A corridor represents a path, such as a road, trail, railroad, or airport runway. The geometry of a corridor is defined by a horizontal alignment and a profile. Together, these form the baseline - the centerline of the 3D path of the corridor. Along the length of the baselines are a series of assemblies which define the cross-sectional shape of the alignment. Common points in each assembly are connected to form feature lines. Together the assemblies and feature lines form the 3D shape of a corridor. A corridor also has one or more surfaces which can be compared against an existing ground surface to determine the amount of cut or fill required.