The contours represent lines on the ground that are horizontal and whose meanderings follow the surface, just as the edge of a flood would follow the irregularities of the hills about it.
Those lines that contours stand for are just as level as the water"s edge of a lake, but horizontally they wander back and forth to just as great a degree.
The line marked 880, at the penitentiary, pa.s.ses through on that particular piece of ground every point that is 880 feet above sea level. Should the Missouri River rise in flood to 880 feet, the penitentiary would be on an island, the edge of which is marked by the 880 contour.
Contours show several things; among them the height of the ground they cross. Usually the contour has labeled on it in figures the height above some starting point, called the DATUM PLANE--generally sea level. If, with a surveying instrument, you put in on a piece of ground a lot of stakes, each one of which is exactly the same height above sea level--that is, run a line of levels--then make a map showing the locution of the stakes, a line drawn on the map through all the stake positions is a contour and shows the position of all points of that particular height.
On any given map all contours are equally s.p.a.ced in a vertical direction, and the map shows the location of a great number of points at certain fixed levels. If you know the vertical interval between any two adjacent contours, you know the vertical interval for all the contours on that map, for these intervals on a given map are all the same.
With reference to a point through which no contour pa.s.ses, we can only say that the point in question is not higher than the next contour up the hill, nor lower than the next one down the hill. For the purposes of any problem, it is usual to a.s.sume that the ground slopes evenly between the two adjacent contours and that the vertical height of the point above the lower contour is proportional to its horizontal distance from the contour, as compared to the whole distance between the two contours. For instance, on the map, find the height of point A. The horizontal measurements are as shown on the map. The vertical distance between the contours is 20 feet. A is about one-quarter of the distance between the 800 and the 820 contours, and we a.s.sume its height to be one-quarter of 20 feet (5 feet) higher than 800 feet. So the height of A is 805 feet.
The vertical interval is usually indicated in the corner of the map by the letters "V. I." For instance: V. I.=20 feet.
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]
On a good many contoured maps a figure like this will be found in one of the corners:
[Ill.u.s.tration]
On that particular map contours separated by the distance
[Ill.u.s.tration]
on the vertical scale show a slope of 1: if separated by the distance
[Ill.u.s.tration]
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 water-mark 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 new 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. Fig. 7. Fig 8. Fig. 9. Fig 10. Fig. 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 that 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 the contour forms.
The contours on an ordinary map seem much more complicated, but this is due only to the number of them, their length, and many turns before they finally close on themselves. Or they may close off the paper. But trace each one out, and it will resolve itself into one of the forms shown in figure 11.
Just as the high-tide line round the continents of North and South America runs a long and tortuous course, but finally closes back on itself, so will every contour do likewise. And just as truly as every bend in that high-tide mark turns out around a promontory, or in around a bay, so will every bend in a contour stand for a hill or a valley, pointing to the lowlands if it be a hill, and to the height if it mark a valley.
If the map embrace a whole continent or an island, all the contours will be of closed form, as in figure 11, but if it embrace only it part of the continent or island, some of the contours will be chopped off at the edge of the map, and we have the open form of contours, as we would have if figure 11 were cut into two parts.
The closed form may indicate a hill or a basin; the open form, a ridge or a valley; sometimes a casual glance does not indicate which.
Take up, first, the contour of the open type. If the map shows a stream running down the inside of the contour, there is no difficulty in saying at once that the ground feature is a valley; for instance, V, V, V, and the valley of Corral Creek on the map. But if there is no stream line, does the contour bend show a valley or a ridge?
First of all, there is a radical difference between the bend of a contour round the head of a valley and its bend round the nose of a ridge,
Compare on the map the valleys V and the ridges R. The bend of the contour round the head of the valley is much sharper than the bend of the contour round the nose of the ridge. This is a general truth, not only in regard to maps, but also in regard to ground forms. Study any piece of open ground and note how much wider are the ridges than the valleys. Where you find a "hog back" or "devil"s backbone," you have an exception to the rule, but the exceptions are not frequent enough to worry over.
To tell whether a given point is on a ridge or in a valley, start from the nearest stream shown on the map and work across the map to the undetermined point, keeping in mind that in a real trip across the country you start from the stream, go up the hill to the top of a ridge, down the other side of the hill to a water-course, then up a hill to the top of a ridge, down again, up again, etc. That is all traveling is--valley, hill, valley, hill, valley, etc., though you wander till the crack o" doom.
And so your map travels must go--valley, hill, valley, hill--till you run off the map or come back to the starting point.
On the map, follow the R-V line, V indicating valley and R ridge or hill. Note first the difference in sharpness in the contour bends; also how the valley contours point to the highland and the ridge contours to the lowland.
The contours go thus:
[Ill.u.s.tration]
The streams flow down the valleys, and the sharp angle of the contour points always _up_ stream. Note also how the junction of a stream and its tributary usually makes an angle that points _down_ stream.
"Which way does this stream run?"
Water flows down hill. If you are in the bed of a stream, contours representing higher ground must be to your right and to your left. Get the elevations of these contours. Generally the nearest contour to the bank of the stream will cross the stream and there will be an angle or sharp turn in the contour at this crossing.
If the point of the angle or sharp turn is toward you, you are going downstream; if away from you, you are going upstream.
If the contours are numbered, you have only to look at the numbers to say where the low and where the high places are; but to read a map with any speed one must be quite independent of these numbers.
In ordinary map reading look, first of all, for the stream lines.
The streams are the skeleton upon which the whole map is hung.
Then pick out the hilltops and ridges and you have a body to clothe with ail the details that will be revealed by a close and careful study of what the map maker has recorded.