Air Fronts: FM 21-25, Elementary Map and Aerial Photograph Reading - CHAPTER 9. Your Map and Compass go to Work

CHAPTER 9: YOUR MAP AND COMPASS GO TO WORK

We know enough now about the compass and the map to start to use them together in a practical way.

Finding the Azimuths of Things on a Map

We have learned how to find the azimuth of an object on the ground by sighting with the compass. Now we come to the problem of learning to find the azimuth of something on a map.

Figure 91

Figure 91 is a section of a map with two points on it, a house and a bridge, where the highway crosses the creek. The question is, what is the azimuth from the house to the bridge? The first thing to do is to draw a line lightly on your map between the house and bridge as in figure 91. Then orient the map. Next, lay your compass flat on the pencil line so that the hair line of the compass cover points at the bridge. You read the azimuth 122º' on the compass, as in figure 92, and you have your answer.

Figure 92

Suppose that you wanted to show on a map that a machine gun was on the path at an azimuth of 278º from the bridge. To find the correct point on a map, you would proceed as follows: First, orient your map; then lay your compass so that the tip of the compass cover is directly over the bridge (fig. 93).  Read the azimuth of 278º on the compass and pivot the compass, keeping the cover tip at the bridge, until the hair line in the cover lines up with that azimuth. Make a mark at both ends of the lined-up compass, the cover tip, and the eyepiece. Remove the compass and draw the line between the two marks until it crosses the path (fig. 93). That is the point you are looking for.

Figure 93

Finding Yourself on the Map

One of the most important things to be learned from the map and compass is finding your location on the map. If your location is somewhere between the airfield and the creek on the Sackville map to find your exact location:

First: Orient your map with a compass.

Second: Find two things on the ground in front of you which you can also find on your map, anything you can draw a bead on.

Third: With your compass, take an azimuth on a point, for example the building at the mine. Sight through the compass, line up the building with the hair line in the compass (fig. 94), and read the azimuth through the glass eyepiece, which is 305º.

Figure 94

Fourth: Lay your compass on your map, with the tip of the cover at the mine (fig. 95). Be sure that your map is oriented with the north arrow of the compass. Then pivot the compass slowly until the azimuth of 305º lines up with the hair line. Make two marks on the map, one at each end of the compass, remove the compass, and connect the two marks with a line.

Figure 95

Fifth: This azimuth is a forward azimuth; however, we are back of the house, therefore, we want a back azimuth. To get this a line is carried 'straight back, as in figure 95. If we put the compass on the line again, we can see that the back azimuth of 305º is 125º.

You are somewhere along this line on your map, but you still need to know exactly where.

Figure 96

Repeat the procedure with a second point, for example the tip of the left-hand runway of the airfield (fig. 96). A forward azimuth reading shows it to be 67º. Lay the tip of the compass cover at the tip of the runway on your oriented map, line up the hair line with 67º', make your two marks, and draw your line straight nack. This I line will cross the first line you drew, at your exact location. (See fig. 97.) Notice that the back azimuth is 247º.

Figure 97

How to March on an Azimuth

Suppose you are somewhere southeast of Sackville and your platoon leader tells you to go to a certain bridge. He points out the bridge on the map and draws a line on the map from your position to the bridge. By laying your compass on the oriented map you find that the bridge is on an azimuth of 51º from where you are (See fig. 98). Your job is to get to the bridge. That means that you must march along that azimuth.

Figure 98

First you take your compass, sight into it along 51º, and discover that you cannot see the bridge at all from where you are. You know, however, that it is somewhere up there ahead, along that line on which you are sighting. You can see a house right on your line of sight, so you walk to the house, go around it, and sight once more on your azimuth. You still can't see the bridge, but you notice that the edge of the woods (fig. 99) is on a line with your sight. So you head for the edge of the woods (fig. 100), and when you get there you find the bridge in front of you (fig. - 101).

Figure 99          Figure 100

Figure 101

If it so happened that you could not see the bridge from the edge of the woods, you would do the same thing, picking out a tree, fence, or other object along your line of march, until you finally reach a point where you could see the bridge.

Taking a Back-check

The ground may not be level and straight, so it is wise to expect to find obstacles in the way and for things to go wrong. For example, suppose, after you have taken your first sighting on an azimuth, you run into a pond and you lose your compass. That's not good, but if you had taken a few safety steps, you would still be all right. One of these safety steps is to take a back-check. This would allow you to get back on your original line of march after you had detoured the pond. It works this way.

Figure 102

When you started turning off your course to get around the pond (fig. 102), if you had looked back and noticed that your starting point at A was right in line with that hill at B, you would be able to come back around the pond to where the starting point and the hill were in line again. You would then be back on your original azimuth. Keeping your starting point and the peak lined up as you went forward would take you to your first aiming point. When you got there you could figure out your original line by taking a back-check and carrying that line forward to choose another. aiming point to walk to. You would finally arrive at your bridge even though you had lost your compass. This is a good point to remember.

It may not always be possible to take a back-check, especially if it is dark or if an obstacle interferes with your line of vision. If this is the case, it is possible to get back on the original azimuth by marching at right angles four times around the obstacle, as in figure 103, counting the number of paces we take and making sure that we march the same number of paces on lines A and B.

Figure 103

The only problem here is how to march at right angles. We can find this direction with our compass. This is the easiest way. A right angle has 90º, which is ¼ of the way around our compass dial, or the distance between any two of the four main directions: N. E. S. and W.  To go off at 90º from your course, just line up one of these points on your azimuth and follow the mark 90º from it. Figure 103 makes clear your course. If, however, you have lost your compass as-we previously supposed, you must estimate the 90º turns.

Using the Compass at Night

There are a number of lines and dots on the compass which we have not talked about so far because they are used mainly at night.

The lensatic compass has two glass faces, one under the other. The topmost glass face rotates, the under one does not. The top glass rotates with a clicking sound; each click means it has turned 3º'. On the top glass are two lines visible at night. One line is about four times as long as the other. These two lines are 45º apart. On the under, or stationary glass face, are three dots and a short line, each 90º from the luminous marks on either side of it, or one-quarter of the way around the face of the dial. On the azimuth dial itself the letters W. E. and S. and the arrow for north are visible at night. On the inside of the cover, lined up with the hair line sight, are two more luminous dots.

We cannot see the numbers on the azimuth circle at night, but these dots and lines are enough to allow us to read the compass.

Setting of Your Azimuth at Night

Before we can start off on an azimuth, the first thing to do is to arrange the compass so that the azimuth can be seen at night. We want to get to the same bridge we walked to in daylight, on an azimuth of 51º. If it is too dark to read the numbers on the compass dial and we are forbidden to use a light, we prepare the compass in four steps:

First: Face in the general direction in which you are going.

Second: Line up the north arrow and the long line on the revolving face with the short mark on the fixed glass face. The compass should look as it does in figure 104(1).

Figure 104

Third: Hold the movable glass dial still by gripping the knurled brass ring with one hand, and turn the compass under it to the tight 17 clicks. Since each click means 3º, that would add up to 51º'. The compass would then look as it does in figure 104(2).

Fourth: Turn the whole compass until the north needle lines up with the long line, and there you are, figure 104(3). Your azimuth is the line formed by the two dots on the inside of the cover and the short line on the fixed glass. You follow that line.

If you can prepare your compass before it gets dark or if you are allowed to use a light to adjust it, you set your compass as follows:

First: Rotate the whole compass until 51º on the dial is directly under the short luminous line on the lower fixed glass.

Second: Holding the compass in that position, rotate the upper movable glass until the long luminous line is over the north arrow of the dial. Your compass is set on a 51º azimuth. Now all you have to do to march on this azimuth in the darkness is to open the compass and rotate it so the north arrow is directly below the long, luminous line. Follow the line formed by the short line on the lower fixed glass and the two dots on the cover.

Following a Compass Line at Night

To follow an azimuth at night, we must be able to pick up aiming points ahead of us, just as we do in the daytime. At night this is not easy to do. You cannot see very far ahead, and your aiming points, if you see them at all, will be very near to you. You must take readings on many more aiming points at night than in the day. If it is so dark that you cannot find an aiming point, send another man out a little ahead of you. Direct him to: move either to the right or left, until he is on your line of sight. Be sure he does not move until you have reached him. Then do the same thing all over again until you can find- some aiming point in the area ahead.

Because you must take so many compass readings at night, it is best to lay aside only easily handled items of metal equipment. But when, because of the need for speed or silence, you. must take a reading with metallic equipment on you, sling your rifle over your left shoulder and hold the compass in your right hand in front of you and away from such metal items as you can.

Notice the way the compass is held at night (fig. 105 (1)). You do not aim through the eye-piece as you do by day. You simply point the compass in the direction you are going, stand behind the compass, and line yourself up with an aiming point which is on a line with the three luminous pointers on the compass (fig. 105 (3)). Then walk to your aiming point.

Figure 105

Other Compasses

There are other types of compasses used by the Army. A newer lensatic compass, figure 106, very much like the one we have been using in this book, is made so that the azimuths on the dial are visible at night. The other differences are that the numbers are printed in plain back instead of white and there is an additional scale on the dial called a mil scale. The mil scale is not used for ordinary compass work.

The wrist compass (fig. 107) is a simple compass which you may often have. It has a fixed arrow on the glass cover and a plastic outer scale which may be rotated. Printed on this outer scale are the four main directions and the divisions for each five degrees from 0º to 360º. The north needle is a straight line. At night you can see the north end of the needle and a dot at the south send, the arrow on the glass, the letters N. E. S. snd W on the plastic ring, and four 45º dots, each half-way between two direction letters.

Figure 107

To use the wrist compass in the daytime set the arrow on the glass cover on the azimuth you want to follow, by turning the plastic ring. Then turn your arm or body until the needle is lined up with the N on the plastic ring (fig. 107), and simply follow the arrow to your aiming point.

At night, if you cannot see enough to read the numbers on the scale, you estimate the degrees between the directions, since there are no clicks on this compass, and use the compass just as you do in the daytime.

Bringing Back the Information

When you are sent out on a reconnaissance or scout assignment you will find information which must be sent back to your commander. That is your mission. It is necessary, then, to know how to send back this information

First of all, we must know the names of things on the land. It is not necessary for anyone to know what the things are called by, until he has to tell someone else about them. Then it is very important that he knows what the names of land forms are. Figure 108 is a scetch showing the names of various land forms

Figure 108

Learn these names. It is important that you be able to describe things accurately and in the right words so that the men you work with can understand what you mean.

Overlays

An overlay is used to send back information obtained by reconnaissance. An overlay is simply a tracing, on a lain piece of paper, of a section of a map. To make an overlay, lay a piece of paper over the section of the map you are interested in, and draw on it the corners of the grid squares, as in figure 109. These crosses are called register marks and they are put on the overlay to show 'who-ever uses it just where to place. Since there is always a chance that anyone using the overlay would not know where to place it on his map, the register marks should be numbered with the numbers of the grid lines.

Figure 110

Once the register marks are on the overlay, you need only to indicate on it, the objects you are concerned with.

Figure 110

Write your message right on the overlay, with the usual information that is commonly put on ordinary written messages. The person who gets the overlay then lays it on his map so that the register marks line upon the grid lines, and is able to use the information received.