The most common military method of expressing a direction is by using azimuths. As stated before, an "azimuth" is defined as a horizontal angle, measured in a clockwise reamer from a north base line. When the azimuth between two points on a map is desired, the points are joined by a straight line. Then a protractor is used to measure the angle between grid north and the drawn line. This measured angle is the grid azimuth of the drawn line (fig. 5-29). When using an azimuth, you imagine the point from which the azimuth originates as

Figure 5-29.-Azimuth angle.

Figure 5-30.-Origin of azimuth circle.

Figure 5-31.-Azimuth and back azimuth.

the center of the azimuth circle (fig. 5-30). 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 (fig. 5-21). Therefore, any one given direction can be expressed in three different ways: a grid azimuth, when measured on a military map; a magnetic azimuth, when measured by a compass; or a true azimuth, when measured from a meridian of longitude.

The BACK AZIMUTH of a line is its forward azimuth plus 180; or if this sum is greater than 360, the back azimuth is the forward azimuth minus 180. For example, if the forward azimuth of a line is 112 (fig. 5-31), the back azimuth is as follows:

Figure 5-32.-Detour around enemy positions or obstacles.

112 + 180 = 292

When the forward azimuth of a line is 310, the back azimuth is as follows:

310 - 180 = 130

Figure 5-32 shows an example of how to bypass enemy positions or obstacles by detouring around them. This allows you to stay oriented by moving at right angles for specified distances. For example, if you are moving on an azimuth of 360 and wish to bypass an obstacle or position, you change direction to 90 and travel for 100 yards; change direction back to 360 and travel for 100 yards; change direction to 270 and travel for 100 yards; then change direction to 360; and you are back on your original azimuth

Bypassing an unexpected obstacle at night is a fairly simple matter. To make a 90 turn to the right, hold the compass as described earlier in the method for night use; turn until the center of the luminous letter E is under the luminous line (do NOT change the setting of the

Figure 5-33.-Types of protractors.

luminous line). To make a 90 turn to the left, turn until the center of the luminous letter W is under the luminous line. The compass setting (bezel ring) does not require changing, and you can be sure of accurate 90 turns. For example, you decide to detour to the right. You turn until E is under the luminous line and move ahead in that direction until you have outflanked the obstacle. You then turn until the north arrow is under the luminous line and move parallel to your original course until you have bypassed the obstacle. You then turn again until the W is under the luminous line and move back the same distance you originally moved out. Finally, you turn until the north arrow is under the luminous line and go ahead on your original course. This method works regardless of what your initial azimuth may be.