A compass is a direction finding instrument that orients the user to magnetic north by means of a magnetized needle mounted on a pivot within a liquid filled capsule. The liquid dampens the movement of the needle which will always point to magnetic north when the compass is held in a steady, level position and away from metal objects which may cause deflection. The parts of a military compass are shown in Figure 1.

HOW TO DETERMINE A MAGNETIC AZIMUTH USING A COMPASS

Another way to determine a magnetic azimuth is by the Compass-to- cheek method. Army Field Manual 3-25.26 gives the directions as follows:

If you determine a magnetic azimuth as described above and follow it, you will be heading, generally, in the right direction. If you walk far enough west, you will eventually come to the Pacific, or if you walk far enough east, you will eventually come to the Atlantic. But you cannot hope to accurately reach any particular point in your travel until you have set your compass for the magnetic declination of the area.

Figure 3.

Figure 4.

The rule for correcting your magnetic azimuth for declination is to ADD the declination if it is to the WEST, and to SUBTRACT the declination if it is to the EAST.

The mnemonic is "West is best." Anything that is added to is best. And, "East is least." Anything subtracted from becomes least.

Some examples of how to calculate the declination are given below.

Figure 5.

72 degrees -- Desired azimuth
+12 degrees -- Add Declination to desired azimuth
84 degrees -- The azimuth to follow

Figure 6.

90 degrees -- Desired azimuth
+20 degrees -- Add Declination to desired azimuth
110 degrees -- The azimuth to follow

Figure 7.

90 degrees -- Desired azimuth
-10 degrees -- Subtract Declination from desired azimuth
80 degrees -- The azimuth to follow

Consider a straight rail road track that disappears over the horizon, and two people standing side by side, each straddling one rail of the track. If both sight on the point where the track disappears at the horizon, each will read the same azimuth. However, if each sights on a point only 50 meters ahead on the rail over which he is standing, the azimuths will be different. The reason for the error is that there are an infinite number of parallel lines that gradually merge as the distance from the point of observation increases. So, for greater accuracy in maintaining your azimuth, site on landmarks at moderate distances.

The second thing to do is to look for a prominent landmark at your present position that you can use to check your back azimuth. When you have completed the first leg of your walk, you can sight on the landmark at your starting point to see if you have deviated from your azimuth. If you have, you can make a correction, sight a new landmark ahead of you, and walk toward it, repeating the process.

On the other hand, if the azimuth you are following is MORE THAN 180 degrees, you SUBTRACT 180 degrees from it to compute your back azimuth. For example, if your azimuth is 200 degrees, your back azimuth would be 200 – 180 = 20 degrees.

READING MAPS AND USING PROTRACTORS

Magnetic north is the north pointed to by the magnetic needle of a compass. True north is where the meridians meet at the earth's axes. This is what we call the north pole. This geographic north is not the same point as the magnetic north (See Figure 3.) The difference between magnetic north and true north is called the magnetic declination and is expressed as a degree angle.

The third north is the grid north of a military map. Gird lines are straight lines that intersect at right angles and form squares. The vertical grid lines of the map point toward grid north. The map grid is superimposed on the Transverse Mercator Projection. See Figure 9. The projection is a representation of the three dimensional curved surface of the globe as a two dimensional plane. The parallels and meridians of the Transverse Mercator Projection are curved lines that create quadrangles of different shapes and sizes, and when the military grid system of squares is superimposed on the quadrangles, the straight vertical lines of the grid do not run parallel to the curved meridians of the projection; consequently, the vertical grid lines point to a grid north which is not congruent with either magnetic north or the true north (Dept. of Army 4-3).

Mercator Projection

Transverse Mercator Projection

The bottom and right sides of the triangular cutout each represent 1000 meters. The one hundred meter distances are indicated by a scale point and a numeral. In addition, each hundred meters is divided in half with the 50 meter points designated with a scale point only. See Figure 10.

Using a protractor to locate a point within a particular grid square is a simple process. Referring to Figure 11 as an example, you would place the triangular cutout on the map with the base of the triangle on the horizontal grid line that forms the base of the grid square containing the "X" (line 00). The right side of the triangular cutout should be aligned with the vertical grid line that is to the left of the "X"(line 78).

With the protractor in position, slide it to the right, being careful to keep the base of the triangular cutout aligned with the horizontal grid line. Continue to slide the protractor to the right until the vertical side of the triangular cutout is lying on top of the "X." Now, look at the bottom of the triangle and read the position of the point on the meter scale. In Figure 11, the protractor would indicate that the point is close to 500 meters from the vertical gird line number 78. Therefore, the east-west position of the point within the grid box would be designated 7853.

While maintaining the position of the protractor on the map, read the meter scale on the right side of the triangular cutout to find the vertical coordinate for the point "X." In Figure 11, the point is about 300 meters above the base line of the triangle, and so its north- south location would be designated 0032.

Finally, the figures designating the horizontal and vertical coordinates for "X" are combined, giving the east-west coordinate first (the run) and then the north-south coordinate (the rise). Thus, the location of the point in the grid square to within 10 meters is 78530032.

The next step is to place the small hole at the center of the protractor over the tick mark that represents your known location and adjust the protractor so that its edges are parallel with the horizontal and vertical grid lines on the map. You can then read the grid azimuth of the line connecting the two points where it intersects the degree scale on the inner edge of the protractor. When you reach the desired destination, you can then plot a new grid azimuth to another location by repeating the same steps.

The difference between the grid north of a military map and magnetic north is called the Grid-Magnetic Angle (G-M Angle). It is another expression of declination but in this case the reference point is not true north, but the "map north" represented by the vertical grid lines of the map. This angle is used to convert a grid azimuth to a magnetic azimuth and vise versa. See Figure 12.

1. Identify your present position on a map using the grid coordinates.
   • Read the numbers from left to right along the horizontal line at the bottom of the map.

   • Read the numbers from the bottom to the top along the left edge of the map.

   • These two numbers are the coordinates of the grid square where you are located. They designate the point at which the horizontal and vertical grid lines intersect at the lower-left corner of that square.

   • To more precisely locate your position within the square, use your protractor as explained above.

2. Locate the point of destination using the same steps.

3. Draw a line intersecting the two points and extending beyond them.

4. Orient the map by placing the site wire of the compass on one of the vertical grid lines. While holding the compass in position on the grid line, rotate both the map and compass until the north pointing arrow is aligned with the black index line on the compass. Both map and compass are now pointed toward magnetic north.

5. Set the declination of the compass according to the Grid-Magnetic angle shown on the convergence diagram of the map. Take Note: Set the declination according to the numbers given on the diagram rather than according to the angle shown. The angle may not be to scale.
   • Because you are converting a grid azimuth to a magnetic azimuth in an area with a westerly declination, you ADD the declination to the compass reading. On the Fort Picket map, you would ADD the 6 degrees of the Grid-Magnetic angle by rotating the bezel ring of the compass two clicks COUNTERCLOCKWISE.

   • Conversely, if you are in an area with an easterly declination, you must SUBTRACT the declination by rotating the bezel ring of the compass in a CLOCKWISE direction.

6. When the declination is set, orient the compass along your plotted course by placing the sight wire over the line you have drawn that connects your present position and your destination.

7. Read the azimuth under the black index line.

8. Next, holding the compass firmly on the map, rotate the bezel ring until the luminous line is directly over the north-pointing compass needle.

9. Remove the compass from the map and hold it in the prescribed military manner. Rotate your body until the degree azimuth of your plotted course is under the black index line and the luminous line is over the north-pointing arrow. Keeping these two points of reference, follow your azimuth.

   How to Convert a Magnetic Azimuth to a Grid Azimuth

   To convert a magnetic azimuth to a grid azimuth in an area with a westerly declination, you must SUBTRACT the Grid-Magnetic Angle from the magnetic azimuth. This conversion is necessary in the processes of resection and intersection which are explained below.

   Conversely, if you are in an area with an easterly declination, you must ADD the Grid-Magnetic Angle to the magnetic azimuth.

   Working from Map to Compass

   On the Fort Pickett map, the Grid-Magnetic Angle is 5 1/2 degrees, or six degrees when rounded up. This figure is the difference between the direction that the magnetic needle of the compass points and the direction indicated by the vertical grid lines on the map.

   When the compass sight wire is placed over a vertical grid line, and both map and compass are rotated to point north, the difference between the grid north and the magnetic north is not apparent. However, we know that the grid north and the magnetic north are not really the same, but do, in fact, diverge by 6 degrees; therefore, we add these hidden degrees to the compass when we convert a grid azimuth to a magnetic azimuth.

   Working from Compass to Map

   Conversely, when we work from the magnetic azimuth we have observed in the field to a grid map, we know that the grid north of the map is off by the same 6 degrees. So to make the magnetic azimuth on the compass conform to the grid map, we must subtract these 6 magnetic degrees.

   In summary, a grid map is a projection that does not represent magnetic declination with its vertical lines, so the magnetic-grid angle must be added when converting a grid azimuth to a magnetic azimuth. On the other hand, to correctly represent a magnetic azimuth on a grid map, the same declination must be subtracted before the azimuth is plotted.

   When the magnetic declination is easterly, the situation is reversed. To convert a grid azimuth to a magnetic azimuth you must subtract the grid-magnetic angle. And to convert a magnetic azimuth with an easterly declination to a grid azimuth, you must add the grid-magnetic angle.

   RESECTION

   The Compass Method

   If you find yourself in the field with a map, protractor, and compass, but do not know where you are, you can locate your position using a method called resection.
   1. Orient the map using the compass.

   2. Identify two or three prominent landmarks in the distance that you can relate to the map.

   3. Using the compass-to-cheek method, take a sighting on one of these landmarks and read the azimuth.

   4. Convert the magnetic azimuth to a grid azimuth.
      • You make this conversion by SUBTRACTING the Grid-Magnetic Angle from the magnetic azimuth.

   5. Convert the grid azimuth to a back azimuth.
      • You make this conversion by ADDING 180 degrees if the grid azimuth is LESS than 180 degrees, or by SUBTRACTING 180 degrees if the grid azimuth is MORE than 180 degrees.

   6. Using your protractor, draw this azimuth to the map by making a line from the landmark back toward your unknown position.

   7. Repeat this procedure with a second and preferably a third landmark. The intersection of these lines will mark your location.

   The Straightedge Method

   If you are in the field with only a map, you can still find your location by using a straightedge.
   1. Orient the map with the terrain as best you can.

   2. Identify two or three prominent landmarks in the distance that you can relate to the map.

   3. Lay a straightedge on the map using the landmark as a pivot point. Or, if you have a pin, stick it through the map at the landmark and hold the far end of the straightedge against it.

   4. Rotate the straightedge until the known position on the map is aligned with the known position on the ground.

   5. When, you have made this alignment, draw a line on the map with the straightedge from the landmark back toward your unknown location.

   6. Repeat this process with two or more other landmarks. The point at which the lines intersect is your location.

   7. You can then determine the grid coordinates of your location.

   INTERSECTION

   The Compass Method

   You use the process of resection when you want to locate your unknown position on a map. You use the process of intersection when when you know your own position and you want to locate an unknown point on the map.
   1. Orient the map using the compass.

   2. Using the cheek-to-compass method, take a sighting on the point you wish to locate on the map.

   3. Read this magnetic azimuth and convert it to a grid azimuth.
      • You make this conversion by SUBTRACTING the Grid-Magnetic Angle from the magnetic azimuth.

   4. Using a protractor, draw a line on the map from your known position to the point you wish to locate on the map.

   5. Move to a second and third location that you can identify on the map and repeat this process.

   6. The point at which the lines representing the grid azimuths intersect locates the unknown point on the map. You can then use the map and protractor to find the grid coordinates of this point.

   The Straightedge Method

   1. Orient the map with the terrain as best you can.

   2. Locate your position on the map and mark it.

   3. Lay a straightedge on the map using your known position as a pivot point. Or, if you have a pin, you can stick it through the map at the your position and hold the end of the straightedge against it.

   4. Sight along the straightedge toward the point you wish to locate. Hold your end of the straightedge stationary over your position on the map and rotate the free end to line up with the unknown point.

   5. When, you have made this alignment, draw a line on the map with the straightedge from your known position toward your unknown point.

   6. Repeat this process by moving to one or more other known landmarks. The point at which the lines intersect will indicate the location of the unknown point.

   7. You can then determine the grid coordinates of this point.

   The Best Angles for Resection and Intersection

   In using resection and intersection, you should select landmarks that will give you angles of not less than 30 degrees nor more than 150 degrees whenever possible. If the angle is less than 30 degrees, "the lines will appear to close up before they come to the actual point, and so that point will not be easy to get down accurately" (Greenwood 73). If the angle is more than 150 degrees, the point will be hard to see because the angle will be nearly flat. Angles of 90 degrees will give you the clearest indication of location.

   Works Cited

   Department of the Army. Map Reading and Land Navigation. FM 3.21-26. 16 Nov. 2003. http://155.217.58.58/cgi-bin/atdl.dll/fm/3-25.26/toc.htm

   "Earth's Magnetic Field." Wikipedia. 19 Nov. 2003. http://en2.wikipedia.org/wiki/Earth%27s_magnetic_field

   Fleming, June. Staying Found. 2nd. ed. Seattle: The Mountaineers, 1994.

   Greenwood, David. Mapping. Chicago: University of Chicago Press, 1964.

   The Compass Store. "How to Use Your Declination Value." 20 Nov. 2003. http://www.thecompassstore.com/howtouseyour.html