Air Fronts: FM 21-25, Elementary Map and Aerial Photograph Reading - CHAPTER 7. Which Way Is It ?

CHAPTER 7: WHICH WAY IS IT ?

Let's leave our map for a moment and talk about the direction of things. We shall come back to our map in chapter 8.

Figure 69

On the ground and on the map, the army has an easy way to point out the direction of things. It is easy because the same idea is used wherever we are. The idea is simply this: We suppose that wherever we happen to be at any given moment, we are in the center of a circle which has "avenues" running off in all directions, as in figure 69. This circle is marked off into 360 avenues (fig. 70). Each of the 360 spaces is called a degree and each avenue has a name called an azimuth. This azimuth is just a name for direction line; each of these direction lines has a number, depending upon which of the 360 avenues it is.

Figure 70

We can, march off on one of these avenues, or azimuths, starting at the center of the circle. The avenues all start where you are, whether you are in a jungle, on a hill, or on a highway, just as with the soldier in figure 71. This soldier has been told to go along the avenue marked "azimuth 60 degrees" (written as 60º), and he is pointing his finger at it. You can think of the soldier in the center of the circle with 360 avenues or azimuths running out from him like the spokes from the hub of a wheel. Every 10th avenue is numbered to make it easy to find those in-between.

Figure 71

Now there are two important things to remember about this circle of avenues.

FIRST: The zero --0- avenue must always point NORTH. In a moment we will talk about how to find north, but right now don't worry about it.

SECOND: The avenues are numbered CLOCKWISE. That means we number them in. the direction that we number hours on a clock. Figure 72 shows what we mean by clockwise direction.

Figure 72

With this knowledge, let's put this circle to work for us. You are told that there is a sniper in a tree in the orchard at Furlough Farm, and also told you that if you crawl along up the Creek to where the railroad crosses it at the foot of the hill, you can see the sniper's tree on an azimuth of exactly 60 degrees (60º'). Figure 73 shows the situation.

Figure 73

You find where the railroad crosses the creek and go there. You remember that the center of the circle of azimuths is right where you are. The first thing to do is to point the zero mark on the circle at north. (You will learn how to do this with your compass later in this manual). Now it is easy to see which tree is on the avenue or azimuth marked 60º on the circle. That is the address of that tree, using the azimuth system.

You take a bead on the sniper and knock him from the tree with your first shot. You move cautiously up to the tree, examine the fellow carefully, and find he is dead. You walk back to the railroad where it crosses the creek. Now what azimuth did you walk back on? Was it 60º ? No, it wasn't. Look at figure 74. It shows you that if you face in one direction you have one azimuth (called a forward azimuth) and if you face in the opposite direction, the azimuth (called a back azimuth) has a different name. The back azimuth is on a straight line back through the center of the circle from the forward azimuth. Figure 75 shows you that at the railroad, with the zero of the circle pointing north, the azimuth on the circle reads 60º from there to the tree, and that at the tree, with the zero of the circle still pointing north, the azimuth on the circle reads 240º from the tree to where the railroad crosses the creek.

Figure 74

Figure 75

The back azimuth is an important thing to know about, because if you know how to use it, it will take you back to your starting point. If you are sent on a mission to a point in strange country and at night, for example, your back azimuth will show you the direction in which you return. The following section, on compasses, show you how to find the back azimuth.

The Compass

We come now to the compass, that useful item which finds north for us and finds our azimuth for us. The compass has on it the circle of numbered avenues or azimuths which we have been talking about in the last few pages. In other words, the compass is our direction finding tool, and it has everything on it to help us find our way.

There are several types of compass, but the one which we shall use here is called the lensatic compass. There are other kinds of compasses, but they all work on the same principle. If you understand how to use the army lensatic compass, you will find it easy to learn to use the others. Later in the manual we shall talk a bit about some of these other army compasses.

Let us look now at figure 76 for a good look at the lensatic compass. The most important thing about that compass is that no matter how you turn it, as long as you hold it level, the white arrow always points in the direction of north. It won't let you down if you remember one thing: never use it near any metal object if you can help it. Metal objects will make the compass needle (arrow on this compass dial) point in the wrong direction. Whenever you use a compass then, make sure you are well away from such metal objects as your helmet, rifle, truck, a wire fence, or any metal object which will bother your compass needle.

Besides the compass needle which points north, there is another important part of the compass. That is the numbered circle of avenues or azimuths right on the face of the dial. Everything else on the compass is designed to help you line up your compass with things on the ground and on your map and to help you read the avenue or azimuth numbers.

Figure 77

Figure 78

Compass reading is easy, if it is done correctly. For example, it is important to hold the compass correctly. Remember to point the compass in the general direction you want to go before you try to use it, and hold it level. Figure 77 shows the compass ready to be used in daytime sighting. Figure 78 shows how to hold the compass so that it is steady. Notice that it is held with the eyepiece close to the eye. Figure 79 shows how to line up a tree. You look at the tree through the slit in the eyepiece and through the slit in the cover with the hair line in it. The glass eyepiece is used only to read the azimuth numbers on the dial. It is there only so you will be able to glance down and read these numbers at the same time you line up an object with the compass. The view you get when you use the compass correctly is shown in figure 80. The soldier here has lined up the tree in his compass sights and found that the tree is on an azimuth of 60º.

Figure 79

With a compass, you can see how easy it is to find a back azimuth.. You can do it in two ways. The first way is to read your back azimuth right off the dial, by taking the number opposite the forward azimuth on a straight line through the center of the dial. The second way is to sight from the point you have reached, back to point you started from. The main difference between these two methods is that in the first way, you do not have to be able to see your starting point in order to get the back azimuth.

Figure 80