By means of this scale of M. D.'s on the map, the map reader can determine the slope of any portion of the ground represented, that is, as steep as 1/2° or steeper. Ground having a slope of less than 1/2° is practically level.
[1868]. Slopes. Slopes are usually given in one of three ways: 1st, in degrees; 2d, in percentages; 3d, in gradients (grades).
1st. A one degree slope means that the angle between the horizontal and the given line is 1 degree (1°). See [Fig. 10], [Par. 1867].
2d. A slope is said to be 1, 2, 3, etc., per cent, when 100 units horizontally correspond to a rise of 1, 2, 3, etc., units vertically.
Fig. 12
3d. A slope is said to be one on one (1/1), two on three, (2/3), etc., when one unit horizontal corresponds to 1 vertical; three horizontal correspond to two vertical, etc. The numerator usually refers to the vertical distance, and the denominator to the horizontal distance.
Fig. 13
Degrees of slope are usually used in military matters; percentages are often used for roads, almost always of railroads; gradients are used of steep slopes, and usually of dimensions of trenches.