Air Fronts: FM 21-25, Elementary Map and Aerial Photograph Reading - CHAPTER 6. Where Is It?

CHAPTER 6: WHERE IS IT?

In a town or city, it is easy for us to tell someone that the church is at the corner of 1st Avenue and 1st Street. Or if you make a date with someone at the circle at the corner of 6th Avenue and 3rd Street, you could be pretty sure that both you and your date would be able to find the place. For example, if you were a stranger in town and did not know that the highway near the church went straight to the corner of 6th Avenue and 3rd Street, you would walk to the right, watching the numbers on the streets, until you reached 6th Avenue; from there you would walk up the avenue until you arrived at 3rd Street (fig. 64).

Figure 63

Figure 64

In the army, however, we are faced with a different problem. We must be able to give to someone else the location of a lone tree in the middle of a large field, or a machine gun or sniper in a woods, or a guard along a stream. There are no streets in those places, but our maps have a system of letting us tell someone else where these points are.

Grids

This, is done by placing on the face of the map a series of lines in the form of squares. These, squares are used somewhat like the street system in a city, and every point on the map is near some "street." All we have to do is, tell someone to go to one of these corners, just as we do in town with real streets.

This is called a grid system, and the pattern is called a grid. Figure 65 shows our map with an army grid on it.

Figure 65

The streets in a grid all have very simple names. They are all numbers. Before we can use these numbers, however, we must learn a few rules about them.

In the first place, each square gets its name from the numbered lines which meet at its lower left-hand corner. This name is made up of two numbers separated by a dash. The first number is the one you read to the right, which marks the line running from the bottom of your map to the top, and the second number is the one you read up, which marks the line running from side to side.

For example, if we told you that an enemy patrol was observed in square (47-33), you would know the square meant is the one whose lower-left-hand corner is made by the crossing of up-and-down line 47 and, side-to-side line 33. Notice that we use only the two most important numbers of the grid line. The whole numbers for the corner squares, 1046000 and 638000 are given once, in the lower left hand comer of the map, but these large numbers are too clumsy to use. For example, here we omit the three zeros at the end of the number and- the 10 and 6 at the beginning. The two remaining numbers are enough to tell someone which grid line we mean.

There is a rule in army map reading which helps to remember which number is given first. The rule is "READ RIGHT UP." For example, if your platoon leader tells you to meet a patrol on the path at (47-33) on our map (fig. 65), first you read right along the numbers at the bottom until you come to 47. Then you read up this line until you come to the line marked 33. Read right up.

Figure 66

If the object we are trying to give the address of is not right at the place where a line crosses another line, we may imagine the sides of the grid square divided into 10 equal parts, so that 100 imaginary smaller squares are formed, as in figure 66. The lines on this grid are numbered to the right from 1 to 9, and up from 1 to 9. You read the numbers of the small squares just as you do the large ones. For example; to find the number for road junction 322 in figure 66, you count right from the lower left corner of the square to the line 4 which forms a corner of the correct small square, and you count up to the correct line, 7. The number you get is (4-7).

This number, however, is not yet complete. It is necessary to show which large square the smaller squares were in. So we use a combination of both numbers, those of the large square and those of the small one. We write the numbers so that the numbers reading to right are together, separated by a period or decimal point, and the numbers reading up are together, separated by a period. For example, to give the location of RJ322 in figure 66, thefir  st number would be 47.4 and the second 33.7. The whole address would then be (47.4-33.7).

There are only two more main things we must know about this army street system. The first is that these numbers are called coordinates. That is a fancy name for a simple idea, but once. you understand how to use coordinates, the word itself should not trouble you. The second point is that on many army maps these lines are 1,000 yards apart, but this may not always be the case; other kinds of grids may be used. If they are, the unit of measure and the distance between lines of the grid are indicated in the margin. Look for these numbers before you use the grid.

For example, figure 65 has the 1,000-yard grid on it. We can tell from the map that the circle in the town is about 1,000 yards from the edge of the airfield, because it is about the same distance between two grid lines. This is another way to tell distances on a map, and you can use it instead of the graphic scale.

Thrust Lines

There is one trouble with grid coordinates. If the enemy gets hold of one of our messages with these numbers on it, he can read exactly what we have written and find the place on a map. So the army uses other kinds of addresses, too--ones with secret keys to them. One of these methods is called the thrust line method, and it works as follows:

In Treasure Island, the famous story of the hunt for buried treasure, the treasure-hunters have a map which says the gold is 10 feet from a certain tree. The only difficulty is that for a long time they don't know where the tree is. It does them no good to know the distance between the tree and the gold unless they know what tree the map is talking about.

A thrust line works on this same idea. Instead of a secret tree, we use a secret line, which is not known to the enemy. This line is a base line, set up by the commander. The line is designated by means of two points, lettered T and L, standing for "Thrust Line": T means the first point (or base point), and L means the second point (or thrust point). From this line we measure forward or backward and at right angles to the right or to the left.

The order which gives the location of the thrust line also gives the unit of measure to use with it, miles, yards, or feet. When the measurement is in miles, we always measure in tenths of this unit, but we leave out the decimal point. For example, if the number we are working with is 136, that means the distance is 13.6 miles, or if the number is 08, that means the distance is 0.8 miles. When yards or feet are used, the unit of measure is designated by the commander; no decimal point is involved in. this case. For example if the commander designates the unit of measure as "tens of, yards," and we measure the distance as 64 yards, we write it 6; or if the distance is 178 yards, we write it 18.

The order telling you the base line will give you the grid coordinates of the points T and L; for example, an order may set up a thrust line from a base point at the juncture of two streams at (63.2-76.0) to forward point CR 13 at (64.0-77.3), measurements in hundreds of yards (fig. 67).

The first thing you do is draw this line on your map lightly, so you can erase it later. Figure 67 shows you the line.

Figure 67

Before we can use this line as our secret base line, we must know: First, that forward means from T to L, which generally means toward the enemy.

Second, that the first part of our coordinate is measured along the thrust line.

Third, that we measure the second part of our coordinate to the right or left as we face forward along the line

Fourth, that we use four simple abbreviations:

F, standing for forward from the base point T along the thrust line toward L.

B, standing for backward from the base point T along the thrust line.

R, standing for right of the thrust line, at right angles, or 90º.

L, standing for left of the thrust line, at right angles, or 90º.

Now we are ready to use our thrust line. Suppose your platoon leader tells you that you are to destroy a. bridge at B18R6. Where is the bridge?

Figure 68

B means backward from the base point. Therefore, first you extend the line straight back so you can work from it (fig. 68). The number 18 means 1800 yards, because you were told that the unit of measure is in hundreds of yards. With your graphic scale, you measure 1800 yards back from the base point. The next item in the address of the bridge is R6, which means 600 yards to the right of the thrust line as you look forward toward L. You measure this at right angles from the spot 1800 yards back of the base point, and get your location.

Now suppose that the enemy intercepted that message.

It would do him no good because he wouldn't know where our starting point or base point is or in what direction our thrust line runs ! Our commander will change this point often, so that the enemy cannot find the location of the thrust line.