Autodesk CIVIL3D - CIVILDESIGNCOMP 2007 SLM CD Tutorials Manual page 695

13 Repeat these steps for each segment of the centerline, starting at the top
and ending at the bottom segment. Dimensions named LenA2 through
LenA5 are created for the centerline of the structure.
NOTE For this exercise, make sure you dimension the segments in the order
shown in the following illustration.
14 Add a Distance dimension to the lower horizontal line by picking the
points at each end. This represents the distance from the structure
centerline to the start of the elbow. This is used to ensure that the drop
stays on the outside of the structure. LenA6 is created.
Exercise 2: Defining the Manhole Geometry | 683