surface of the
ground. In other words, it is a line on a map which shows the route
one might follow on the ground and walk on the absolute level. If, for
example, you went half way up the side of a hill and, starting there,
walked entirely around the hill, neither going up any higher nor down
any lower, and you drew a line of the route you had followed, this
line would be a _contour line_ and its projection on a horizontal
plane (map) would be a _contour_.

By imagining the surface of the ground being cut by a number of
horizontal planes _that are the same distance apart_, and then
projecting (shooting) on a horizontal plane (map) the lines so cut,
the elevations and depressions on the ground are represented on the
map.

It is important to remember that the imaginary horizontal planes
cutting the surface of the ground must be the same distance apart. The
distance between the planes is called the _contour interval_.

2. The word _contour_ is also used in referring to _contour
line_,--that is to say, it is used in referring to the line itself in
which a horizontal plane cuts the surface of the ground as well as in
referring to the projection of such line on a horizontal plane.

An excellent idea of what is meant by contours and contour-lines can
be gotten from Figs. 5 and 6. Let us suppose that formerly the island
represented in Figure 5 was entirely under water and that by a sudden
disturbance the water of the lake fell until the island stood twenty
feet above the water, and that later several other sudden falls of the
water, twenty feet each time, occurred, until now the island stands
100 feet out of the lake, and at each of the twenty feet elevations a
distinct water line is left. These water lines are perfect
contour-lines measured from the surface of the lake as a reference (or
datum) plane. Figure 6 shows the contour-lines in Figure 5 projected,
or shot down, on a horizontal (level) surface. It will be observed
that on the gentle slopes, such as F-H (Fig. 5), the contours (20, 40)
are far apart. But on the steep slopes, as R-O, the contours (20, 40,
60, 80, 100) are close together. Hence, it is seen that contours far
apart on a map indicate gentle slopes, and contours close together,
steep slopes. It is also seen that the shape of the contours gives an
accurate idea of the form of the island. The contours in Fig. 6 give
an exact representation not only of the general form of the island,
the two peaks, O and B,