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Old 30 November 2007, 16:22   #1
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Best way to record a compass deviation table

I have a Plastimo Offsore 75 compass.

It does not have compensation (although you can get an optional compensation box)

I have been comparing it against the GPS and there seems to be about 10-15 degrees between the two. I guess the compass is likely to need a deviation table to compensate for any interference.

I am thinking the best way to do this would be to wait util I am next out and use a handheld compass as a point of reference to compare. Is this going to get me a fairly accurate table?

Chris
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Old 30 November 2007, 17:26   #2
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Is your GPS set to show true north up (which, I believe, is the default for most units)? If so, that's probably the reason for the large deviation. A chart of your local waters should have the true-to-magnetic deviation, which will at least get you to the difference between the two units. At that point you can plot out your actual error.

One thing to note (not saying that you don't already know this, but some people don't), is that a gps will only show heading while it's moving. Once stationary, it knows where it is, but not where it's pointing.

So, to bounce the GPS against the compass, you need to steer a known heading (off the GPS) and see what the compass reads. To do it right, you do that for 8 or 16 different headings, and map out the error for each. Unless you are doing manual chartplotting via compass and timer, I'd say that's probably overkill. I say that becuase I can hardly hold a course seeing where I'm going. Having to do it blind and by compass and chart, well...

jky
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Old 30 November 2007, 17:59   #3
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Rather than compare against a hand held that it subject to its own errors compare aginst something more definate.

Drive up and down as many charted transits as you can (ideally 30-36 headings) and note the results, allow for variation and remaining error is your deviation.

Best of luck, it takes hrs
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Old 30 November 2007, 18:01   #4
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Quote:
Originally Posted by jyasaki View Post
Is your GPS set to show true north up (which, I believe, is the default for most units)? If so, that's probably the reason for the large deviation. A chart of your local waters should have the true-to-magnetic deviation, which will at least get you to the difference between the two units. At that point you can plot out your actual error.

jky
difference between mag and true is variation not deviation
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Old 30 November 2007, 18:11   #5
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Quote:
Originally Posted by jyasaki View Post
Is your GPS set to show true north up (which, I believe, is the default for most units)? If so, that's probably the reason for the large deviation. A chart of your local waters should have the true-to-magnetic deviation, which will at least get you to the difference between the two units. At that point you can plot out your actual error
Variation is only about 3deg W here (Solent/UK) currently so this is unlikely to be the
whole story.

Is the apparent offset constant on all boat headings?

Deviation caused by magnetic elements in the boat should normally alter as the heading goes round the compass (prob by +/- 15 deg in your case.

The technique for working out deviation is known as 'swinging the compass'. Quiet an interesting exercise if you have the patience. Prerequisite is a calm day at slack tide - using a transit, or bearings to a variety of known objects is the most reliable way. Most nav textbooks explain the theory - practice as ever is a bit different.
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Old 01 December 2007, 03:38   #6
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When me old man had his Moonraker 36, we used to swing the compass every season. And every season a minor correction was made and a deveation chart completed.

We used to use a Pelorus (sp?) tripod mounted on the back deck. Calm day with little or no wind and manipulation of the throttles to keep it in the right place. Used to take us a good hour every time.

Although we had GPS and the old Racal Decca system on board, there was nothing like having an accurate compass
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Old 01 December 2007, 06:31   #7
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There should have been full instructions supplied in the box with the compass, complete with a deviation card (unless it came with the boat of course). Can send you a copy if you PM me your details.
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Old 02 December 2007, 05:20   #8
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The compass in my boat is out by 45 degrees, I just ignore it now.
Its a decent compass (ritchie), there just too much stuff around it.

Nick.
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Old 02 December 2007, 05:47   #9
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The compass in my boat is out by 45 degrees, I just ignore it now.
Its a decent compass (ritchie), there just too much stuff around it.

Nick.
But what happens if your electronics fail, and you find yourself in low visibility????
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Old 02 December 2007, 12:08   #10
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so far I have been boating in familiar waters. But I have an idea which way the compass is wrong, so if I have to, I can work it out.
I usually have a handheld compass as well.
But I am not sure if you can correct the compass for 45 degrees.

Nick.
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