Most people were never taught that a compass doesn’t point to the north pole.   What?? 

It points at the Earth’s north magnetic pole.  Guess what?  The Earth’s north rotational pole and the Earth’s north magnetic pole are NOT in the same place!

In fact, the Earth’s magnetic pole is constantly moving around erratically because of changes inside the molten iron core.  Since this point is moving around on the Earth’s surface constantly, you need to stay updated on where it is.  Depending on your location, the angle at which you measure the north magnetic pole will vary.

The angle between true north and magnetic north is called Magnetic Declination.  To compound matters, maps are generally oriented north-south on a third line called Grid North.  In other words, the vertical edge of a map is parallel to grid north, but not parallel to true north or magnetic north.  That means if you orient the G.I. Lensatic Compass’ flat straightedge along the vertical edge of the map while it is pointing due magnetic north, you will have to “cock” the map a few degrees left or right to put the map oriented to Grid North.  Grid North is usually, but not always, somewhere in between true north and magnetic north.

If you didn’t figure in for this small angular deviation, and you are traveling a good distance, you are going to end up in the wrong place!  It’s just like a small deviation on the angle of the barrel of a rifle can cause a bullet to land a mile away from where it should.

To check the Magnetic Declination for your neck of the woods, go to NOAA’s Geophysical Data Center (NGDC) and type in your zip code: http://www.ngdc.noaa.gov/geomagmodels/struts/calcDeclination

AZIMUTH

Now would also be a good time to get you familiarized with the term Azimuth.  The most common military method of expressing a direction is by using azimuths.  An azimuth is defined as a horizontal angle, measured in a CLOCKWISE manner from a north base line.  When the azimuth between two points on a map is desired, the points are joined by a straight line and a protractor is used to measure the angle between grid north and the drawn line.

For example, if you had basecamp Alpha and basecamp Bravo and you wanted to know the azimuth to establish the correct angle to point directional antennas for communication.  One camp would use the azimuth and the other the back-azimuth to get them pointed at each other. In the diagram below, basecamp Alpha is used at the apex of the azimuth measurements so it would set its antenna direction at an azimuth of 112 degrees and tell basecamp Bravo to set theirs at a back-azimuth of 292 degrees.  Both azimuths are based off the north baseline, which in this case could be a magnetic north baseline since we are talking about using compasses.

The measured angle is the grid azimuth of the drawn line.  When using an azimuth, the point from which the azimuth originates is imagined to be the center of the azimuth circle.  Azimuths take their name from the base line from which they have been measured; true azimuths from true north, magnetic azimuths from magnetic north, and grid azimuths from grid north.

Therefore, any one given direction can be expressed in three different ways:  a grid azimuth if measured on a military map, a magnetic azimuth if measured by a compass, or a true azimuth if measured from a meridian of longitude.

BACK-AZIMUTH

A back azimuth is the reverse direction of an azimuth.  It is comparable to doing an “about face.”  To obtain a back azimuth from an azimuth, add 180 degrees (3200 mils) if the azimuth is 180 degrees or less, or subtract 180 degrees (3200 mils) if the azimuth is 180 degrees or more.  The back azimuth of 180 degrees may be stated as either 0 degrees or 360 degrees.

DECLINATION DIAGRAM

A declination diagram is placed on most large-scale maps to enable the user to orient the map properly.  The diagram shows the interrelationship of magnetic north, grid north, and true north.  On medium-scale maps declination information is shown by a note in the map margin.

Declination is the angular difference between true north and either magnetic or grid north.  There are two declinations, a magnetic declination and a grid declination.

The declination diagram contains three prongs representing magnetic north, grid north, and true north. 

G-M ANGLE

The G-M Angle is an arc, indicated by a dashed line, that connects the grid north prong with the magnetic north prong of a declination diagram.  The value of this arc – the grid-magnetic angle (G-M ANGLE) – states the size of the angle between grid north and magnetic north and the year it was prepared.  This value is expressed to the nearest 1/2 degree, with mil equivalents shown to the nearest 10 mils.  This G-M Angle is what you need to add or subtract when converting back and forth from a grid angle to a magnetic angle.  In other words, you have to add or subtract the G-M Angle when going from a map to a compass or compass reading to a map.  A rule to remember when solving such problems is this:  no matter where the azimuth line points, the angle to it is always measured CLOCKWISE from the reference direction (base line).  With this in mind, the problem can be solved in three easy steps:

(1) Examine the declination diagram on the map (use the numerics, the graphic illustration may be exaggerated for clarity so don’t measure off of that with a protractor).

(2) From the apex of the declination diagram, draw a line in the general direction of the azimuth you are contemplating (it doesn’t need to be exact, this is just a visualization technique to help you understand the relationships between the different angles, we’re not shooting for accuracy here, only understanding how they relate to one one another).

(3) Now you have a diagram to use to figure out which angles you need to add or subtract.  For instance, if you have a compass measurement of 50 degrees, you know that is a magnetic azimuth and is therefore measured off the magnetic north baseline in a clockwise fashion.  Knowing that is 50 degrees because you measured it, and knowing that the G-M angle is 10 degrees because the declination diagram told you so, subtracting 10 degrees from the 50 degrees will give you the azimuth relative to the Grid North baseline, or 40 degrees.  Since the prongs on the declination diagram can be in different positions at different planetary locations, you should always draw a “cheat sheet” sketch like the diagram above to visualize the relationships.

Regardless of the order the prongs are in, always remember to measure azimuth CLOCKWISE.

And always know your G-M ANGLE for anywhere you are operating!!!

Don’t forget to convert your magnetic compass reading to a grid (map) reading and visa-versa or you will hose the whole operation!!

This lesson was adapted from Army Field Manual FM 21-26: Map Reading. You can get this manual and a shitload more from this site all bundled together into one secure download for only $6.99. Click the banner below for more information:

Bookmark It

Filed under: Tutorials by Patriot75