chart, and measure the bearing from that point to a distant point (a tall antenna is ideal). Historically, the sun was used.
(2) Construct a 'sight line' (two sticks aligned on the vessel with the centerline or a gunsite or such) aligned down the heading of the vessel
(3) Use the 'Sight line' to point the vessel directly at the distant point.
(4) Compare the charted bearing to your compass heading. This will be more accurate than the COG
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Old 14-03-2011, 05:04   #6
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Re: Deviation table from gps

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Originally Posted by estarzinger View Post
Perhaps. Although, most professionals aim for .5 degree accuracy in a deviation table, and most decent helmsmen can steer +- 2 degrees on a compass marked at 5 degree increments.

There is a relatively easy way to use GPS that eliminates this COG error.
(1) Plot your gps location on a chart, and measure the bearing from that point to a distant point (a tall antenna is ideal). Historically, the sun was used.
(2) Construct a 'sight line' (two sticks aligned on the vessel with the centerline or a gunsite or such) aligned down the heading of the vessel
(3) Use the 'Sight line' to point the vessel directly at the distant point.
(4) Compare the charted bearing to your compass heading. This will be more accurate than the COG
good tip...but still overkill for the average boater in my "pro" opinion.
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Old 14-03-2011, 06:34   #7
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Re: Deviation table from gps

Overkill? Why settle for inaccuracy when it's so easy to do better?
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Old 14-03-2011, 06:50   #8
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Re: Deviation table from gps

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Overkill? Why settle for inaccuracy when it's so easy to do better?
The why not go all the way to charted ranges????...not that much more work for even a tiny bit more accuracy. My point is that most small boaters NEVER use a compass to steer a long distance course or even COULD to 2.5 degree accuracy. The simple function of small boats and normal motion on the water...it all turns out to be an average. Helmsmen using small boat compasses are looking for a gross streer to get back to land or the other side of a lake..etc...

If the OP had a large (20 meter or better) vessel doing lot's of offshore work...then OK...2.5 degree accuracy becomes a big deal.

When the final product is incapable of close tolerances...then the process doesn't need them either. In this case...the small inaccuracies aren't additive...they become averaged in my experience....

No matter what theoretical or paper argument is made...I know reality. Having watched steersmen on large ships and small boats for the last 45 years of going to sea/small boating...there's no comparison...

Be precise on the paperwork if you want...when the hands hit the wheel/tiller...most of that accuracy goes out the window. Successful small boat DR has a lot more to do with experience than hard numbers from tables and charting.
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Old 14-03-2011, 07:04   #9
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Re: Deviation table from gps

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...
My point is that most small boaters NEVER use a compass to steer a long distance course or even COULD to 2.5 degree accuracy

...
OK. But a 2.5 degree error in steering + a 2.5 degree error in the compass equals a possible 5 degree error.

Why add additional error into the equation if it's easy to take it out?

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Old 14-03-2011, 07:08   #10
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Re: Deviation table from gps

I agree with you that it's mostly an issue for long-distance offshore passage making, and not something most need to be concerned about, but it can come into play for the coastal cruiser, too.

I recall a trip down the Chesapeake Bay back in the 1960s (no GPS, no LORAN). Visibility was about a quarter mile pretty much the whole 50 nm. We were hand steering using the compass, and paying pretty close attention to keeping on course. When the buoy we were shooting for came into view, just a little off the port bow, I was pretty glad we'd gone through the routine of swinging the compass. We'd have been 2-1/2 miles off had we not made the correction, as there was a 3 degree deviation on the heading we were traveling.
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Old 14-03-2011, 07:43   #11
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Re: Deviation table from gps

IMHO, "back in the day" it WAS more important to swing the compass on a small vessel. I used to run offshore in a 20' Mako with only a compass and a depth finder. I'd fish all day, and cover a lot of ground while out of land sight. 100% dead reckoning, and upon my first glimpse of land was within 1nm of my inlet latitude.

Today? I don't so much bother for the reasons stated above... between set, drift, and lazy helmsmanship, I can't steer better than my compass error anyhow. On the astronomically small chance that all the GPS sats fell from the sky tomorrow, or my triple redundant receivers all failed, and all three compasses fell overboard, I bet I could still find land just by celestial orientation.

BUT, keep in mind in my part of the world, 99.9% of the destinations never require you to be more than 100nm from SOME landmass!
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Old 14-03-2011, 08:08   #12
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Re: Deviation table from gps

I'm not arguing that the compass shouldn't be swung as accurate as possible...again to a reasonable degree...but worrying about a deviation table is another story...every time you shift the tools in your tool box the table could be off as much as just using the GPS and a decent sight/point to steer to make the table to begin with....again most mariners (no offense to the OP) couldn't zero out the drift and keep an accurate enough steer to even run a 2 degree error range.

Most people can't estimate the tide well enough to argue a 2 degree steer when dead reconing.

Like I said..experience versus "theoretical" is a huge difference and thinking the theoretical is actually going to save you...well let's just say I've hoisted out of the water or searched for enough boaters in my day to know "practical" navigation.
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Old 14-03-2011, 08:11   #13
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Re: Deviation table from gps

This is another thread that smacks of a "Tradition" versus "Practical" real life debate. There is nothing wrong with "Traditional" ways and they can be fun but in real long distance full time cruising they are not practical or even practiced.
- - There are many good ways to "swing" your magnetic compass utilizing a GPS system and even some older GPS manuals used to outline how to do it. Many of the ways have been outlined by posters above. And of note, some surveyors get bent out of shape if you cannot produce a current deviation table for your magnetic compass.
- - In real life cruising practice the binnacle magnetic compass is - as some posters have said - used only as a very raw guide to whether you are heading in one cardinal direction or another. Out in the oceans and seas the boat heading varies dramatically with waves, swells, and wind variations usually in the 5 to 10 degree range and sometimes more.
- - 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. In real cruising life we set the autopilot to a particular heading and thereafter adjust things by "one click; two clicks (degrees)" port or starboard to hold a course line.
- - Like the required "bell", the magnetic binnacle compass looks very traditional but is of very limited use - unless in some rare occurrences - all the modern electronic/automatic navigation system fail or shut down. Although quite rare, I have had that happen and then that old magnetic compass is worth its weight in gold.
- - But trying to hold a magnetic compass heading tight enough that a couple of degrees or less of deviation makes any difference is downright silly. And in big seas and swells it is also dangerous. There is a rhythm of port and starboard heading changes as you climb and descend waves/swells that goes a long way to preventing broaching and/or burying the bow. Modern autopilots have this feature built into the program and is known as "sea state" compensation.
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Old 14-03-2011, 08:46   #14
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Re: Deviation table from gps

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!
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Old 14-03-2011, 08:52   #15
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Re: Deviation table from gps

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.
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