Air Fronts: FM 21-25, Elementary Map and Aerial Photograph Reading - CHAPTER 5. How Far Is It?

CHAPTER 5: HOW FAR IS IT?

We have now put a map together and looked at many of its parts, so that at this point we can learn a good deal about a region by reading a map. There are still more things which a map can tell us. One of these things is: How far is it from one place to another?

Distances on a map can be measured. The reason for this is that a map is a true picture of the land. For example, we have a picture of the M1 bayonet, and we want to find out how long the blade is. We know the picture is smaller than the real bayonet. If we knew how much smaller the picture is than the real bayonet blade, we could find out how large the blade is.

By measuring the blade on the picture in figure 55, we find that it is 2 1/2-inches long in the picture.

Figure 55

Now suppose someone tells us that the picture is onequarter (1/4) the size of the real bayonet. We could then figure out how long the blade is. If the picture is onequarter the size of the real bayonet, the real bayonet is 4 times as large as the picture.

Let us put these figures to work. The picture of the blade is 2 1/2-inches long. The real blade is 4 times as long (as someone has told us); so the real blade is 4 times 2 1/2 inches, or 10 inches long.

4 X 2 2/1 = 10"

In the same manner as this, a map always tells you how much smaller it is than the real land. A map can tell you this in two ways.

One way is the same as we have just used on the picture of the bayonet.

A distance is measured on the map, and then the map tells you how much smaller this distance is than the actual ground, by means of a number found in the bottom margin of the map, about in the center (figure 56). This number is called the SCALE.

Figure 56

The scale number may be shown in two ways, both meaning the same thing. It may be written as a fraction: 1/25,000; or it may look like this 1:25,000. In either case, it is the same as saying that 1 inch on the map is equal to 25,000 inches on the read ground, just as in our picture of a bayonet, 1 inch on the picture was equal to 4 inches on the real blade. The scale of that picture of the. bayonet would have been called 1/4 or 1:4.

Let us try this out on our map. We want to find ot how long the right-hand runway of Liaison Field is. First, place an ordinary ruler along the runway, as figure 57.

Figure 57

It reads 1 inch. Now our scale reference says 1/25000 or 1 inch on the map equals 25,000 inches on the field. So for each inch we have measured on the runway, must substitute 25,000 inches. That means 1 times 25,000 inches, or 25,000 inches. But such long distanc are usually stated in yards or feet. Let us divide by 36 and by 12 and find out the number of yards or feet the runway: 25,000 divided by 36 equals about 700 yards. 25,000 divided by 12 equals about 2,100 feet.

Graphic Scale

Another method for finding distances is by the use of the Graphic scale. This method is even easier to use than the one we have just discussed. Just below the notation of scale, 1/25,000, is something which looks like a ruler (fig. 58). It is a ruler, a special one made just for that particular map. It is a ruler which has already done your arithmetic for you.

Figure 58

Let's look at our bayonet picture again with such a ruler and see how it works. The ruler or graphic scale is a special one made just for this particular picture. All we have to do is place this ruler on the picture of the bayonet with the zero at the tip of the blade, as in figure 59. We can see at a glance that the real bayonet blade is 10 inches long. The special ruler has shown the real distance of the bayonet.

Figure 59

This special ruler is called a graphic scale. But it does not matter what you call it as long as you know what it looks like on a map and how it is used. It is used with a map in the following manner:

First, a straight strip of paper is placed on our map alongside the airfield (fig. 60(1)). We then place marks on the paper at both ends of the field. The paper is then placed alongside our graphic scale on the map which shows the field really is (fig. 60(2)). There is another thing to notice about this scale.

Figure 60 (1)

Figure 60 (2)

It has two parts (fig. 61). From the zero mark to the right it reads in large numbers, 500 yards apart (The A part of fig. 61). From the zero mark to the left it breaks down this large distance into smaller distances (the B part of fig. 61), 100 yards apart, so that we can measure more accurately.

Figure 61

For example, in figure 60(2) the marks on our strip of paper are farther apart than the distance between the zero and the 500-yard marks on the graphic scale. If we place the right-hand mark at the 500-yard point on the graphic scale, the left-hand mark overlaps into the, "B" part of the graphic scale. We see that it is about at the second mark to the left of the zero, or at the 200-yard mark. By adding 200 yards to the first 500 yards we can judge that the runway is about 700-yards long.

For purposes of allowing different units of measure to be used on a map, there are sometimes found more than one graphic scale on the same map. For example, one of the scales may be measured in miles, another in yards, or another in kilometers. In any case, there is a note on the scale which tells you what the unit of measure is, and you can use any of the scales shown on that map.

In order to measure a curved or irregular line, for example, a section of the Burma Road (fig. 62(1)), we divide the curved line of the road section, between C and A into small straight sections (fig. 62(2)). Then we lay the edge of a strip of paper on the tick marks, one after the other, adding each section to what we already have marked. We finish with a straight piece of paper with the total length of the curved road on it. We can measure this with the graphic scale, in the same way as we measured the runway.

Figure 62 (1)

Figure 62 (2)

Scale of Maps

It is important to be sure which is the larger scale, a 1/25,000 map or a 1/50,000 map. The answer is: the 1/25,000 map, because the number is a fraction and 1/25,000 of something -is bigger than 1/50,000, just as 1/2 is large than 1/4. That is clear enough to us, but these numbers are a little tricky, and it is easy to make mistakes and forget that the larger the number in the lower part of the fraction, the smaller the scale of the map.