Air Fronts: FM 21-25 - Elementary Map and Aerial Photograph Reading

Air Fronts: FM 21-25, Elementary Map and Aerial Photograph Reading - CHAPTER 8. Two Ways to Line Up Your Map with the Ground

CHAPTER 8: TWO WAYS TO LINE UP YOUR MAP WITH THE GROUND

Before the compass and the map are ready to work together, the map must be placed in a position so that the directions on the map are lined up with the directions on the ground. There are two ways to do this, one of them without the aid of a compass and the other with the help of a compass or of some other way of finding north. This act of lining up your map with the ground is called orienting the map.

By Inspection

The first way of lining up your map is called "by inspection," which simply means "by looking at the ground with the map in front of you." This can be done when you have found objects on the ground which you recognize on your map and which you can see. For example, in figure 81 you hold your map so-that the crossroads on it line up with the crossroads on the ground in front of you. Your map is then oriented.

Figure 81

If the objects on the ground are not as simple as crossroads, such as in figure 82, you can still line up your map by inspection. You must know your approximate positions on the map, and then turn your map in front of you until the distant object (the tower) on the ground lines up with the same object on the map and with your position on the map. The dotted line in figure 82 illustrates this lining up process. Your map is then oriented.

Figure 82

By Compass

A second way to line up your map, and a much easier method, is by using the compass. First you must know that the top of your map points in the general direction of north. In the margin of the map, or on the map itself, you will always find a small diagram which looks like figure 83(1); this is called a declination diagram. The only line on that diagram in which you are interested now is the one with the half arrowhead. That is called the magnetic north line, and is the line on your map with which you must line up the needle on your compass. You do it as in figure 83(2). Lay your map on a flat surface, extend the magnetic north line, and lay your compass down over it. Now turn your map, with the compass on it, until the north arrow on the compass and the arrowhead line on the diagram are in a straight line. Your map is then lined up, or oriented, as in figure 84. If the declination angle is 3 degrees or less, however, this method will not work and should not be used.

Figure 84

On some maps you will find a device which you can use to orient your map in place of the declination diagram. When you find this device on a map use it, because it gives you a long north line on your map to work with.

Here is the way you use this device:

Figure 85

At the bottom margin of your map you will find a small circle marked P (See fig. 85). This is called the pivot point. At the top margin you will find a scale marked off in degrees. To find your magnetic north line with this scale, read the number of degrees, called the G-M angle, on the declination diagram between the magnetic north line and the plain line marked "y" (fig. 85), and draw a line from the circular mark P to that number of degrees on the scale, as in figure 85. This line is the correct magnetic north line, and that is the line you use to orient your map with the compass. figure 86 is the map of Sackville with the declination diagram and scale.

Figure 86

Before you use a map you should ask your platoon leader to check the magnetic north line on your map and see that it is correct.

How to Find North Without a Compass

Even without a compass, however, you can orient. your map with a north line. There are ways to find north without a compass.

By Day

North Temperate Zone. In the north temperate zone, one way to find north is with an ordinary watch in good running order. Simply point the hour hand at the sun. Halfway between the hour hand and 12 o'clock is due SOUTH (fig. 87 (1)). Directly opposite from south is NORTH, of course. The only tricky thing about this is that the watch should read on sun, or standard time. If your watch is running one hour ahead (such as on wartime, or daylight-saving time), use the 1 o'clock number instead of the 12 o'clock number.

Figure 87 (1) and (2)

South Temperate Zone. In the south temperate zone we use the watch, too, but a bit differently. In this case you point the 12 o'clock on the watch at the sun. Halfway between 12 o'clock and the hour hand is due NORTH (fig. 87 (2)). Again, be sure it is standard time. If your watch is on wartime or daylight-saving time use the 1 o'clock number on the watch instead of the 12 o'clock number.

At Night

Northern Hemisphere. At night we must use another way to find north without a compass. We do this by means of the stars. In the northern hemisphere one way to use the stars is to find the Big Dipper. The Big Dipper is made up of seven fairly bright stars in the shape of a dipper with a long curved handle, as in figure 88. If you can see the Big Dipper, use as pointers the two stars which form the side of the cup farthest from the handle. These point in the direction toward which you would pour from the dipper. These pointers aim at a bright star which is about five times the distance between the two stars of the Dipper cup. This bright star is the North Star, and is directly over the North Pole. .If you hold a finger away from your eyes so it just fits between the two pointers, and then, keeping your hand the same distance from your eyes, measure 5 finger widths away from the end pointer, your farthest finger will just touch the North Star (See fig. 89).

Figure 89

Sometimes, however, you cannot see the Big Dipper, although you may be able to see other stars. In that case we use a star pattern called the Big "W" or the Big "M." Look at it in figure 88. Notice that it is on the other side of the North Star from the Big Dipper. The top of the "W" points about at the North Star. On a clear night you will always be able to see, in the northern hemisphere either the Big Dipper or the Big "W" Sometimes you will be able to see both. In either case, you will be able to find the North Star.

Southern Hemisphere. In the southern hemisphere you find true South in the direction of the South Pole Star. This is not as bright as the North Star and often you won't be able to see it at all. Look first for the Southern Cross, which is made by four stars in the form of a cross, as in figure 90. These four stars look like a kind of kite. Now if you figure the length of the kite from tip to tail and put a straight tail on the kite 4½ times as long as the length of the kite, the end of the tail will be about at the South Pole Star, which is directly over the South Pole. By using the width of your finger as a measuring stick, as with the Big Dipper (fig. 89), you can find where this star is.

Figure 90

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