Compass Heading - True vs Magnetic

[barnakiel]

Originally Posted by SVTatia

As long as you stay in the same hemisphere. If you cross the equator you need to adjust the compensating magnets.

Which one?

You are not talking about THE equator, are you?

What is worth having onboard is another compass that has been built for another inclination.

Otherwise one can go from UK to Aus and suddenly find their compass card is at 45 degs to the horizon ;-))))

b.

[svHyLyte]

Originally Posted by Randyonr3

Without using a GPS, or electronics, how do we create a compass card
for different areas of the world?

Randy--

Before leaving on a trip you can download the Isogonic Charts for the region in which you expect to be sailing. You then make up (or cut from an old chart as did I) a Compass Rose. Paste that to a cardboard backing. pumch a hole in the center and affix it to a slightly larger square piece of clear acetate with horizontal and transverse lines enscribed through the center. With this device, you align the acetate with the lat-lon lines on your chart (for the region your in), rotate the compass rose to the variation of the region (say 6º west) and, with a set of parallels, walk your desired course line to the Compass Rose to read off the magnetic heading you should be steering for the course you want to lay.

I assume you have a deviation table already made up for R3 and it's easy enough to interpolate deviation adjustments between major headings. (Our deviation table at 10º increments is on a laminated card hanging from a lanyard on our binnacle. With that, for example, I know if I want to steer 45º magnetic, I need to be looking at a heading of about 50º on my Compass as the 5º difference is my deviation while on a magnetic course of 45º.

I don't do much in "True" any longer as adjusting for magnetic variation is quite easy and the ship's deviation table remains valid all around the clock regardless of where we are.

The foregoing is easier in practice than it is to describe.

FWIW...

[mdvick]

I was flying from a Russian Icebreaker in Antarctica - The difference in some places betwen True and Magnetic was 120 degrees! I had a Garmin in the helicopter and it was able to show True or Magnetic north, thank God. The ship used True and so did I. Worked great...

[skipmac]

Originally Posted by Cheechako

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

Then to each card add the deviation corrections for your boat's compass and you're gold.

[nigel1]

If you have the cash to spare, a GPS compass will remove the problem of deviation. It will also be a back up for position as well

[Randyonr3]

Originally Posted by nigel1

If you have the cash to spare, a GPS compass will remove the problem of deviation. It will also be a back up for position as well

we carry more electronics than a "Radio Shack" aboard our boat.
the idea is to take a step back and opperate within basic parrameters.

[Adelie]

Originally Posted by Exile

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?

Only if you do it on a lake during a dead calm day with no waves running.

Any body of water with any current would be unsuitable. Trying for slack current doesn't work either, the net water movement is zero but there will be plenty of eddies.

Opps, next post said what I just said.

[Adelie]

Originally Posted by Exile

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.

A fluxgate compass is subject to the same errors as a standard compass but will provide a slightly different reading than the standard compass. Because it is located in a different part of the boat than the standard compass the influences causing deviation such as wiring, the engine block, etc., have a different effect. Locating them together would not work since their operation would mutually interfere with each other.

On the other had there is technique for creating a 'deviation card' for a fluxgate that is not possible with a standard compass. The boat is steered in a wide circle (very flat water, no currents) at a consistent rate of turn for one, two, three rotations, whatever the instructions say. The electronics of the compass look at how how consistent the rate of change of magnetic heading is, assumes that the rate of change in heading is due to deviation and creates an internal electronic 'card' that it automatically applies to readings until the boat is swung again and a new card is created. The accuracy of the 'card' is dependent on how consistently the boat was turned and the accuracy of the electronics.

It might be reasonable to create a deviation card for a standard compass by comparing the readings to a fluxgate compass that has already been swung. I would not be happy with this but as a stopgap I think it would be fine.

[Exile]

Originally Posted by Adelie

A fluxgate compass is subject to the same errors as a standard compass but will provide a slightly different reading than the standard compass. Because it is located in a different part of the boat than the standard compass the influences causing deviation such as wiring, the engine block, etc., have a different effect. Locating them together would not work since their operation would mutually interfere with each other.

On the other had there is technique for creating a 'deviation card' for a fluxgate that is not possible with a standard compass. The boat is steered in a wide circle (very flat water, no currents) at a consistent rate of turn for one, two, three rotations, whatever the instructions say. The electronics of the compass look at how how consistent the rate of change of magnetic heading is, assumes that the rate of change in heading is due to deviation and creates an internal electronic 'card' that it automatically applies to readings until the boat is swung again and a new card is created. The accuracy of the 'card' is dependent on how consistently the boat was turned and the accuracy of the electronics.

It might be reasonable to create a deviation card for a standard compass by comparing the readings to a fluxgate compass that has already been swung. I would not be happy with this but as a stopgap I think it would be fine.

I was thinking about my radar which gets NMEA data for its EBL display but that likely relies on the GPS or the fluxgate as well. What about a calm day at anchor, perhaps using a hand-bearing compass that is held clear of potential interference? Not perfect, but might allow deviation correction for at least one point on the binnacle compass.

[belizesailor]

Originally Posted by barnakiel

Which one?

You are not talking about THE equator, are you?

What is worth having onboard is another compass that has been built for another inclination.

Otherwise one can go from UK to Aus and suddenly find their compass card is at 45 degs to the horizon ;-))))

b.

Come on guys, lots of misinformation being posted on this thread:

The compensating magnets on a compass are used to adjust for "deviation" NOT "variation". If you don't know what these terms mean then please look them up before posting.

DEVIATION does NOT change based upon a vessels location on the surface of the earth. Crossing the equator is totally irrelevant. Also look up, "isogonic chart" -- you will notice that the plotted lines of magnetic "VARIATION" do NOT change relative to the equator.

[Adelie]

Originally Posted by belizesailor

Come on guys, lots of misinformation being posted on this thread:

The compensating magnets on a compass are used to adjust for "deviation" NOT "variation". If you don't know what these terms mean then please look them up before posting.

DEVIATION does NOT change based upon a vessels location on the surface of the earth. Crossing the equator is totally irrelevant. Also look up, "isogonic chart" -- you will notice that the plotted lines of magnetic "VARIATION" do NOT change relative to the equator.

While deviation and variation do not specifically change in relation to the equator DIP does. Dip can affect the reading of a standard compass, especially a hand bearing compass when dip starts to cause the compass card to drag on the housing. In a gimballed ship's compass dip can dignificantly tilt the card confusing users.

[belizesailor]

Originally Posted by Exile

... What about a calm day at anchor, perhaps using a hand-bearing compass that is held clear of potential interference? Not perfect, but might allow deviation correction for at least one point on the binnacle compass.

Yes, you can do this (at least approximately) with a hand bearing compass because you can assume no deviation for the hand bearing compass (safe assumption if you are well away from any large chunks of ferrous metal or electromagnetic interference, but still not as accurate as fixed physical ranges).

I usually do this as a rough check on the steering compass when preparing to run any boat I have not run before (or one which has had related work done).

Delivered a boat a few years ago following an extensive refit. They had installed an impressive new Ritchie Navigator compass -- sure was pretty....but had up to 10 degrees of deviation on some headings! Sure glad I checked.

[Adelie]

Oddly enough recently go an article related to this topic.

Separation and Redundancy

[Exile]

Originally Posted by belizesailor

Yes, you can do this (at least approximately) with a hand bearing compass because you can assume no deviation for the hand bearing compass (safe assumption if you are well away from any large chunks of ferrous metal or electromagnetic interference, but still not as accurate as fixed physical ranges).

I usually do this as a rough check on the steering compass when preparing to run any boat I have not run before (or one which has had related work done).

Delivered a boat a few years ago following an extensive refit. They had installed an impressive new Ritchie Navigator compass -- sure was pretty....but had up to 10 degrees of deviation on some headings! Sure glad I checked.

In my case, there was no deviation card to be found when I bought the boat. All was well until I had to remove the binnacle compass to service the steering chain and forgot to mark it off. I think it's still close but will try and remember to compare with the hand bearing compass while at anchor. I suppose if I only managed to create ONE additional issue by fixing another I'm doing well!

[belizesailor]

Originally Posted by Adelie

While deviation and variation do not specifically change in relation to the equator DIP does. Dip can affect the reading of a standard compass, especially a hand bearing compass when dip starts to cause the compass card to drag on the housing. In a gimballed ship's compass dip can dignificantly tilt the card confusing users.

True, but as I understand it, dip is more of an issue near the poles. Have you ever heard of it being a practical issue anywhere other than extreme N/S latitudes?