The other important function of the site plan is to highlight the special surface conditions, or topography, of the lot. This will indicate to the builder the slope or flatness of the site. The topography of a particular lot may be indicated on the site plan (Figure 6.6). For some projects the topography needs to be shown separately for clarity, and a grading plan has to be used. Topographical information includes changes in the site's elevation, such as slopes, hills, valleys, and other variations in the surface. These changes in the surface conditions are shown on a site plan by means of a contour, which consists of a line connecting points of equal elevation. An elevation is a distance above or below a known point of reference, called a datum. This datum could be sea level or an arbitrary benchmark established for the particular project.
Architects normally adjust the existing contours of a site to accommodate the building construction and site-improvement requirements. Adjusting existing contours is one of the stages in the site-improvement process, in which the architect or designer requires a topography map to study the slope conditions that may impact the design. This map is usually prepared by a civil engineer and is meant to show in drawing form the existing contour lines and their accompanying numerical elevations. Normally, existing contour lines are illustrated by a dashed line, and new or proposed contours are normally shown as a solid line. The topography map can therefore be considered to be a plat map in which its broken lines and numbers indicate the grades, elevations, and contours of the site. The distance between the contour lines is at a constant vertical increment, or interval. Typically, an interval of five feet is used, but other intervals may be substituted, and one-foot intervals are not uncommon for site plans requiring greater detail (or where the change in elevation is more dramatic).
When reading a site plan, note that contours are continuous and often enclose large areas in comparison to the size of the building lot. This is why contours are often drawn from one edge to the other edge of the site plan. Contours do not intersect or merge except in the case of a vertical wall or plane. For example, a retaining wall shown in plan view would show two contours touching, and a cliff that overhangs would be the intersection of the contours. When contour lines are spaced far apart, the land is relatively flat or gently sloping. When the contour lines are close together, the land is much steeper.
A benchmark is normally required, which is basically a known reference point such as an elevation on the construction site. The benchmark is established in reference to the datum and is commonly noted on the site print with a physical description and its elevation relative to the datum. For example, "Northeast corner of catch basin rim - elev.1085'" might be found on a site plan. When individual elevations or grades are required for other site features, they are noted with a "+" and the grade. Grades differ from contours in that a grade registers accuracy to two decimal places, whereas a contour is shown as a whole number. A grading plan shows existing and proposed topography (Figure 6.7) and is used to delineate elevations and drainage patterns. Grading-plan requirements will vary from one location to the next, but a final grading plan will typically show the site boundaries, existing landform contours with a benchmark, existing site features, and proposed site structures. In some instances the grading plan may also show a cross section through the site at specified intervals or locations to more fully evaluate the surface topography.
The north arrow is used to show the direction of magnetic north as a reference for naming particular sides or areas of the project. Moreover, surveyors label the property lines in accordance with the directions normally found on a compass. This reference, in the form of an angle and its corresponding distance, is called the bearing of a line. The bearings of the encompassing property lines often provide the legal description of the building lot. Larger projects usually need several site plans to show the different scopes of related or similar work including drainage, utility plans, and landscaping plans.
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