Take the 2-minute tour ×
Geographic Information Systems Stack Exchange is a question and answer site for cartographers, geographers and GIS professionals. It's 100% free, no registration required.

I have a very basic question that will seem very elementary to most who observe this website. The reason I ask is because I have been told to difference answers and I'm now a little confused. So, here I go.

Assume that Magnetic North is +14.62e & True North is +13.113e and Grid North is -1.49w. If I wanted to correct gyro surveys (in True North) and then converted them to Grid North, what values would be needed to do so correctly. I'been told to add 1.49 deg to the True North surveys which would convert them to Grid North.

But, I've also been told the opposite and subtract the 1.49 deg from the True North. I've also been led to believe that since you are going from True to Grid (in a counterclock wise direction) you would therefore add the -1.49 deg. Perhaps someone can better explain.

share|improve this question

closed as unclear what you're asking by Devdatta Tengshe, PolyGeo, Fezter, iant Jan 6 at 12:18

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question.If this question can be reworded to fit the rules in the help center, please edit the question.

isn't true north at 0 - by definition? –  iant May 24 '13 at 10:41
yes, I believe True North can be considered to be about the 0 degree reference point. I should have said that the declination is 13.113 deg. Hope this explains better. –  Mr. Cox May 24 '13 at 14:25
@Mr.Cox -- As you say, the answer is simple, and i can provide it below. However, there appears to be a problem with the count of the angles you give or your exact use of terminology. Dang, it's easier to explain the problem via the answer than doing it here! But, according to The Rules, i'm supposed to try... –  martin f Jan 6 at 6:29
North is just a reference direction. By itself, it doesn't have an angular value. (You and @iant have already hit on that concept.) The difference between two Norths does have an angular value -- it is the correction you refer to. A bearing (or azimuth) is an angle from (some) North to some direction of interest. The confusion for us may be that you're mixing corrections and bearings. Even if you've moved on with your life, kindly re-read and then edit the question. –  martin f Jan 6 at 6:41
show 1 more comment

1 Answer 1

If you are, as you say, "going from True to Grid (in a counterclock wise direction)" and if you are converting True bearings to Grid bearings then you must add the value 1.49°. Put another way, you subtract the negative value 1.49°. Clear? Of course not! So let's have a look at a declination diagram:

    GN      |     MN
      \     |     /
       \    |    /
        \ MC|MD /
         \  |  /
          \ | /
           \|/  MB
            o ----------- OH

GN: grid north -- parallel to grid's Y-axis
TN: true north -- direction of local meridian (aka geographic or geodetic north)
MN: magnetic north -- direction of local magnetic force field

OH: our heading -- direction we're going, or looking, towards (aka bearing, azimuth or course)
    Note: this could be anywhere, of course (excuse the pun), 
        it just happens to point east in the diagram

MD: magnetic declination -- angle between true north & magnetic north
    Note: MN may be West or East of TN (ie, MD may be -ve or +ve)

MC: meridian convergence -- angle between true north & grid north
    Note: GN may be East or West of TN (ie, MC may be +ve or -ve)

MB: magnetic bearing -- angle between magnetic north & our heading

Not shown

TB: true bearing -- angle between true north & our heading
GB: grid bearing -- angle between grid north & our heading

Some simple relationships

TB = MB + MD
GB = MB - MC
GB = TB - MC

Returning to your situation, where the meridian convergence is negative

GB = TB - - 1.49° = TB + 1.49°

thus there was some truth in each of your possible answers.

The moral of the story is: draw or examine the declination diagram (for the situation at hand) and things will be clear. But remember, the relative directions of True, Grid and Magnetic Norths will depend entirely on where on Earth you are and where on the map projection you are.

share|improve this answer
add comment

Not the answer you're looking for? Browse other questions tagged or ask your own question.