Air Fronts: FM 21-25, Elementary Map and Aerial Photograph Reading - CHAPTER 4. How High Is It?

CHAPTER 4: HOW HIGH IS IT?

So far, everything on our map is flat. We must now find a way to learn something about the different ground levels. It means something to us to know that a hill is in a certain place, but we would further like to know how high it is. A picture of a hill taken from above will not show us this, but there is a way for our map to give this information.

Since a hill is broader at its base than it is at its top, let us take an object which is like a hill, a cone, for example, and see what we can do to a picture of it frorn above to let us know how high it is.

Height

First, suppose there are two boulders on the side of the cone, as in figure 41 (1). When we look at this from above (fig. 41(3)), we can still tell that there are two boulders there, but if we had not already seen the cone from the side, we would not know which boulder was higher and we would not know how high up the cone either boulder is.

Figure 41

Next, let us suppose that we walk up this cone until we are 10 feet higher than the base (fig. 42). Now let us walk around the cone, staying 10 feet high all the way around. Finally, we will come back to the place we started from. If we had a leaky bag of flour with us as we walked, we would have left a mark, on the hill which would look like figure 42(1). From above, this line would look like it does in figure 42(3).

Figure 42

Now what do we know about this mark as we look at it from above? We know that anything on it is 10 feet higher than the base of the cone. Notice that one of the boulders is right on this line; therefore its elevation, or height, is 10 feet. We know that anything outside it is lower than 10 feet.

Figure 43

Let's move up the, cone until we are 10 feet higher and do the same thing, as in figure 43.- The result is another flour line. What do we know about this line in figure 43 (3) ? We know that everything on it is 20 feet high, and everything between it and the first line is somewhere between 10 and 20 feet high. Now the second boulder is about halfway between the two lines, so we can judge that the second boulder is about 15 feet. high.

In this way we can tell the height of objects.

Shape

These lines tell us still more. They tell us that the cone is round, for example. If the object were not round, an object other than a cone, these lines can tell us that, also. For example, suppose we stretch one side of the cone so that it looks like figure 44(1). If we do our-flour-bag stunt again, from above, the lines look like figure 44(3). We can tell which side it bulges on.

Figure 44

We find, then, that these lines can tell us-two things: height and shape. Maps have many such lines, and if you understand what they mean, they are easy to read and very helpful. On maps these lines are called contours or contour lines.

Let us see what happens when we place contour lines on our -map of Sackville. Figure 45(1) outlines the hill which overlooks the town. In the, picture in figure 45(2) we have lifted the hill right out of the ground so we can talk about it and examine it more closely.

Figure 45 (1) and (2)

First, let us make our flour marks on the hill, just the way we did on the cone. From the side, it looks like figure 46(1). As we rise into the air we can see more of these lines, as in figures 46(2) and (3).

Figure 46

Finally, we have a view from directly above, and we can see that the shape of the hill is shown (fig. 47) just as the shape of the cone was shown in figure 44. If we take these lines out of the photograph and put them on a map, as in figure 47, we can tell which parts of the map are high and which are low.

Figure 47

It would clutter up the map too much to have the number showing the height of each contour line, so only a few of them are numbered. To make it easy to count these lines, every fifth line is made heavier than the others. The distance between contour lines on each map is shown by means of a note at the bottom of the map. This note may read, for example: Contour Interval: 10 feet

This means the distance between any two neighboring contour lines on that map is 10 feet in an up-and-down (vertical) direction. Figure 48 shows how the contour interval is noted on a map of Sackville: on this map the contour lines are 100 feet apart.

Figure 48

Since the highest point on the hill in figure 47 does not fall exactly on a contour line, this point is labeled with the exact height. On maps, such height numbers are found often. Some things take their names from these numbers. For example, if there are a number of road junctions on a part of a map, and we wish to name one of them in particular, we may call it RJ 124 (that is, the road junction at a height of 124 feet).

Figure 49

Height Above What?

It may be noticed that the height of the base of the hill in figure 46 is 300 feet rather than 0 feet. The reason for this is, height on all maps is figured from sea level. In other words, we compare the height of all land anywhere to the average level of the sea.

For example, in figure 49, although the hill is far from the sea, yet the base of the hill on the land is 300 feet above sea level. Sea level is the zero for our height measurements.

Ground Forms

It is clear that these contour lines are very helpful things to have, and they can help us in other ways. For example, suppose we have a high spot of ground that breaks off suddenly -and becomes a cliff. From the ground, as in figure 500), it is easy to tell this. It is also easy to tell this by examining the contour lines on the map. When a hill or cliff is steep, the contour lines appear close together, as in figure 50(2). This arrangement of contour lines on a map, always shows a sharp rise in the ground.

Figure 50 (1) and (2)

If, however, the lines are spaced gradually and fairly far apart, it means that the hill rises gradually and evenly.

If there are two hills with a saddle between them, as in figure 51 (1), the contour lines, as in figure 51(2), tell us exactly what these hills and valley look like.

Figure 51 (1) and (2)

A special kind of valley is formed by a stream, and contour lines in the area are usually regularly spaced (see fig. 52(2)). They form "V's" where they cross the stream. It is important to remember that these "V's" all point uphill, or upstream.

Figure 52 (1) and (2)

Contours, then, can tell us several things about streams. They can tell us the location of a stream or valley; they can tell us which way the stream is flowing, .which shows the slope of the ground; and by the spaces between the contour lines they can tell us how steep the valley is.

Remember, then, that V-shaped contours show valleys or streams.

If we had a lake surrounded by mountains, as in figure 53(1), a first glance at the contours (fig. 53(2)) would make it appear to be a hill; however, by a closer look at the height numbers on the contour lines, we would see that as they get nearer the center they get smaller. We would know, therefore, that the, land is going downhill and forms a kind of basin.

Figure 53 (1) and (2)

Figure 54 shows our familiar map of Sackville and vicinity with contour lines on it. Our map is becoming more complete. The contour lines show the shape of the ground. We can tell now that the land has a hill, with the ground sloping very gently from its base toward the town and toward the airfield.

Figure 54