From the above, we have this rule:
To construct a scale of M. D. for a map, multiply 688 by the contour interval (in feet) and the R. F. of the map, and divide the results by 1, 2, 3, 4, etc., and then lay off these distances as shown in [Fig. 11], [Par. 1867a].
FORMULA
M. D. (inches) = 688 × V. I. (feet) × R. F./Degrees (1, 2, 3, 4, etc.)
[1867a]. Scale of Map Distances (or, Scale of Slopes). On the [Elementary Map], below the scale of miles and scale of yards, is a scale similar to the following one:
Fig. 11
The left-hand division is marked 1/2°; the next division (one-half as long) 1°; the next division (one-half the length of the 1° division) 2°, and so on. The 1/2° division means that where adjacent contours on the map are just that distance apart, the ground has a slope of 1/2 a degree between these two contours, and slopes up toward the contour with the higher reference number; a space between adjacent contours equal to the 1° space shown on the scale means a 1° slope, and so on.
What is a slope of 1°? By a slope of 1° we mean that the surface of the ground makes an angle of 1° with the horizontal (a level surface. See [Fig. 10], [Par. 1867]). The student should find out the slope of some hill or street and thus get a concrete idea of what the different degrees of slope mean. A road having a 5° slope is very steep.