Grid North vs True North? Calculating difference?

Discussion in 'General GPS Discussion' started by Dale Atkin, Jan 29, 2005.

  1. Dale Atkin

    Dale Atkin Guest

    I'm working on a Geocache. As part of the exercise, one has to calculate an
    offset from a point on a particular bearing (relative to true north). I know
    I could just use some built in function for this, but I'd really like to do
    it long hand (better workout for the brain). The only thing is, I don't know
    how to convert between grid north and true north, and google is strangely
    uncooperative... Anyone have any good links? Or helpful suggestions? I
    should I just cheat, and tell the GPS to do it for me?

    Dale Atkin, Jan 29, 2005
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  2. Dale Atkin

    web1000 Guest

    isnt it just grid north = magnetic declination + true north ...?
    sounds too simple

    web1000, Jan 29, 2005
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  3. The math will depend on what map projection the grid system is using. If
    you're using UTM, here's a link from Google:

    that has some information, but quick look doesn't show the right equation.
    You want to find the convergence angle. I know the equations are either in
    TM8358.2 or TM8358.1. Both are available at

    Melita Kennedy, Jan 29, 2005
  4. If it is a conformal projection (like TM is), a decent
    approximation is the difference in longitude times the sin
    of the latitude. That will be good to a couple of minutes
    of arc, much better than any magnetic compass (since we do
    not know declination to better than a degree or so).

    steve robertson, Jan 29, 2005
  5. too simple, indeed. No, grid north has nothing to do with magnetic

    True north points towards the geographic north pole.

    Grid north points along the northing grid lines of the projection used, and
    the difference to true north depends on the projection *and* on the present
    position. I have no fomula available, sorry.

    Magnetic north points towards the magnetic north pole. The difference to
    true north is small in many countries, e.g. in most parts of Europe, but it
    can be up to 30 degrees or more in other parts of the worls, e.g. America.
    And, if you are close to the magnetic north pole (Hudson Bay, I think) then
    the magnetic north is useless at all.

    Here is a link for calculation of magnetic declination at your location:
    Heinrich Pfeifer, Jan 29, 2005
  6. Dale Atkin

    KBH Guest

    I'm working on a Geocache. As part of the exercise, one has to calculate
    Well, you can calculate an ellipsoidal direction inverse between latitudes
    and longitudes, convert the latitudes and longitudes to (UTM I suppose)
    grid, calculate a grid direction inverse using basic trig on the rectangular
    coordinates, and subtract one direction from the other...

    Of course the GPS unit might give an accurate direction inverse between
    latitudes and longitudes and will convert the latitudes and longitudes to
    UTM grid...
    KBH, Jan 29, 2005
  7. Dale Atkin

    web1000 Guest

    hmm .. 18 Degrees here ... might as well toss that compass sucker ...
    web1000, Jan 29, 2005
  8. Dale Atkin

    Wayne R. Guest

    As I understand the function of a magnetic compass, it doesn't
    actually point to the Magentic North Pole, but aligns itself parallel
    to the local magnetic flux lines.

    So, an internal GPS declination table will let you know what the
    usable offset would be while using a GPS - but a plain vanilla compass
    could show something very different. The kicker: Both could be
    accurate, but the users would have to depend on their understanding of
    what they're looking at.

    Very cool:
    Wayne R., Jan 30, 2005
  9. Dale Atkin

    Annapress Guest

    As part of the exercise, one has to calculate an
    If you're in the USA, take a look at a USGS 7.5' topo map. It shows the
    differences between grid north, true north, and magnetic north. Magnetic north
    could have moved slightly in the years since the map was produced, but the
    relation between grid north and true north remains constant.

    Annapress, Jan 30, 2005
  10. Dale Atkin

    Graham W Guest

    Maybe I'm missing the bleeding obvious here.

    Grid north is parallel to the line of longitude up the centre of the UM

    True north is parallel to the line of longitude you're sitting on.

    If you're at a pole, the angle between these two lines will be equal to
    the difference in angular longitude. The angle will be equal to 1 x
    (long<zone centre> - long<point of interest>)

    If you're at the equator, they're going to be parallel. The angle will
    be equal to 0 x (long<zone centre> - long<point of interest>)

    There's some function for latitudes in between which will tell you what
    factor, call it Z, to apply to the same basic equation such that the
    angle will be equal to Z x (long<zone centre> - long<point of interest>).

    Without even drawing up complicated diagreams and working it all out,
    I'm pretty confident that Z = sin (latitude).
    Graham W, Jan 30, 2005
  11. The thing that is bing discussed is known as "grid convergence". See
    (Go down first column until the subject starts.)

    Most conversion programs like Corpscon (free and recommended) will output
    when converting.

    A more technical reference is

    A related topic is the "scale factor" which corrects for distances in UTM.
    David L. Wilson, Jan 30, 2005
  12. Dale Atkin

    Karl Pollak Guest

    x-no-archive: yes
    That should read "magnetic SOUTH" points towards the magnetic north pole.
    Karl Pollak, Feb 2, 2005
  13. Dale Atkin

    Gary S. Guest

    One caveat:

    The sketch showing their relation should not be relied on for angle.
    Instead, use the numbers printed.

    Also, the map gives the declination for Magnetic North vs True North,
    and also the rate of change in declination as of when the map was
    produced. If it has been more than 10 years, it would be best to use
    an up-to-date resource, perhaps one of the web sites mentioned by

    Also, before you get too deep into the difference between grid North
    and True North, you might figure a ballpark idea of the discrepancy
    over the distance you are doing. If it is rather small, you might not
    need to bother depending on the overall accuracy of what you doing.

    If you think it through, the difference will increase as you travel
    from the Equator to the Pole.

    Happy trails,
    Gary (net.yogi.bear)
    Gary S., Feb 7, 2005
  14. Dale Atkin

    sawers Guest

    .........a wee thing they taught us back in the pre gps days was 'mag to
    grid, get rid.......................grid to mag, add'........of course you
    have to know the amount to 'get rid' or 'add' when converting from
    magnetic bearing to grid bearing, subtract the declination....and reverse
    for magnetic to grid conversion............using prismatic compasses over
    distances typically less than ten miles in a leg, it was not worth bothering
    with for normal nav purposes.
    sawers, Feb 10, 2005
  15. Dale Atkin

    Gary S. Guest

    The little sayings like that are specific to which side of the zero
    declination line you happen to be on. Here in New England the
    declination ranges from 14 to 17 degrees one way, and in the Cascades
    it is about 15 degrees the other way. I have also seen advice from
    people living near the zero line that declination is not a major

    Many people hiking will do everything in magnetic bearings, even
    drawing parallel lines aligning with MN. As long as your whole group
    is consistent, this works out fine.

    Happy trails,
    Gary (net.yogi.bear)
    Gary S., Feb 10, 2005
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