The rate of rise or fall of a landform is known as its slope and may be described as being gentle or steep. The question arises as to how gentle or how steep? The speed at which equipment or personnel can move is

Figure 5-16.-(a) Cut; (b) Fill.

Figure 5-17.-Depression.

Figure 5-19.-Graphic scale.

affected by the slope of the ground. Most equipment has a limit on the steepness of slope it can negotiate. So a more exact way of describing a slope is demanded. Slope may be expressed in several ways, but all of them depend upon a comparison of vertical distance (VD) to horizontal distance (HD) (fig. 5-18). VD is the difference between the highest and lowest elevations of the slope and is determined from the contour lines. HD is the horizontal ground distance between the highest and the lowest elevations of the slope.

The VD and HD must be expressed in the same units. Both measurements must be made with extreme accuracy in order to have a valid determination of steepness. The computations normally are made for only the steepest part of a slope.