Agriculture Reference
In-Depth Information
The first step is to make a visual
assessment of the topographic map with
the idea of grouping areas where the con-
tours fall within a similar range of distance
apart. Looking at Figure 3.4, you can see
that the USGS map has been separated into
zones: A, B, C, and D. The contours in zone
A are generally spaced at a greater distance
apart than the contours in the other zones.
Zone B contours are closer together than
those in zone A, and the contours in C are
even closer together than those in B. The
contours in zone D are generally the closest
together, and indicate the terrain is steeper or
at a greater percent of slope than in the other zones.
The contours in Figure 3.4 were first visually grouped into four cat-
egories. Category A contours appear to be the least densely spaced con-
tours (that is, the contours are spaced farther apart than other groups of
contours) in the topographic map, creating terrain that slopes from 0 to
5 percent. Group D contours are the most densely spaced and therefore
result in a steeper average slope of over 20 percent. The zones shown as
groups B and C fall somewhere in the mid-
dle of slope percentages, with the B area
terrain averaging 5 to 10 percent and group
C averaging 10 to 20 percent.
Figure 3.5 illustrates where the slope
groups would occur in the field if each
individual slope category were marked
out on the ground. Figure 3.6-A shows
another way to visualize the comparison
between different degrees of slope in
the landscape. The letter S noted on the
triangles in Figure 3.6-A is the slope of
the terrain expressed as a percent. Slope
Figure 3.3
Portion of a USGS quad map
D
D
D
B
A
D
D
C
B
A
B
D
C
B
C
Figure 3.4
USGS map divided into four categories of slope
from shallow (A) to steepest slope (D). Areas B and C are
medium slope.
