Part 56 (1/2)

On maps of very small pieces of ground, the V. I. is usually small--perhaps as small as 1 foot; on maps of large areas on a small scale it may be very great--even 1,000 feet.

Contours also show =slopes=. It has already been explained that from any contour to the next one above it the ground rises a fixed number of feet, according to the vertical interval of that map. From the scale of distances on the map the horizontal distance between any two contours can be found. For example: On the map the horizontal distance between D and E is 90 yards, or 270 feet. The vertical distance is 20 feet, the V. I. of the map. The slope then is 20/270 = 1/13.5 = 7-1/2% = 4-1/2, in all of which different ways the slope can be expressed.

[Ill.u.s.tration: Slope.]

On a good many contoured maps a figure like this will be found in one of the corners:

[Ill.u.s.tration: Scale.]

On that particular map contours separated by the distance

[Ill.u.s.tration: Scale.]

on the vertical scale show a slope of 1; if separated by the distance [Ill.u.s.tration:

__2__

] they show a 2 slope, etc. A slope of 1 is a rise of 1 foot in 57. To use this scale of slopes, copy it on the edge of a piece of paper just as you did the scale of distances and apply it directly to the map.

You will notice that where the contours lie closest the slope is steepest; where they are farthest apart, the ground is most nearly flat.

It has already been set forth how contours show height and slope; in addition to this they show the shape of the ground, or GROUND FORMS.

Each single contour shows the shape at its particular level of the hill or valley it outlines; for instance, the 880 contour about the penitentiary shows that the hill at that level has a shape somewhat like a horse's head. Similarly, every contour on the map gives us the form of the ground at its particular level, and knowing these ground forms for many levels we can form a fair conception of what the whole surface is like.

A round contour like the letter O outlines a round ground feature; a long, narrow one indicates a long, narrow ground feature.

Different hills and depressions have different shapes. A good many of them have one shape at one level and another shape at another level, all of which information will be given you by the contours on the map.

One of the ways to see how contours show the shape of the ground is to pour half a bucket of water into a small depression in the ground. The water's edge will be exactly level, and if the depression is approximately round the water's edge will also be approximately round.

The outline will look something like figure 6.

Draw roughly on a piece of paper a figure of the same shape and you will have a contour showing the shape of the bit of ground where you poured your water.

Next, with your heel gouge out on one edge of your little pond a small, round bay. The water will rush in and the watermark on the soil will now be shaped something like figure 7.

Alter your drawing accordingly, and the new contour will show the new ground shape.

Again do violence to the face of nature by digging with a stick a narrow inlet opening out of your miniature ocean, and the watermark will now look something like figure 8.

Alter your drawing once more and your contour shows again the hew ground form. Drop into your main pond a round clod and you will have a new watermark, like figure 9, to add to your drawing. This new contour, of the same level with the one showing the limit of the depression, shows on the drawing the round island.

Drop in a second clod, this time long and narrow, the watermark will be like figure 10, and the drawing of it, properly placed, will show another island of another shape. Your drawing now will look like figure 11.

It shows a depression approximately round, off which open a round bay and a long, narrow bay. There is also a round elevation and a long, narrow one; a long, narrow ridge, jutting out between the two bays, and a short, broad one across the neck of the round bay.

[Ill.u.s.tration: Fig. 6 to 11.]

Now flood your lake deeply enough to cover up the features you have introduced. The new water line, about as shown by the dotted line in figure 11, shows the oblong shape of the depression at a higher level; the solid lines show the shape farther down; the horizontal distance between the two contours at different points shows where the bank is steep and where the slope is gentler.

Put together the information each of these contours gives you, and you will see how contours show the shape of the ground. On the little map you have drawn you have introduced all the varieties of ground forms there are; therefore all contour forms.