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Old 15-12-2017, 17:53   #1
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Deviation Table with GPS

Is it possible to make a deviation table running courses using a GPS? Have the GPS output True compass headings, adjust for local variation, the remainder will be your deviation. Any issues with doing it this way?
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Old 15-12-2017, 18:24   #2
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Re: Deviation Table with GPS

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Is it possible to make a deviation table running courses using a GPS? Have the GPS output True compass headings, adjust for local variation, the remainder will be your deviation. Any issues with doing it this way?
One problem comes immedietly to mind... GPS COG is not really a heading, it is a direction of motion. Any cross wind or current would set up a COG different than your heading. You would have no way of knowing or correcting for this.
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Old 15-12-2017, 18:41   #3
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Re: Deviation Table with GPS

What he said ^^^^^

And also..... I have just conducted a very unscientific test involving a magnetised screwdriver and my iphone.

The compass in the phone would appear to be a simple fluxgate.... approach with screwdriver and compass swings happily.. so... conclusion .... it will be experiencing the same ( or similar ) deviation as your steering compass.
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Old 15-12-2017, 20:52   #4
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Re: Deviation Table with GPS

Post's 2 and 3 are correct..but. You can if you know with certainty there is no current or wind...like inside an enclosed marina on a windless day. Then your heading and COG are effectively the same. It's close enough for government work.
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Old 16-12-2017, 04:20   #5
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Re: Deviation Table with GPS

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Originally Posted by billknny View Post
One problem comes immedietly to mind... GPS COG is not really a heading, it is a direction of motion. Any cross wind or current would set up a COG different than your heading. You would have no way of knowing or correcting for this.
Couldn't the same be said for traditional methods of swinging a compass? You are essentially measuring your COG. Any time you swing your compass you want it to be in a protected body of water presumably.
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Old 16-12-2017, 09:56   #6
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Re: Deviation Table with GPS

yep.... easy peezy....

calm day little ot no current.....

have done it a lot...

new compasses have it right in their instruction manual.

as always, you should know the equipment you are using.
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Old 16-12-2017, 11:48   #7
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Re: Deviation Table with GPS

If you have an auto pilot check the deviation it records. Mine is under 1deg so I don't bother with a compass card. Also the readout on the auto pilot is recording heading not COG.
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Old 16-12-2017, 13:47   #8
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Re: Deviation Table with GPS

On our MFD (Raymarine E120) you may read the heading in addition to COG. This of course gives true heading only if the equipment was properly calibrated initially at setup.
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Old 16-12-2017, 14:11   #9
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Re: Deviation Table with GPS

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Couldn't the same be said for traditional methods of swinging a compass? You are essentially measuring your COG. Any time you swing your compass you want it to be in a protected body of water presumably.
Not at all. With all traditional methods of compass swinging you point the bow of boat at something. Thats a heading, not a COG.

Here is a long list of ways to do it...
How to swing a compass - Practical Boat Owner

My personnel choice is to use a hand bearing compass. Easy, quick to check, and more than accurate enough for small boat work.
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Old 16-12-2017, 14:13   #10
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Re: Deviation Table with GPS

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On our MFD (Raymarine E120) you may read the heading in addition to COG. This of course gives true heading only if the equipment was properly calibrated initially at setup.
Of course that heading is only as accurate as your installed fluxgate compass... How do you get a deviation table for that???
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Old 16-12-2017, 14:24   #11
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Re: Deviation Table with GPS

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Originally Posted by billknny View Post
Of course that heading is only as accurate as your installed fluxgate compass... How do you get a deviation table for that???
Best practice is not to produce a "readable" deviation table for the fluxgate, but to use the table to modify the formula inside the device so that it will show an "adjusted" heading that has the table already built in. Look for "linearization" in the Raymarine manual.

The process in the manual accomplishes two objectives: a) measure the change in deviation around the compass rose (based on keeping constant rate of turn ie locked helm) and linearize relationship between actual heading and displayed heading based on those measurements; and b) correct the average difference between linearized reading and actual heading.
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Old 16-12-2017, 14:42   #12
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Re: Deviation Table with GPS

Amazing how this becomes complicated.

Go to a compass manufacturer website, download a manual and go to deviation or compensation.

The rest is SO simple if you pick a good area and conditions to do it in.

To compensate your compass, follow the below....then just modify it to do your deviation table by doing it in 10 to 20 degree or so pie cuts,

Straight frim Ritchie compass website....

1. While at sea, with the compass in its intended position, but not
fully secured, obtain the Loran/GPS bearing to a visual buoy or
landmark that is within 10 degrees of the North/South line.
2. Position your boat along that line and steer it directly at the
mark. Turn the port/starboard compensator until the compass
heading matches the Loran/GPS bearing.
3. Check the Southerly course by steering away from the mark,
to a bearing 180 degrees from step 2. If an alignment error
is detected, rotate the compass itself to correct for half of this
error. Then repeat steps 1 and 2 and recheck.
4. Repeat steps 1-3 for the East/West course, using the fore/aft
compensator. Any alignment error should have been eliminated
during the initial part of the procedure.
5. When finished, secure the compass in its final position.
*When using this method, always use the bearing
"To or From" on the Loran/GPS. Do not use the
heading information because it is inaccurate in
real time.
If you have trouble achieving compensation,
contact a reputable compass adjuster. Once
you've achieved proper compensation, this
would be the ideal time to check for deviation
and complete your final deviation table.
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Old 17-12-2017, 02:06   #13
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Re: Deviation Table with GPS

On my boat we are all heathens, rough necks and alcoholics. And we will tolerate no deviation from this lifestyle.
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Old 17-12-2017, 03:07   #14
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Re: Deviation Table with GPS

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Originally Posted by billknny View Post
Of course that heading is only as accurate as your installed fluxgate compass... How do you get a deviation table for that???
See svlamorocha post.
Basically - during calibration procesd the inner computer, using connected GPS, builds deviation table for the flux compass.
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Old 17-12-2017, 04:09   #15
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Re: Deviation Table with GPS

I dont beleive all, especially older APs/flux gates have this capability.
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