When you compute the materials for a sample line group, you can use three different volume calculation methods: Average End
Area, Prismoidal, or Composite.
- If the cut/fill areas between two successive stations are of similar shape, then the Average End Area Method can be used.
- If the terrain has greater changes between stations, then the Prismoidal Method can be more accurate.
- The Composite Method uses the actual surface data and does not use formulas to interpolate the volume between sample lines. This method is particularly
useful if the any of the surface triangle edges are smaller than the sample line interval.
You can specify which volume method to use in the following dialog boxes:
Average End Area Method
The Average End Area method calculates volumes by adding the area of a material type at one station to the area of the material
type at the next station and dividing the sum by two, then multiplying the result by the distance between the sections (L).
The Prismoidal method is similar to the Average End Area method but uses an additional cross section at the middle of the
two successive stations.
The Composite method is limited to material lists that have only two surfaces and cannot be used for material lists that contain
To calculate composite volumes, AutoCAD Civil 3D creates polygons between sample lines and then computes the bounded volumes of those polygons. The polygons are created by
joining the offsets of the sample lines between two successive stations, as shown in the following illustration.
- If the offsets are uniform, then a line that matches the curvature of the centerline is used to join the sample lines (1).
- If the offsets are not uniform, then a straight line is used to join the sample lines (2).
- The composite volume is then calculated for the area within the polygon (3) for each material in the material list of the
sample line group.
The volume is recorded at the second sample line of the polygon. The first sample line in the sample line group will therefore
have zero volume associated with it.