depending on whether its reference
number is greater or smaller than that of the outer contour. A hilltop
is shown when the closed contour is higher than the contour next to
it; a depression is shown when the closed contour is lower than the
one next to it.

If the student will first examine the drainage system, as shown by the
courses of the streams on the map, he can readily locate all the
valleys, as the streams must flow through valleys. Knowing the
valleys, the ridges or hills can easily be placed, even without
reference to the numbers on the contours.

=For example:= On the Elementary Map, Woods Creek flows north and York
Creek flows south. They rise very close to each other, and the ground
between the points at which they rise must be higher ground, sloping
north on one side and south on the other, as the streams flow north
and south, respectively (see the ridge running west from Twin Hills).

The course of Sandy Creek indicates a long valley, extending almost
the entire length of the map. Meadow Creek follows another valley, and
Deep Run another. When these streams happen to join other streams, the
valleys must open into each other.

=1867. Map Distances (or horizontal equivalents).= The horizontal
distance between contours on a map (called map distance, or M. D.; or
horizontal equivalents or H. E.) is inversely proportional to the
slope of the ground represented--that it to say, the greater the slope
of the ground, the less is the horizontal distance between the
contours; the less the slope of the ground represented, the greater is
the horizontal distance between the contours.

[Illustration: Fig. 10]

+-----------+--------+--------------+
| Slope | Rise | Horizontal |
| (degrees) | (feet) | Distance |
| | | (inches) |
+-----------+--------+--------------+
| 1 deg. | 1 | 688 |
| 2 deg. | 1 | 688/2 = 344 |
| 3 deg. | 1 | 688/3 = 229 |
| 4 deg. | 1 | 688/4 = 172 |
| 5 deg. | 1 | 688/5 = 138 |
+-----------+--------+--------------+

It is a fact that 688 inches horizontally on a 1 degree slope gives a
vertical rise of one foot; 1376 inches, two feet, 2064 inches, three
feet, etc., from which we see that on a slope of 1 degree, 688 inches
multiplied by vertical rises of 1 foot, 2 feet, 3 feet, etc., gives us
the corresponding horizontal distance in inches. For example, if the