Agriculture Reference
In-Depth Information
Figure 7.18 presents a sequence of diagrams, each with the same
set of three contour lines but different additional information. Diagram
No. 1 in Figure 7.18 has simply three wavy lines but is absent of any addi-
tional graphic or alphanumeric information. Without any additional
information, we can only guess what the three lines represent. For these
contours to be useful or even recognizable as contour lines, additional
information is required, as shown in diagram No. 2. The contours in
diagram No. 2 include an elevation above each contour. Now we can see
that the contours represent a partial slope with elevation 101 as the low
point and elevation 103 two feet higher. With the addition of a scale (in
this case 10˝ = 200´), the percent of slope can be calculated (see diagram
No. 3). The steepness or degree of slope can be determined by dividing
the vertical elevation difference between the contours (for instance, con-
tour 101 and 102) by the horizontal distance between the two contours.
In diagram No. 4 of Figure 7.18 a grid has been overlaid on the
contour lines. The grid is useful in a several ways. Assuming the grid
is referenced to site property lines, a contractor would be able to pin-
point the exact location of any existing or proposed object or feature,
including a precise elevation at any intersection within the grid. The
contractor would place stakes in the ground with the desired elevation
for each grid intersection. This grid pattern of stakes would guide the
earth-moving operations to achieve the grading shown in a site-grading
plan prepared by the designer. The method used to calculate spot eleva-
tions and slope will be covered in the Chapter 10, "How to Calculate Spot
Elevations."
SLope in pLan and Section
One of the tools used in developing a site-grading plan is a section draw-
ing. A section drawing aids the designer to quickly visualize existing
terrain under study, as well as to see what one or more proposed grading
strategies look like before deciding which design alternative is the bet-
ter solution. A section should be drawn to show relations of the terrain
