A Contour Map And Site Plan

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.

Click on the image to open in a PDF

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 by the 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 bold lines.
  • Dimensions of the lot and location of the building on it are also shown.

If you’re wanting a Site Plan, what I’ll need is simple to locate if you are someone other than the surveyor.

  1. Locate an up to date survey of the property. This can usually be found at the court house. One with the existing topography would be great if you’re wanting before and after contour lines. If a survey isn’t available, a tax map will do, although they aren’t the most reliable medium for boundary lines. The reason that the survey is better than the tax map is that certain information like property set backs, easements, etc. are usually already on them, saving time and research.
  2. Make a couple of copies of that survey or tax map, and sketch the building or buildings on it with any notes you feel I need to complete the drawing. If you have a CAD file of the building or buildings, please send them to me. This will save drawing time, and you money.