Deviation and Variation on Compass

[Sailaway Kid]

Where do you find the degree of deviation in the area where cruising? Is that something on the charts?

[rebel heart]

Variation is affected by the area you're cruising and is accountable on charts (it's what's referenced on a compass rose); it increases with latitude. Deviation is localized magnetic changes that cannot be accounted for on charts. Deviation is like having a cast iron pan too close to a compass.

Compass - TheKeel

[skipmac]

Deviation is an "error" in the compass itself. You might be able to adjust the compensating magnets to eliminate or at least reduce the deviation in your compass. If the deviation is very much you will make a deviation card that gives you a correction to add or subtract to the compass reading at different headings to give the correct magnetic course.

Variation is the difference between true north and magnetic north. This varies depending on your geographical location. The variation will be found on the chart on the compass rose.

[Sailaway Kid]

Ok, now that I have made a fool out of myself. I was actually confusing the two.

Variation on the compass rose. So you take the lines where they are drawn on the compass rose and use that to plot your course. How do you plot a course if you are not on one of those lines? Add to it/or subtract like in the formular CDMVT.

[Tempest245]

Deviation is specific to the vessel. The effects that certain metals, or electronics have on your compass. Deviation can change with the heading of the vessel. You can create a Deviation table for your vessel by steering through a variety of courses, or swinging the vessel.. There are good methods for doing this already in print. Try Chapmans.

COMPASS ERRORS | VARIATION | DEVIATION

Variation changes with Location, and the time..Annual changes. Your chart should give you the variation for the area in question.

Some people refer to Deviation when talking about the dipping of the compass at higher, lower latitudes......I believe you are talking about a deviation table for the ships compass.

[David M]

Originally Posted by Sailaway Kid

How do you plot a course if you are not on one of those lines?

You don't necessarily need a compass rose on your chart to correct for variation. You just need to know the variation for your area. If you are not on one of the lines of variation then do an estimate. If your estimate gets you within a degree or two then thats close enough.




There is declination for the Earths magnetic field which is the vertical element of the Earths magnetic field. This is relevant in that the more vertical the magnetic field, the less effective is a magnetic compass. Fortunately, we only need to worry about this at high latitudes in certain parts of the world as seen in the chart below.

[amarinesurveyor]

Going from true to compass you use the formula: TVMDC+W, which starting with True heading and applying the local Variation, gives you the Magnetic heading ; and applying the Deviation, gives you the Compass heading, (westerly variation is added, eastlerly variation is subtracted). I was taught an easy way to remember this is True Virgins make Dull Company add Whiskey.
Converting from compass to true is the opposite, and the easy way to remember this is Can Dead Men Vote Twice.
Kind of silly but it's something I've remembered for a long time.
Brian

[captain58sailin]

True Virgins Make Dull Companions + Whiskey to make them fall down.

[David M]

Or... TVMDC where "West is best and East is least".