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Magnetic compasses - deviation

Discussion in 'Back to Basics' started by ghrit, Sep 12, 2008.

  1. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    The care and feeding of magnetic compasses is touched upon here:

    Magnetic compasses have a failing. They don't point north except in very specific locations on earth. The magnetic pole is not the same as the geographic pole. The gp does not move, the mp does. (But you knew that since you are here.)

    Imagine a line connecting the mp and the gp, and run it around the world. If you happen to be on that line, your magnetic compass points true north, meaning it points at the gp. If you are off that line, there is a difference between true north and what the compass says. That difference is "deviation" and for you to know where true north is, you have to provide a correction to the magnetic reading. As a practical matter, the further south you are, the deviation assumes less importance, but along the higher latitudes, the difference is signifcant. You northern Canucks have to be very conscious of this, you could be north of the mp --

    All that said, there are other things that affect deviation. Large masses of iron ore, for only one.

    So far as I know, there is only one way to know the correction factor that you need to find true north. The Coast and Geodetic Survey can supply you with charts that show which way to apply the correction (this matters because you can be on either side of the line where there is no deviation) and the amount. It is critically important that you obtain a recent issue of the charts, because the mp moves at a variable rate and direction. All this presumes you need very high accuracy for some reason, like finding your cache buried 15 years ago before the area was logged over, the mines played out, and the landscape was modified. (Hopefully not where you dug the hole.)

    Another thing to remember when hiking in the Mesabe Range or other iron ore areas (like northern Michigan.) A mass of magnetic material will also cause the needle to dip in the direction of the mass. You may need to tilt your compass a bit to allow the needle to swing free. Level only counts as long as the needle doesn't drag. Watch for this when you take sightings with engineer's or lensatic compasses. (Okeefenokee dwellers can ignore this last.) Geodetic survey charts may or may not address dip, I've never had to deal with it.

    Put your GPS in the fridge (saves the batteries while you are out) and take a hike with your compass and chart, it can be fun.


    Edit to add, slightly off topic but related:

    Here is a link to a "how to" run down on map reading. With all the resources on the web today, I won't try to make map reading any easier than it is, and it truly is easy. This one happens to be British, but to a large extent the Brits follow the same conventions as the CGS charts.


    I found this as well. Again, British, but the same thinking applies. I guess, as preppers, all of us should have some basic skills at finding our way out of dodge and into unknown regions. For me, reliance on GPS seems off the radar, as I've become firmly convinced that satellite nav aides work both ways, they can (and sometimes do) transmit your location. Not interested in that at all. I want to be under the radar if SHTF, so knowing charts, maps and compasses is important to me. YMMV.

  2. Seacowboys

    Seacowboys Senior Member Founding Member

    Down add west...most charts show the variation near the compass rose or on the legend. You can't correct for magnetic deviation without knowing the variation where you are.
  3. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Nor can you know which bearing to follow without knowing the starting point (where you are.) One more very good reason to have a map or chart when the GPS batteries die. The variation is good only for the chart you are using, and it is approximate for the whole chart. That is good for small scale, but not nearly right for large scale if any sort of precision is needed. For all practical purposes (short of large area surveys) using the deviation shown in the legend or on the rose is plenty good enough.

  4. overbore

    overbore Monkey++

    As a retired airline pilot and former AF pilot, we had to learn some simple phrases:

    C an
    D ucks
    M ake
    V erticl
    T urns? or

    T rue
    V irgins
    M ake
    D ull
    C ompany followed by

    East is least and West is best meaning subtract
    Easterly variation and
    Add Westerly.

    Since variation is from the molten iron core of the earth and it is rotating at a different rate than the surface, we have about .5 increase yearly in Easterly variation. More data than that will make your head spin ---also.:lol::lol::lol:

  5. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Good scoop. It is a bit more than us ground dwellers need for basics, but it is always good to know more than you have to, and we are here to learn from each other. Thanks.
  6. gunbunny

    gunbunny Never Trust A Bunny

    If you are worried about magnetic deviation, all you need to get is a current sectional chart from your local FBO. It will have the deviation lines clearly marked, (and laid out as E or W deviation, just use the saying Overbore said) as they are needed for flying any distance if you want to arrive at your chosen destination.

    A sectional chart is what we puddle jumpers use for airborn navigation. The scale is a little too small for close ground navigation, but you can use it to plan routes, as they have terrain, rivers, large roads, towers, etc clearly marked for VFR navigation.

    You can get a sectional chart by visiting your local (small) airport that has services available (or what we call an FBO- Fixed Base Operator). They will generally be able to sell you a sectional chart for the (gasp) cheap price of $20!

    There is a lot of info on a sectional chart. Don't get visual overload when you first take a look! Everything is laid out, in a key, for easy reference on the map margins. Lattitude and longitude work the same, etc.

    Navigating by air with a compass isn't much different than on the ground with a compass, just 50 times faster.
  7. sniper-66

    sniper-66 Monkey+++ Moderator Emeritus Founding Member

    Or you can just tell me the area you want and when they go out of date, I will mail it to you for the price of the postage.
  8. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member


    Just in case you missed it, you could be spoofed right into a less than desirable location if you depend on your GPS without checking what it's telling you. One more reason to go with the tried and true rather than gadgetry.
  9. overbore

    overbore Monkey++

    Minor/ Major "Korrection" Sir;;

    Draw a line on a chart (from where you are to where you want to be? ghrit) - that line is (your intended direction/course?)
    Apply local variation usually in red on navigation charts
    correct for iron in your rifle barrel that is too near- that is the deviation correction.

    After applying all three, that is the course you walk, stalk, drive or fly; period. OVERBORE

    Yep, that'll do it for you in the woods. But as above, set your magnetic metals down and walk away to get accurate N indications. Your rifle, steel core ammo, pocket knife and side arm will affect the indication of a magnetic compass if you aren't far enough away to negate the effects. Gunbunny can set a much straighter course in the air than us woods rats can, but Overbore's "Korrection" is spot on. Very often, knowing where you are is more difficult than figuring out what direction to take. That is where charts come in handy, land marks are easy to identify if you can see them. Storms (or anything else) that restrict visibility are best handled by hunkering down and riding it out. -

    Overbore -
    My apologies for editing rather than responding. Mental lapse.
  10. zulu54

    zulu54 Monkey++

  11. overbore

    overbore Monkey++

    Back when- about two or three years ago when I was a student pilot--I forget how long ago that was, 1954, HA!, we were taught this silly little mind jogger; 'CAN DUCKS MAKE VERTICAL TURNS Which means, Compass course on the map, +- deviation, +- variation = True or what you fly or what you walk. KISS- works every time Laus deo overbore
  12. bnmb

    bnmb On Hiatus Banned

    If the standard deviations are still valid...heard some reports that something was going on with compasses and deviations...certainly should be checked.
  13. Georgia_Boy

    Georgia_Boy Monkey+++

    A minor point, re: maps & charts.
    Maps are generally land based. ie, you follow roads or go any where you want.
    A chart on the other hand identifies where you do not want to go, ie shallow water, hazards, reefs, etc.
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