When it’s necessary to design a building for a site that’s not level, the designer needs to know just how high each point is above the lowest part of the site. This information is obtained by the surveyor and delivered in the form of contours on a property boundary, and is called a contour map.
To give you an explanation, imagine a flooded condition of the neighborhood where the water is gradually rising. The part of the lot first covered with water would be that at the corner of Salem Avenue and North Harvard Boulevard. Here the shore line would follow the dotted line marked “101“. After the water has risen 4 more feet the shore line would follow the dotted line “105” and the water would have almost reached the location of the building. So you can see that each dotted line represents points of the same level across the lot.
Notice on the curb near the street intersection the bench mark which is marked “100“. This is always established on a fixed object, and the various levels measured from it using an instrument called a surveyor’s level.
Look at the note on the building plan that says that the basement floor elevation is “105.0“. This means that the basement floor is to be 5′ – o” above the bench mark. So all the levels of the building and site are measured from this fixed elevation point.
The contour map is very important, because it tells the designer just how far below the first floor the ground is at any point around the building. This information is needed for properly placing doors, windows, steps, etc., in the outside walls at the ground level. It’s also useful in determining the amount of excavating that has to be completed for the basement, and for the fill for grading outside when the building’s completed.
This site plan also shows:
- The location of the existing trees and their approximate size. They’re indicated by the spots on the drawing and are sometimes noted as to kind and size.
- Location and depth of sewers and location of the water main are also shown.
- The points of the compass are given on the map when they are needed.
- The lines showing the original contour of the lot are shown on this map by dotted lines and the future or proposed grade is shown by solid freehand lines.
- Dimensions of the lot and location of the building on it are also shown.
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