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Old 25-07-2012, 08:00   #1
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Compass Heading - True vs Magnetic

Without using a GPS, or electronics, how do we create a compass card
for different areas of the world?
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Old 25-07-2012, 08:10   #2
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Re: Compass Heading - true vs magnetic

Easy pease Do a google search...Find this web page with a calc and RUN IT!!!

Page has moved
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Old 25-07-2012, 09:56   #3
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Re: Compass Heading - true vs magnetic

go to the paper chart and swing the boat based on ranges
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Old 25-07-2012, 10:15   #4
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Re: Compass Heading - true vs magnetic

Your compass card should be consistent no matter where you are. A compass card shows the deviation for your compass (boat). Only create a new one if you put on or take off any new electronics, large metal objects etc. which affect deviation. What does change for different geographic areas is variation and that should be on any nautical chart. Most GPSs should also show the deviation for the area you are in (my garmin Map76 does when I set it to magnetic).
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Old 25-07-2012, 10:47   #5
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Re: Compass Heading - true vs magnetic

A deviation table "compass card" corrects for deviation and not variation.

It's an easy Google search to learn how to to create a deviation table. A GPS is not used to create one.

GPS's do not show deviation, they can't possibly know what local iron might be introducing an error into your magnetic compass. A GPS might have the option for showing variation in your area. All GPS's that I know of have the option of displaying true or magnetic north.
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Old 25-07-2012, 10:54   #6
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Re: Compass Heading - true vs magnetic

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Originally Posted by ptlones View Post
Your compass card should be consistent no matter where you are.).
PT
As long as you stay in the same hemisphere. If you cross the equator you need to adjust the compensating magnets.
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Old 25-07-2012, 11:12   #7
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Re: Compass Heading - true vs magnetic

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As long as you stay in the same hemisphere. If you cross the equator you need to adjust the compensating magnets.
If the compass is not properly corrected, the deviation will change with location, usually large changes in latitude.
In compass correction, there are two types of magnetism to contend with, permanent and induced.
On a large magentic compass in a binnacle, the permanant magetism is corrected for using hard iron magnets, placed fore and aft and athwartships. Induced magnetism is corrected for using a flinders bar and the spheres.
Most yachts wont have these, so you cant correct for it, and have to adjust the permanent magnets when changing location.
The change in deviation is usually brought about by vertical magnetic induction in "vertical soft iron". At the magnetic equater, the vertical component of the earths field is zero, at the magentic poles, its at its maximum. This vertical component is known as magentic dip. As your latitude increases, the more induced magnetism takes place in vertical iron.
The deviation most effected is on east/west headings.
A properly sized flinders bar will compensate for this, but as said before, you most likely aint got one.
For more bed time reading see
Maritime Safety Information
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Old 25-07-2012, 11:13   #8
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Re: Compass Heading - true vs magnetic

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A deviation table "compass card" corrects for deviation and not variation.

It's an easy Google search to learn how to to create a deviation table. A GPS is not used to create one.

GPS's do not show deviation, they can't possibly know what local iron might be introducing an error into your magnetic compass. A GPS might have the option for showing variation in your area. All GPS's that I know of have the option of displaying true or magnetic north.
Since the GPS will always presumably show the correct magnetic heading (taking into account variation), couldn't you compare this to your compass heading to create your compass card? Your compass, properly corrected for deviation, should match the GPS, no? Or is this more theoretical?
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Old 25-07-2012, 11:22   #9
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Re: Compass Heading - true vs magnetic

It would not work very precisely with GPS, because a GPS does not show heading. GPS shows course over ground (COG). These are two very different things, although at times your COG and HDG can coincidently be the same reading. But you would be guessing and assuming that they are always the same if you were to try to create a deviation table with your GPS. Your compass shows heading so you need to create your deviation table ranging your compass off a known range, which is information you would get from the chart.

The only time creating a deviation table might work with a GPS is on a lake with no current on a day with no wind....but that's not realistic for most of us.

There is also a second way to create a deviation table, using the Magneuto method, but you have to hire a professional with the Magneuto device to do that. I did that once but I thought it ended up being too expensive. In the end, the Coasties approved the deviation table. After that I ended up using the traditional and bullet proof method of swinging the boat off of two objects on the chart, creating a range.

None of this is qualitative, it is an exact quantitative science.
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Old 25-07-2012, 11:27   #10
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Re: Compass Heading - true vs magnetic

Guys, a suggestion, no offense intended, but some of you need to research the difference between "deviation" and "variation". The terms are misused in several posts on this thread.
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Old 25-07-2012, 11:29   #11
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Re: Compass Heading - true vs magnetic

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Originally Posted by Exile View Post
Since the GPS will always presumably show the correct magnetic heading (taking into account variation), couldn't you compare this to your compass heading to create your compass card? Your compass, properly corrected for deviation, should match the GPS, no? Or is this more theoretical?
GPS shows course over ground. If there is any wind or current the two courses are not the same. Also GPS course is an average of past positions not where you are currently pointing.

All that said I have made a deviation table on a windless day on a lake with no boat wakes using an autopilot to hold a steady course. I should probably check it against traditional means.

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Old 25-07-2012, 11:32   #12
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Re: Compass Heading - true vs magnetic

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No, because a GPS does not show heading. GPS shows course over ground (COG).
Just to add to the confusion: My chart plotter shows both heading and course, indicating each with a different colored arrow, but I believe its getting the heading info from the fluxgate compass.
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Old 25-07-2012, 11:54   #13
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Re: Compass Heading - true vs magnetic

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Just to add to the confusion: My chart plotter shows both heading and course, indicating each with a different colored arrow, but I believe its getting the heading info from the fluxgate compass.
Yes, ditto for me, i.e. my GPS displays heading as well as COG which I do understand are rarely the same. The boat needs to be moving for COG to be accurate, of course. Both numbers are set for magnetic, and the GPS will also display the amt. of variation in any particular location. While COG is from the satellite, it makes sense that the displayed heading will come from the fluxgate compass since all that basically amts. to is which direction the bow is pointed at any particular point in time. This should then match the heading display on the auto-pilot. If this is the case, then you'd really be comparing the fluxgate vs. the ship's compass in trying to determine deviation, which may or may not be a viable way to go about it. I really know nothing about the accuracy or much else about fluxgate compasses.
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Old 25-07-2012, 17:19   #14
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Re: Compass Heading - true vs magnetic

My mistake, your gps can give you variation not deviation. Variation is consistent throughout a geographic area and is the same for everyone in that area (it is the angular difference between true north and magnetic north). Deviation is caused by the "stuff" on your boat and is the angular difference between magnetic north and what your compass reads.
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Old 25-07-2012, 17:26   #15
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Re: Compass Heading - true vs magnetic

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Originally Posted by David M View Post
A deviation table "compass card" corrects for deviation and not variation.

It's an easy Google search to learn how to to create a deviation table. A GPS is not used to create one.

GPS's do not show deviation, they can't possibly know what local iron might be introducing an error into your magnetic compass. A GPS might have the option for showing variation in your area. All GPS's that I know of have the option of displaying true or magnetic north.

You could create a variation card for each of the areas you want to travel ahead of time I suppose....
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